s-block ~ I II [I Li Be 3 4 d -bl ock Na Mg A..-- 11 12 I K Ca 19 20 Rb Sr 37 38 Cs Ba 55 56 Fr Ra 87 88 Periodic Table of the Elements o 2
I He II ill] III IV V VI VIII 4.0 3 4 5 6 7 8 9 10 Li Be B C N 0 F Ne 6.9 9.0 10.8 12.0 14.0 16.0 19.0 20.2 11 12 13 14 15 16 17 18 Na Mg Al Si P S CI Ar 23.0 24.3 27.0 28.1 31.0 32.1 35.5 39.9 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 3! 36 K Ca Sc Ti V Cr Mn Fe Co Ni eu Zn Ga Ge As Se BrlKr 39.1 40.1 45.0 47.9 50.9 52.0 54.9 55.9 58.9 58.7 63.5 65.4 69.7 72.6 74.9 79.0 79 83.8 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 S4 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe 85.5 87.6 88.9 91.2 92.9 95.9 99.0 101.1 102.9 106.4 107.9 112.4 114.8 118.7 121.8 127.6 126.91 ' 3 ' . 3 55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 Cs Ba La 4 Hf Ta W Re Os If Pt Au Hg Tl Pb Bi Po AtlRn 132.9 137.3 138.9 178.5 181.0 183.9 186.2 190.2 192.2 195.1 197.0 200.6 204.4 207.2 209.0 210.0 210.01222.0 87 88 89 Fr Ra Ac~ 223.0 226.0 227.0
58 59 60 61 62 63 64 65 66 67 68 69 70 71 Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 140.1 140.9 144.2 (147) 150.4 152.0 157.3 158.9 162.5 164.9 167.3 168.9 173.0 175.0 90 91 92 93 94 95 96 97 98 99 100 101 102 103 "--- Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lw 232.0 231.0 238.1 (237) 239.1 (243) (241) (247) (251 ) (254) (253) (256) ( 254) (257)
A value in brackets denotes the mass number of the most stable isotope. UNIT lJ s-Block Elements
1IIIIill[~llrlllll~lilr~11111 N27910 L.-
E
~ Inner London Education Authority 1983
First published 1983 by John Murray (Publishers) Ltd 50 Albemarle Street London W1X 480
Reprinted 1984~ 1985~ 1987
All rights reserved. Unauthorised duplication contravenes applicable laws
Printed and bound in Great Britain by Martin's of BerwicK
British Library Cataloguing in Publication Data Independent Learning Project for Advanced Chemistry s-BlocK Elements. - (ILPAC; Unit 11) 1. Science I. Title II. Series 500 0161.2
ISBN 0 7195 4049 6
1/ CONTENTS
PREFACE v ACKnowledgements vi Key to activity symbols and hazard symbols vii
INTRODUCTION 1 Pre-Knowledge 2 PRE-TEST 3
THE s-BLOCK ELEMENTS 5 PHYSICAL PROPERTIES 5 Electronic configurations 5 Atomic and ionic radii 6 Ionization energies 7 Electronegativity B Melting points and boiling points 9 Hardness 10 CHEMICAL PROPERTIES 11 Reaction with water 11 Reaction with dilute acid 13 Reaction with oxygen 14 Experiment 1 - reaction between sodium peroxide and water 15 Reaction with nitrogen 19 General properties related to electron loss 20 Reaction with hydrogen 20 s-BLOCK COMPOUNDS 22
THE EFFECT OF HEAT ON SOME s-BLOCK COMPOUNDS 22 Experiment 2 - heating nitrates and carbonates 22 The hydrogencarbonates, sulphates and hydroxides 26 The hydrogencarbonates 27 The sulphates 27 The hydroxides 2B THE SOLUBILITY OF THE s-BLOCK COMPOUNDS IN WATER 29 Experiment 3 - the solubility of some salts of Group II elements 30 THE DIAGONAL RELATIONSHIP BETWEEN LITHIUM AND MAGNESIUM 33 END-OF-UNIT CHECKLIST 36 END-OF-UNIT TEST 37
APPENDIX 41 SOME INDUSTRIAL CHEMISTRY OF THE s-BLOCK ELEMENTS 41 The Downs process for the extraction of sodium 41 The extraction of magnesium from sea-water 42 The manufacture of sodium hydroxide 42 Some uses of s-blocK elements and compounds 44
ANSWERS TO EXERCISES 46
ii i
PREFACE
This volume is one of twenty Units produced by ILPAC, the Independent Learning Project for Advanced Chemistry, written for students preparing for the Advanced Level examinations of the G.C.E. The Project has been sponsored by the Inner London Education Authority and the materials have been extensively tested in London schools and colleges. In its present revised form, however, it is intended for a wider audience; the syllabuses of all the major Examination Boards have been taken into account and questions set by these boards have been included.
Although ILPAC was initially conceived as a way of overcoming some of the difficulties presented by uneconomically small sixth forms, it has frequently been adopted because its approach to learning has certain advantages over more traditional teaching methods. Students assume a greater responsibility for their own learning and can work, to some extent, at their own pace, while teachers can devote more time to guiding individual students and to managing resources.
By providing personal guidance, and detailed solutions to the many exercises, supported by the optional use of video-cassettes, the Project allows students to study A-level chemistry with less teacher-contact time than a conventional course demands. The extent to which this is possible must be determined locally; potentially hazardous practical work must, of course, be supervised. Nevertheless, flexibility in time-tabling makes ILPAC an attractive propo- sition in situations where classes are small or suitably-qualified teachers are scarce.
In addition, ILPAC can provide at least a partial solution to other problems. Stude~ts with only limited access to laboratories, for ex~mple, those studying at ev~ning classes, can concentrate upon ILPAC practical work in the laboratory, in the confidence that related theory can be systematically studied elsewhere. Teachers of A-level chemistry who are inexperienced, or whose main discipline is another science, will find ILPAC very supportive. The materials can be used effectively where upper and lower sixth form classes are timetabled together. ILPAC can provide 'remedial' material for students in higher education. Schools operating sixth form consortia can benefit from the cohesion that ILPAC can provide in a fragmented situation. The project can be adapted for use in parts of the world where there is a severe shortage of qualified chemistry teachers. And so on.
A more detailed introduction to ILPAC, with specific advice both to students and to teachers, is included in the first volume only. Details of the Project Team and Trial Schools appear inside the back cover.
LONDON 1983
v ACKNOWLEDGEMENTS
ThanKs are due to the following examination boards for permission to reproduce questions from past A-level papers:
Oxford and Cambridge Schools Examination Board Teacher-marKed exercise, p17(1976)
Oxford Delegacy of Local Examinations Teacher-marKed exercise, p35(19B1) End-of-Unit Test 6(1977)
University of London Entrance and School Examinations Council Exercise 5 (N1974) Teacher-marKed exercises, p21(L19BO), p35(L197B) End-of-Unit Test 2(L1981), 4(L1979), 5(N197B), 7(L19B1)
Welsh Joint Education Committee End-of-Unit Test 1(1979)
Where answers to these questions are included, they are provided by ILPAC and not by the examination boards. Questions from papers of other examining boards appear in other Units.
Photographs are included by permission as follows: Nuclear reactor, p9 - Popperfoto Cutting potassium metal, p10 - Tony Langham Potassium reacting with water, p12 - Tony Langham Stalagmites and Stalactites, p27 - Popperfoto X-ray photograph of stomach, p32 - Southend General Hospital Photographs of students - Tony Langham
vi SYMBOLS USED IN ILPAC UNITS 0 ill Reading Revealing exercises
Exercise Discussion ~ ® ~\;'"Test ~ Computer programme
~ Experiment o 'A' Level question I[ 11 Video programme 'A' Level part question 00
- [Q8 Film loop 'A' Level question Special paper ~ Model-making
Worked example ~
[] Teacher-marked exercise
INTERNATIONAL HAZARD SYMBOLS
Radioactive [Xl Harmful I~I Toxic
! ~ I Flammable ~.f_~ Explosive
~ ':'4 Corrosive !~! Oxidising
vii
INTRO DUCTION
In this Unit we deal with the elements and compounds of Groups I and II (the s-blocK). To remind you of the elements in these groups, we include their symbols and group names in the outline Periodic Table in Fig. 1 below.
s-block p-block ~ II OJ III IV V VI VII 0 Li Be 3 4 d-block .I.
K Ca 19 20 Rb Sr 37 38 Cs Ba 55 56 Fr Ra 87 88 0 -
Fig.1.
We deal with Groups I and II concurrently so that we can maKe comparisons between them, as well as consider the trends in properties within each group.
Since this is a short Unit, and there is no obvious progression of difficulty in the subject matter, we have not divided it into two Levels. Consequently there is no Level One Test.
In the first part of the Unit we deal with the physical and chemical proper- ties of the elements. (The distinction between physical and chemical properties is not rigid; we simply use the terms as convenient labels for organising information.) Then we consider the properties of some of the compounds of the s-blocK elements.
In an appendix we consider briefly some of the industrial chemistry of these elements and their compounds.
There are three experiments in this Unit.
1 PRE-KNOWLEDGE
Before you start work on this Unit, you should be able to:
(1) identify the B-. p- and d-blocks in the Periodic Table; (2) state the names and symbols of the first four elements in Groups I and II; (3) describe the usual appearance of the elements lithium. sodium, potassium. magnesium and calcium; (4) describe the reactions of sodium. potassium and calcium with water, and write equations; (5) write general equations for the reactions of s-block monoxides with water; (6) write equations for the reactions between magnesium and dilute acids; (7) state the effect of heat on sodium carbonate and calcium carbonate; (8) name the brown gas given off when certain nitrates are heated; (9) describe simple chemical tests for (a) oxygen. (b) hydrogen, (c) carbon dioxide. (d) sulphate ions, (e) carbonates and hydrogencarbonates; (10) perform flame tests and state the flame colours for lithium, sodium. potassium, calcium, strontium and barium; (11) state the meaning of the terms (a) ionization energy. (b) electron affinity, (c) electronegativity, (d) atomic and ionic radii, (e) acidic, basic and amphoteric oxides, (f) oxidation and reduction, (g) oxidation number, (h) polarizing power of a cation, (i) polarizability of an anion; (12) state and apply the following simple solubility guide-lines: (a) all metal nitrates are soluble in water. (b) most metal chlorides are soluble in water, (c) most metal sulphates are soluble in water, (d) all sodium, potassium and ammonium salts are soluble in water.
PRE-TEST
To find out whether you are ready to start, try the following test which is based on the pre-knowledge items. You should.not spend more than 45 minutes on this test. Hand your answers to your teacher for marking.
2 PRE-TEST
1. D is a white crystalline solid which, when heated, produces a mixture of two gases. One is brown and poisonous; the other is colourless and relights a glowing splint. The solid gives an orange-red flame test. Suggest what D may be and give an equation for its decomposition. (3)
2. E is a white crystalline solid which is soluble in water and gives a yellow flame test. Its aqueous solution gives a white precipitate with acidified barium chloride solution. Identify E and give an equation for its reaction with barium chloride solution. (3)
3. Describe the effect of dilute hydrochloric acid on
(a) calcium carbonate, CaC03, (b) magnesium ribbon, Mg, ( 6 ) (c) solid sodium hydrogencarbonate, NaHC03•
4. (a) Complete and balance the equations shown below. State the oxida- tion number for each atom. appearing in each equation.
(i ) Na (s ) + H20 (1 ) -+
(ii) Na(s) + C12(g) -+ ( 4 ) (b) For each of the above reactions s·tate which atom is oxidized and which is reduced. ( 2)
5. Y is a soft metal, easily cut with a knife. When placed in water it floats, melts and ignites with a lilac-coloured flame. Identify Y. (1)
6. This question concerns the following ions:
2 +, 2 +, +, 2-, - Li 8e K C03 0 and r (a) Place the cations in order of increasing polarizing power. (1) (b) Put the anions in order of increasing polarizability. (1) (c) Which combination of cation and anion will produce a compound with the greatest covalent character? Explain your answer. (2)
7. State whether the solution formed in each of the following cases is acidic, alkaline or neither. Give a reason for each answer. (a) Calcium oxide, CaD, added to water. (b) Sodium chloride, NaCl, added to water. ( 6) (c) Lithium oxide, Li20, added to water.
8. Write thermochemical equations to specify: (a) the first ionization energy of sodium, (b) the first electron affinity of oxygen. ( 4 )
9. What two simple tests would help you to decide whether or not an oxide is amphoteric? (2) (Total 35 marks)
3
THE s - BLOCK ELEMENTS
To obtain a simple overall picture of the s-block elements we suggest that you read the introduction to the s-block [or to Groups a I and II if they are treated se~arately) in your text-book(s). Also, skim the rest of the chapter(s) so that you know how the W detailed information on these elements is organised.
In the first part of the Unit we are concerned with the chemistry of the elements themselves. For convenience, we consider physical and chemical properties separately. We shall regard as physical properties those which are not necessarily linked with specific chemical reactions, and which can be given numerical values, but these distinctions are rather arbitrary.
PHYSICAL PROPERTIES
We begin with some properties associated with the behaviour of single atoms. For convenience, we refer to these as atomic properties.
Objectives. When you have finished this section, you should be able to:
(1) write down the electronic configurations of the first four elements in Groups I and II, using the s, p, d notation; (2) explain, in terms of atomic structure, the trends in Groups I and II, in (a) atomic and ionic radii, (b) ionization energy, [c) electronegativity; (3) state the oxidation numbers shown by the elements of Groups I and II in their compounds.
Most of the physical and chemical properties of an element are directly related to its atomic structure. You now spend a short time revising the electron arrangements of the s-block elements.
Electronic configurations
In the first exercise we want you to recall what you have already learned about writing electronic configurations and explore some of their implications in the case of the s-block elements.
Exercise 1 (a) Write down the electronic configurations of the first four elements in Groups I and II using the s, p, d notation. (b) Why are these elements placed in Groups I and II?
Cc) How do the configurations of lithium and beryllium differ from those of the other members of their groups? Cd) What are the charges on ions of Groups I and II? Explain. (Answers on page 46 )
5 In th8 last 8x8rcis8, th8 8mphasis was on th8 similariti8s b8tw88n 818m8nts in th8 sam8 group. Equally important, how8v8r, ar8 th8 r8gular chang8s that tak8 plac8 down a group as 8ach 8xtra sh811 of 818ctrons is add8d. Th8s8 tr8nds h8lp you to pr8dict th8 prop8rti8s of 818m8nts which app8ar low8r down th8 groups and which you may not study in any d8tail.
W8 now go on to id8ntify tr8nds in atomic and ionic radii of Groups I and II.
Atomic and ionic radii
You should b8 familiar with th8s8 t8rms from Unit S4: Structur8 and Bonding. Th8 n8xt 8x8rcis8 conC8rns th8 variation of th8s8 radii in th8 a-block.
EX8rcis8 2 study Fig. 2 and th8n anSW8r th8 qU8stions b8low. ~
Atomic Ionic Atomic Ionic (covalent) (M+) (covalent) (M2+) radius/nm radius/nm radius/nm radius/nm C0 0.123 e 0.060 @ 0.089 ® 0.031 G 0.157 e 0.095 G) 0.136 ~ 0.065 8 0.203 0.133 8 0.174 0.099
;,.~ G 0.216 0.148 G 0.191 '% 0.113 8 0.235 0.169 8 0.198 0.135 Fig.2. Atomic and ionic radii ofthe s-block elements.
( a) From your know18dg8 of atomic structur8 8xplain why: (i ) atomic and ionic s i ze incr8as8 down 8ach group; (ii) Group I atoms and ions ar8 larg8r than th8ir Group II oount erper-t s. (b ) Which of th8 Group II atoms is similar in siz8 to th8 lithium atom? ( c ) Which of th8 Group II ions is similar in s Ize to th8 lithium ion? l Answe r-s on pag8 46 J
Th8 similarity in siz8 of th8 lithium and magn8sium ions l8ads to a numb8r of ch8mical similariti8s in th8 compounds of th8 two 8l8m8nts. W8 discuss this lat8r in th8 Unit, und8r th8 h8ading diagonal r8lationships.
6 Now that you have seen the trends in atomic size we go on to examine ionization energies.
Ionization energies
In Unit S2: Atomic Structure, you learned how ionization energy is related to the structure of the atom. In the following exercise you will see how this property varies in Groups I and II.
Exercise 3 Copy the following table and enter the values using your data booK. Then answer the questions below. Table 1
Ionization energy KJ/mol-1
Element First Second Third
Li Na K Rb Cs 3300
Be Mg Ca Sr Ba 3390
(a) For each element in Group I, explain why the first ionizatidn energy is so much smaller than the other two. (b) Explain the relatively large difference between the second and third ionization energies for all Group II elements. (c) Describe and explain the trend in ionization energies as each group is descended. (d) How do you expect the reactivity of the elements to change on going down each group? Explain your answer. (e) How would you expect the reactivity of the elements to change going across from Group I to Group II in the same period?
(Answers on page 46 )
The predictions you have made in Exercise 3 Cd) and (e) will be tested when you deal with the chemical properties of the elements later in this Unit.
A property directly related to ionization energy is electronegativity, which we now discuss.
7 Electronegativity
You were first introducted to this term (and electropositivity), in Unit S4: Bonding and structure, where you used it to predict the ionic and covalent character of bonds, and linked it to polarizing power.
You also learned in that Unit (Exercise 54) that electronegativity decreases down each group and increases across each period. In the next exercise you use your knowledge of ionization energies to explain these trends in the s-block elements.
Exercise 4 (a) Explain why electronegativity decreases as both Groups I and II are descended. (b) Why does electronegativity increase from Group I to Group II in the same period? (c) Which of the s-block elements is the most electropositive? (d) Which of the s-block elements is least likely to form an ionic compound with chlorine (electronegativity 3.D)? (Ahswers on page 46
In Unit S2 you considered another atomic property of the s-block elements - the light emitted from their excited ions. You will use these flame colours to identify some s-block cations in Unit 12: The Halogens.
You should now be able to do the next exercise which is taken from an A-level paper.
Exercise 5 (a) What is the electronic configuration, in terms of s, p and d electrons, of (i) the beryllium atom; (ii) the barium atom? (b) (i) What is the oxidation state common to both elements in their compounds? (ii) State, in terms of ionization energies, why there is little likelihood of a higher oxidation state for either element. (iii) To what Group of the Periodic Table do these two ele- ments belong? (c] (i) Which of the two elements has the higher first ioniza- tion energy? Give a reason for this. (ii) Would their ions be smaller or larger than their atoms? Give a reason for this. (Answers on page 46 )
Now we look at those physical properties of the elements which depend not only on the nature of individual atoms, but also on the way they are linked together in the bulk material.
8 Objectives. When you have finished this section, you should be able to:
(4) state and explain the trends in melting-points and boiling-points within Groups I and II; (5) describe and explain the difference in h~rdness for the a-block metals.
We start this section by considering the trends in melting-points and boiling- points of the a-block elements.
Melting-points and bOiling-points
Most of the differences in bulk properties can be interpreted in terms of the strengths of metallic bonds. The strengths of these bonds ar~'dependent on the following factors: (i) atomic size; (ii) number of electrons in the outer shell of each atom; (iii) . metallic crystal structure.
Remem~ering these points, and what you have already learned about metallic bonding in Unit S4: Bonding dnd Structure, attempt the next exercise.
E"ercise 6 Look up the melting-points of the elements in Groups I and II using your-data book. For each group in turn, plot the melting-point values on the vertical axis against atomic number on the horizontal axis, on the same piece of graph paper. Then answer the questions below. (a) What is the general trend in each group? (Look for a simple overdll trend, e.g. increase or decrease, ignoring the occasional 'hiccup'.) (b) Suggest an explanation for the trends in terms of 'atomic' size. (Hint: think of the delocalized electrons as a type of 'cement' holding the ions together. Thin 'cement' leads to weak metallic bonding.) (c) Why does each Group II element have a higher melting-point than the Group I element in the same period? (d) Liquid sodium is used as a coolant in some nuclear reactors. Give reBsonswhy you think it is preferable to water. (Use your data book.) (Answers on page 47 )
9 The graph in Fig. 3 shows that similar trends exist in the boiling-points of the s-block elements.
Be Ba 1500 ~ C 1000 '0 a. en c 500 '0 .0
10 20 30 40 50 60 atomic number
Fig.3. The boiling-points of the s-block elements.
The changes in Group II are irregular and this is partly due to differences in crystal structure of the metals in this group. The following exercise deals with the types of crystal structure found in the s-block elements.
Exercise 7 Look up the structures of the metals in Groups I and II in your data book. How does this help explain the irregularities in Fig. 3 for Group II?
(Answer on page 47 )
You should remember that there is no simple relationship between crystal structure and melting-point and boiling-point. We can only conclude that crystal structure affects physical properties.
We now turn to one of the more easily observed properties of the s-block elements - their hardness as metals.
Hardness
You probably already know that all the alkali metals are soft enough to be cut with a knife, whereas the first three alkaline earth metals are not. In the following exercise you suggest explanations for this.
10 Exercise 5 ( a ) Give a reason, based on the simple model of metallic bonding, to explain why Group II elements are much harder to cut than those of Group I.
(b) Explain why the metals from each group become easier to cut with increasing atomic number. (Answers on page 47 )
The last few sections hBve given you a picture of the physical nature and characteristics of the s-block elements. We now go on to consider their chemical properties, and test the predictions you have made, on the basis of ionization energies, concerning the reactivity of the elements.
CHEMICAL PROPERTIES
The s-block elements are the most reactive metals in the whole Periodic Table and combine directly with the majority of non-metals. We have selected four reactions to illustrate trends in reactivity. (You will not handle beryllium or its compounds because they are very toxic.)
Objectives. When you have finished this section, you should be able to:
(6) illustrate trends in reactivity by reference to the reactions of s-block elements with water, dilute acids, oxygen and nitrogen; (7) identify the types of oxide formed by the s-block elements as basic or amphoteric; (5) state how the various oxides of s-block elements react with water, giving equations for the reactions.
Most text-books have a section on the chemical reactions of the s-block elements, either taken together, or in a separate chapter for each group. Read about these reactions, noting particularly the ones mentioned in Objective 6.
We start this section with the reactions of the s-blocK elements with water.
Reaction with water
This reaction is often used to illustrate the general trend of increasing reactivity from the top of Groups I and II to the bottom. If you haven't seen the reactions of lithium, sodium, potassium, magnesium and calcium with water, aSK your teacher to demonstrate them. Now do the next exercises, which are concerned with the reactivity of the s-blocK metals towards water.
11 Exercise 9 ( a ) Write a general equation for the reaction between an alkali metal and water, using M as the symbol for a metal.
(b) When the alkali metals react with water, a flame is seen for potassium but not for sodium. Why is this? (Answers on page 47 )
Exercise 10 (a) Describe the reaction between calcium and water, and write an equatio~. (b) Magnesium can also be shown to react with water, using the apparatus shown in Fig. 4.
magnesium Fig.4.
(i) Why is hot water used in the experiment? (ii) What is the gas produced and how is it tested? (c) How would you expect beryllium and barium to react with water? (Answers on page 47 )
12 Exercise 11 Fig. 5 shows an experiment which demonstrates that hot magnesium reacts vigorously with steam.
ceramic wool soaked in WI le;J
t t HEAT HEAT Fig. 5. (X) (Y)
(a) Select from the following the best position(s) for the Bunsen burner: (i) position (Y) only; (iv) (X) Lrrl t i elly then (Y); (ii) position (X) only; (v) (Y) to (X) then back to (YJ. (iii) (Y) initially then (X); (b) Explain your answer to part (a). (c) Write a balanced equation for the reaction taking place. (Answers on page 48 )
Next, we consider the reaction of the s-block elements with dilute acids.
Reaction with dilute acids
We here restrict our discussion to dilute, non-oxidizing acids, namely hydrochloric acid and sulphuric acid. Some organic acids such as dilute ethanoic (acetic) acid would react in much the same way. Dilute nitric acid reacts with s-block elements in a more complex way as you will find out in Unit 16.
You already know how magnesium reacts with dilute acids from your pre-A-level course. In the next exercise, you are asked to predict the reactions of other elements with dilute acids.
Exercise 12 ( a) HGW would you expect beryllium to react with dilute sulphuric and hydrochloric acids? ( b ) Which of the following acids will produce hydrogen at a faster rate when added to magnesium ribbon? (i) 0.1 M hydrochloric acid (ii) 0.1 M ethanoic acid. Explain your answer. (c) Why is it dangerous to place the alkali metals in dilute acids? (Answers on page 48 ) Another sign of the high reactivity of s-block elements is the ease with which they combine with oxygen, forming various oxides.
Reaction with oxygen
You have probably seen sodium burning in air with a yellow flame, but the white product remaining is not pure sodium oxide, Na20, as you might expect. In fact, it is mainly sodium peroxide, Na202.
With the exception of lithium and beryllium, all s-block elements produce more than one oxide. In addition to the monoxides you met in your pre-A-Ievel course, there are also some peroxides and superoxides. The three types of oxide are all ionic, and the ions are related as follows:
o 2- 2 monoxide peroxide superoxide
Read about these oxides in your text-book(s). (Superoxides are sometimes called hyperoxides.) You should know which oxide(s) is/are formed when each element is burned in an adequate supply of oxygen or air. Also look for the reactions of these oxides with water and find out which monoxide is amphoteric. You should then be able to do the exercises which follow.
Exercise 13 (a) Use the flow scheme given above to: (i) write an equation for the conversion of sodium monoxide to sodium peroxide, (il) suggest the conditions under which sodium monoxide can be prepared. (b) (i) Which of the oxides shown in the flow scheme is not normally formed by Group II elements? (ii) Write down the names and formulae of the oxides formed directly by the alkaline earth metals. (iii) When barium peroxide is heated, it decomposes to produce a colourless gas. Write an equation for this decomposition. (Answers on page 48 )
An important point about most s-block monoxides is that they are basic. The next exercise is concerned with one s-block monoxide which is amphoteric.
Exercise 14 (a) Which s-block monoxide is amphoteric? (b) Give equations to show how this oxide reacts with: (i) sodium hydroxide solution, (ii) dilute nitric acid.
(Answers on page 48 )
14 Peroxides are also basic, although when they react with water, they not only produce the hydroxides of the metals but also other products. The next short experiment illustrates this point.
EXPERIMENT 1 Reaction between sodium peroxide and water
Aim The purpose of this experiment is to identify the products formed when sodium peroxide, Na202, reacts with water.
Introduction When the s-block monoxides dissolve in water, they produce hydroxide ions which make the resulting solutions alkaline. For example, CaO(s) + H20(1) ~ Ca(OH)2(aq) In this experiment, you will note the pH of the resulting solution when sodium peroxide, Na202, reacts with water, and identify other products which are also formed.
One of the products you will be asked to test for is hydrogen peroxide, H202• A very sensitive test for this is one which involves shaking it with orange potassium dichromate(VI) solution, K2Cr207, dilute sulphuric acid and pentan-l-ol, C5H~~OH. If a blue colour develops in the organic layer, then hydrogen peroxide is present. (The blue colour is thought to be due to Cr05, which is stable in pentanol but not in water.)
Requirements safety spectacles 4 test-tubes in rack Bunsen burner and bench mat spatula sodium peroxide, Na202------distilled water wood splints 3 dropping pipettes universal indicator I .~ I potassium dichromate( VI) solution, 0.02 M K2Cr207 ------0' dilute sulphuric acid, 2 M H2S04 pentan-l-olCamyl alcohol L C5HUOH ------1&1
15 Hazard warning Pentanol is highly flammable. Therefore you MUST: KEEP THE STOPPER ON THE BOTTLE WHEN NOT IN USE. KEEP THE LIQUID AWAY FROM FLAMES. Sodium peroxide i~ corrosive and a powerful oxidant. Therefore you MUST: WEAR SAFETY SPECTACLES AVOID CONTACT WITH SKIN
Procedure 1. Cautiously add about 0.1 g of sodium peroxide to about 3 cm3 of distilled water in a test-tube, and immediately test the gas evolved. (What are the possibilities?)
2. To the resulting solution add a few drops of universal indicator.
3. Put a few grains (less than 0.1 g) of sodium peroxide in a test- tube and add the following reagents, in order: 3 cm3 of distilled water, 3 drops of potassium dichromate solution, 1 cm3 of pentan-1-ol and 3 cm3 of dilute sulphuric acid. Shake gently and let the two layers separate. 4. Record your observations and conclusions in a copy of -Results Table 1.
Results Table 1
Experiment Observation Conclusion
Sodium peroxide Colour of gas ...... The gas is ...... added to water and Effect on glowing universal indicator splint ...... pH of solution ......
Sodium peroxide Colour of organic (top) The colour of the plus water, layer ...... pentanol layer added to acidified Colour of aqueous (bottom) ...... dichromate solution layer ...... plus pentanol
(Specimen results on page 48 )
Questions 1. At 0 DC, sodium peroxide reacts with water without the evolution oflxygeh. oxygen. Hydrogen peroxide is detected at this temperature. (a) Write an equation for this reaction. (b) The evolution of oxygen at higher temperatures is believed to be due to a secondary reaction. Write an equation for this secondary reaction. (c) What products are likely to be obtained if sodium peroxide is added to very hot water? Give a balanced equation.
16 2. (a) Predict the reaction between barium peroxide and ice-cold water. Give a balanced equation in your answer. (b) Hydrogen peroxide can be prepared in the laboratory by adding barium peroxide to ice-cold dilute sulphuric acid. (i) Write an equation for this reaction. (ii) Why do you think dilute sulphuric acid is used in the place of water? 3. All s-block oxides (apart from BeD) are basic OXloes and thus react with water to form hydroxides. and with acidic substances to form salts. How would you expect the following oxides to react with carbon dioxide?
(Answers on page 48 )
You should now be able to do the following Teacher-marked Exercise. which is taken from a past A-level paper. We suggest that. before you start. you discuss with fellow students and/or your teacher what is meant by the phrase 'most economical to carry' in the context of the question.
Teacher-marked A. ~80 is a stable oxygen isotope. By means of Exercise equations. show how it would be possible to obtain from H2~80 samples of (aJ ~802(g). (bJ a solution of H2~802 in H2~80. B. The atmosphere in a manned spacecraft needs to be purified from exhaled carbon dioxide and replenished with oxygen. Write equations for the reactions between carbon dioxide and (aJ lithium oxide. Li20, (bJ sodium peroxide, Na202' and (c) potassium superoxide. K02. Which of the following systems would be the most economi- cal to carry in a spacecraft for regeneration of the atmosphere? Give your reasons. (i) lithium oxide and liquid oxygen. (ii) sodium peroxide and liquid oxygen, (iii) potassium superoxide alone. (Use relative atomic masses to the nearest whole number.)
We have already mentioned that lithium and beryllium are not typical of their groups. A further illustration of this is provided by the formation of their oxides. Table 3 summarises the facts about the s-block oxides. Study this table and the following paragraphs in preparation for a Revealing Exercise.
17 Table 2 Oxides of the g-block elements
Elements which form Elements which form oxides directly oxides indirectly Type of oxide in an adequate supply of air. by other methods
monoxide, 02- Li 8e Mg Ca Sr 8a all Group I and II
2- peroxide, O2 Na Ba all except 8e
- superoxide, O2 K Rb Cs Na (with difficulty)
Atomic and ionic size play a very important role in determining both the chemical behaviour and structure of the s-block compounds. For example, as you learned in Unit S4, small cations have a high polarizing effect on large anions. This leads to a degree of covalent character in ionic compounds and so affects the chemical properties of such compounds (e.g. those of Li and 8e).
Furthermore, the relative sizes of the cations and anions can determine the type of structure adopted - in many cases the radii are such that no stable structure can form at all.
In the following Revealing Exercise, you explore some possible reasons for the variety of oxides formed by the s-block elements.
01. How does the polarizing power of the cation change as Groups I and II are descended? Explain your answer.
A1. Polarizing power decreases as each group is descended. The smaller the cation the greater its polarizing power (for the same charge). Since ionic size increases down each group then polarizing power decreases.
02. How does the polarizing power of Li+ compare with that of 8e2+? Explain your answer.
A2. 8e2+ has a greater polarizing power than Li+. This is because 8e2+ is smaller and more highly charged than Li+. 8e2+, therefore, distorts the charge clouds around an anion more.
03. Which of the following anions is more easily polarized - the peroxide 2 ion, 02 -, or monoxide ion, 02-? Explain your answers.
A3. Peroxide ion. It is much larger than the monoxide ion and its electron cloud is therefore more easily distorted.
04. Why is it extremely difficult to prepare lithium peroxide, and why are there no reports at all that beryllium peroxide has been prepared? (Explain in terms of ionic size and polarizing power.)
18 A4. The small cations greatly distort the electron cloud around the per~xide ions. The more the electron cloud is distorted, the more likely it is for the compound to decompose. Because Be2+ distorts the anion more than Li+, beryllium peroxide is even more unstable than lithium peroxide.
It is also rather difficult for very small cations such as Li+ and Be2+ to be surrounded by large peroxide ions in a stable crystal lattice.
Q5. Why does barium form a peroxide more readily than any other element in its group?
AS. It is the largest cation in its group and therefore has the least pola- rizing power. It is also more able to form a stable crystal structure with large peroxide ions, since its larger size gives more room for large peroxide ions to pack around it.
Q6. Suggest reasons why, of all the s-block elements, only K, Rb and Cs readily form superoxides.
A6. The larger size of K+, Rb+ and Cs+ allows superoxide ions to pack com- fortably around them. Ba2+ is very slightly larger than K+ but still does not easily form a superoxide because it is more polarizing than K~~ (See Fig. 2, page 6 for ionic radii.)
Other anomalies of lithium and beryllium can be seen by considering the reactions between nitrogen and the s-block elements. Furthermore, these reactions illustrate the diagonal relationship between lithium and magnesium.
Reaction with nitrogen
Some of the s-block metals, when burned in air, form nitrogen-containing compounds called nitrides. Such two-element compounds are often called binary compounds.
Look up nitrides in your text-book(s). Find out which s-block ·0 elements form them and note their formulae. This will help you to do W the next exercise.
Exercise 15 (a) Write down the formulae of the nitrides formed by direct reaction between the s-bloCK elements and nitrogen. (b) Give balanced equations for the reactions between nitrogen and (i) lithium, (ii) magnesium. (c) Are lithium and beryllium typical of their groups regarding nitride formation? Explain. (d) How does nitride formation illustrate the diagonal relationship between lithium and magnesium? (Answers on page 48 )
19 Most of the reactions you have studied so far in this Unit involve reduction and oxidation (redox)~ with the a-block metals behaving as reducing agents. The chemical reactions of the a-block metals can be summarised by the following half equation: M ne- where n 1 for Group I metals t t metal electron n 2 for Group II metals
In the next section~ we consider the trends in reducing power of the a-block elements and also link electron loss to metallic character.
General properties related to electron loss
Objectives. When you have finished this section~ you should be able to:
(9) illustrate the powerful reducing properties of the a-block elements by referring to their reactions with hydrogen; (10) state and explain the trends in reducing power of the Group I and II elements; (11) state and explain the trends in metallic character in each group.
You will know from your pre-A-level course that hydrogen is a good reducing agent. Only a more powerful reducing agent will be able to force an electron into a hydrogen atom and thus reduce it. Most of the a-block elements are able to do this when they react with hydrogen gas at temperatures up to 700 DC. The resulting binary compounds are called hydrides.
Reaction with hydrogen
Read about the a-block hydrides in your textbook(s). Find out which a-block hydrides are formed by direct combination with hydrogen. and what type of structure they possess. Then you should be able to do the following exercise.
Exercise 16 (a) Which a-block elements react directly with hYdrOgen?~ • ( b) Write equations for the reactions between hydrogen ~~ and IL) sodium. (ii) calcium. (c) Which of the s~block hydrides are classed as (i) ionic and (ii) covalent? (d) How would you demonstrate that the ionic hydrides contain the H ion? (Answers on page 49
You will deal with the reactions of the hydrides with water in Unit 13: The Periodic Table.
20 The next exercise links ionization energies to the reducing powers of the elements in Groups I and II.
Exercise 17 Use ionization energies to predict the trends in reducing ~ power as Groups I and II are descended. ~ (Answer on page 49 )
The s-block elements have many uses as reducing agents in both organic and inorganic chemistry: e.g. sodium in ethanol, and sodiu~mercury amalgam are commonly used in organic chemistry while sodium and magnesium arB used in the manufacture of titanium. Lithium aluminium hydride, LiAIH~, and sodium borohydride, NaBH4, which contain hydride ions, H-, are powerful reducing agents.
In Unit S4 (Bonding and Structure), you learned that metals are usually defined in terms of their chemical properties, i.e. a metal is an element which tends to form positive ions by losing electrons in chemical reactions. It follows from this that the greater the tendency to lose electrons, the more metallic the element becomes. With this statement in mind, attempt the next exercise.
Exercise 18 ( a ) How does metallic character change as Groups I and II are descended? (b) In the s-block, which is: (i) the most metallic element, (ii) the least metallic element? (c) Support your answer in b(ii) by referring to the acid/base nature of its oxide. (Answers on page 49 )
In order to consolidate your knowledge about the elements, attempt the following Teacher-marked Exercise which is taken from an A-level essay paper. Before you start answering the question, look through your-notes to remind yourself of the important points.
Teacher-marked Explain the meaning of the terms 'first electron Exercise affinity', 'first ionization energy I and 'electro- negativity'. Discuss the trends in first ionization energy and electronegativity in Groups I and II of the Periodic Table. In each case, relate your answer to the chemical properties of the elements. Mention and explain three other trends which occur in one or both of these periodic groups.
Discuss briefly the relationships which exist between the two groups of elements.
Having studied the chemistry of the elements and a few compounds we now concentrate on some other important s-block compounds.
21 s - BLOCK COMPOUNDS
In your study of the elements the atypical behaviour of lithium and beryllium became apparent and, furthermore, similarities between lithium and magnesium were beginning to emerge. These points will also be illustrated as you work through the compounds of the s-block metals, starting with their thermal decompositions.
THE EFFECT OF HEAT ON SOME s-BLOCK COMPOUNDS
In this section you will test the effect of heat on the solid nitrates and carbonates of Groups I and II, and briefly refer to their hydrogencarbonates, hydroxides and sulphates.
Objectives. When you have finished this section, you should be able to:
(12) state the effect of heat on the nitrates and carbonates of the s-block elements; (13) state and explain the trends in thermal stabilities of the nitrates and carbonates for each group.
You start this section with an experiment. We advise you to work in pairs and divide the tasks so that you do not use too much laboratory time.
EXPERIMENT 2 Heating the nitrates and carbonates of the s-block elements.
Aim The aim 6f this experiment is two-fold; to identify the products of thermal decomposi- tion, and to estimate the order of thermal stability of these compound$.
Introduction In this~xperiment you heat small samples of the nitrate and carbonate of each element, using the same size flame. You then note the time taken to detect the products of decomposition. Your pre-A-Ievel experience should enable you to recognise nitrogen dioxide, N02 (brown gas), produced from nitrates, and carbon dioxide, CO2, from carbonate~. In order to save time, we suggest that you work in pairs, one student on each part of the experiment.
22 Requirements safety spectacles 30 test-tubes 2 test-tube racks labels for test-tubes solid nitrates and carbonates (anhydrous) of Groups I and II 2 spatulas retort stand, clamp and boss bent delivery tube to fit test-tubes lime-water 2 Bunsen burners and mats 2 stop-clocks (or watches) 2 test-tube holders wood splints hydrochloric acid, dilute, 2 M HCI
Hazard warning
Barium compounds are poisonous if swallowed or absorbed through the skin. AVOID CONTACT WITH SKIN. Nitrogen dioxide is a poisonous gas. HEAT NITRATES IN A FUME CUPBOARD. Nitrates are strong oxidizing agents. DO NOT ALLOW PIECES OF GLOWING SPLINT TO DROP ON TO HOT NITRATES.
Procedure A. Effect of heat on carbonates 1. Set up two rows of four or five test-tubes each. 2. Label the test-tubes in the first row with the names of the Group I carbonates, and the second row with the names of the Group II carbonates. 3. To each test-tube add a spatula-measure of the appropriate carbonate. 4. For the first carbonate, set up the apparatus as shown in Fig. 6.
carbonate
lime water Fig. 6.
5. Start the clock at the moment you begin heating the carbonate by holding the end of the tube just above the inner blue cone of a roaring Bunsen flame. 6. Note the time for the lime-water to just turn milky (if it does at all) and, before removing the flame, prevent 'suckback' by lifting the delivery tube out of the lime-water. /
23 7. Repeat the heating procedure for each carbonate in turn, making sure that the tube height and th2 flame size are the same for each one.
8. Record your results in a copy of Results Table 2.
Results Table 2 Effect of heat on the s-block carbonates
Carbonate Time to detect CO2 Observations
Li2C03 ./
Na2C03
K2C03
Rb2C03 *
CS2C03 *
MgC03
CaC03
SrC03
/ BaC03
*If available (Specimen results on page 49 )
Procedure B. Effect of heat on nitrates. (To be done in a fume cupboard) 9. Heat small quantities of sodium nitrate and magnesium nitrate separately, in clean test-tubes. In each case, test for oxygen and note the appearance of any brown fumes. (This will give you an idea of the products you can expect when Groups I and II nitrates are heated.) 10. Set"up two rows of four test-tubes each containing the appropriate nitrates, as you did for the carbonates. 11. Start the clock at the moment you begin heating the first nitrate by rolding the end of the test-tube just above the blue cone of a roaring Bunsen flame. 12. Tes~ for oxygen at short regular intervals (~lace the glowing splint in the same part of the test-tube) and also look for the first signs of a brown gas. (A white background is helpful.) 13. In a copy of Results Table 3, record the time for the first appearance of brown fumes or when oxygen is detected. (Whichever method of detection you choose for one member of a group you must also apply to the other members of the same group.) Continue heating for another minute. 14. Repeat steps 11, 12 and 13 for the other nitrates in turn. 15. To the cold solid residue remaining after each nitrate is heated add a few drops of dilute hydrochloric acid and warm.
24 Results Table 3 Effect of heat on the s-block nitrates
Time to detect Effect of adding dilute
Nitrate O2 or N02 Observations HCl to cold residue
LiN03
NaN03
KN03
"
RbN03 *
CsN03 * I
"
r MgCN03)2
CaCN03)2 i
SrCN03)2
BaCN03)2
*If available (Specimen resul ts on page 49 )
Questions 1. Explain why many of these nitrates rapidly turn into colourless liquids on first heating, whilst on further heating become white solids again, before they decompose. (Hint: look at the formulae of the nitrates on the reagent bottles.) 2. Which of the Group I nitrates and carbonates most resemble those of Group II in their reaction to heat? 3. Describe the trend in thermal stability of ~bB nitrates and carbonates as each group is descended. 4. How do Group I nitrates and carbonates compare with their Group II counter- parts in terms of thermal stability? 5. Write balanced equations, where applicable, for the thermal decomposi- tions of: (a) lithium nitrate and lithium carbonate; (b) potassium nitrate and potassium carbonate; (c) magnesium nitrate and magnesium carbonate. 6. Why are brown fumes produced when dilute HCl is added to the cold residue obtained when many of Group I nitrates are heated?
(Answers on page 49 )
The reactions you have just dealt with are those of the anions. The more the cation distorts the anion the more likely the anion will undergo decomposition. Using this idea and what you already know about the polarizing effect of cations on large polarizable anions, do the following exercises.
11-8 25 EX8rcis8 19 ( a ) Explain why the thermal stabilities of the nitrat8s ~ and carbonates of Groups I and II increase as each group is descended. ~~ (b) Why is sodium carbonate much more stable to heat than magnesium carbonate? (Answers on page 50 )
Exercise 20 The decomposition temperatures* of the Group II carbonates are shown below, in °c.
BeC03 MgC03 CaC03 SrC03 BaC03 ~100 540 900 1290 1360 *D8composition temperature is the temperature at which the pressure of CO2 reaches one atmosphere. Some CO2 can be detected below this temperature. (a) Suggest why beryllium carbonate is particularly unstAble. (In fact, it must be Kept in an atmosphere of CO2,) (b) Why is lithium carbonate (which decomposes at approximately 900°C) much more stable than beryllium carbonate? (c) Why do you thinK carbon dioxide is not detected if barium carbonate is heated in a Bunsen flame? (Answers on page 50 )
The model we used previously to explain the comparative thermal stabilities of the s-block nitrates and carbonates can easily be applied to other s-block compounds with large polarizable anions. These include hydrogencarbonates, sulphates and even hydroxides, all of which we discuss below.
The hydrogencarbonates, sulphates and hydroxides
Objectives. When you have finished this section, you should be able to:
(14) predict the trends in thermal stabilities for the s-block of the following solids - (i) hydrogencarbonates, (ii) sulphates, (iii) hydroxides; (15) state the products of thermal decomposition (if any) of the following s-blocK compounds - (i) hydrogencarbonates, (ii) sulphates, (iii) hydroxides.
We start by considering the hydrogencarbonates.
26 The hydrogencarbonates
Read about the effect of heat on hydrogencarbonates and find out which s-block hydrogencarbonates exist as solids. You should then be able to do the following exercise.
Exercise 21 (a) Suggest a reason why lithium hydrogencarbonate is the only Group I hydrogencarbonate which does not exist as a solid. (b) Write an equation to show the effect of heat on sodium hydrogencarbonate.
(c) Explain the following facts: (i) self-raising flour, which is often used to make cakes, contains sodium hydrogencarbonate (sometimes called by its old name - sodium bicarbonate, or baking powder).
(ii) Stalagmites and stalactites form in limestone (CaC03) caves.
(d) In what way is lithium hydrogen carbonate similar to magnesium hydrogen- carbonate? (e) How would you distinguish between sodium carbonate and sodium hydrogen- carbonate? (fJ Predict the trend in thermal stability of the Group I hydrogencarbonates. (gJ Suggest reasons why no Group II hydrogencarbonates exist as solids (they do exist in aqueous solution).
(Answers on page 50 )
We now consider the s-block sulphates.
The sulphates
Read about the effect of heat on the s-block sulphates so that you can do the exercise which follows.
27 Exercise 22 All Group I sUlphates are thermally stable whereas some ~ decomposition does occur with Group II sulphates at high ~~ temperatures. (a) Magnesium sulphate decomposes at about 600°C. Write the equation for this decomposition. (b) What trend would you expect in the thermal stability of the sulphates as Group II is descended? Give a reason for your answer. (Answers on page 50 )
We now ask you to make some predictions about the s-block hydroxides.
The hydroxides
You are probably already familiar with the solid hydroxides of sodium and potassium from your pre-A-level work. They are white deliquescent solids and are sold as pellets, sticks or flakes.
Read about the effect of heat on the s-block hydroxides so that you o can answer the questions in the exercise which follows. W
Exercise 23 (a) Predict which of the two Groups, I or II, have the more thermally stable hydroxides. (b) Predict the trend in the thermal stability of the s-block hydroxides as each group is descended. (c) Does the data given below back up the trend y~u predicted in (b)? Explain. Table 3
~C298 K) for the reaction Compound M COH 2) Cs ) -+ MOCs) + H20CIJ
BeCOHJ2 +54 kJ mol -1 MgCOHJ2 +81 kJ mol -1 CaCOHJ2 +109 kJ mol -1 SrCOH)2 +127 kJ mol -1
BaCOH)2 +146 kJ mol -1
Cd) Do you think that lithium. hydroxide will be different from the other Group I hydroxides in terms of thermal stability? Give an equation if you think it is relevant. (Answers on page 50 )
28 The s-block hydroxides have many commercial uses, e.g. sodium hydroxide is the cheapest industrial alkali and is used in the manufacture of rayon, paper, soap, oven-cleaners, etc. We include a short study of the manufacture of sodium hydroxide in an appendix to this Unit (pages 41-45).
A suspension of magnesium hydroxide in water (sometimes called milk of magnesia) is used to treat acid indigestion, because it is a weak alkali.
The fact that magnesium hydroxide forms a suspension in water indicates that, unlike most Group I hydroxides, it is not very soluble in water. We take a closer look at the solubility of s-block compounds in the next section.
THE SOLUBILITY OF THE s-BLOCK COMPOUNDS IN WATER
Most of the alkali metal salts are soluble in water but this is not true of the alkaline earths, which have a large number of insoluble salts.
In this section we will examine the trends in solubility for various Group II compounds and illustrate more anomalous properties of lithium.
Objectives. When you have finished this section, you should be able to:
[16J state the trends in solubility of the following Group II compounds - [aJ hydroxides, (c) sulphates, (eJ carbonates; [bJ chlorides, (d) sulphites,
(17J explain why (aJ barium chloride solution is used to detect the presence of sulphate ions; (b) barium hydroxide solution can be used to detect small quantities of carbon dioxide.
We begin this section with an experiment in which you examine quali-. tatively the trends in solubility of various Group II compounds.
29 EXPERIMENT 3 The solubility of some salts of Group II elements
Aim The aim of this experiment is to demonstrate the trends in solubility of the Group II carbonates~ sulphate5~ sulphites and hydroxides.
Introduction In this experiment~ you add each of the anion solutions to 1 cm3 of each cation solution provided~ drop by drop~ until the first sign of a precipitate appears. For each salt~ the solubility is proportionai to the number of drops of anion added.
Requirements 16 test-tubes 4 test-tube racks labels for test-tubes 5 teat pipettes (marked off at 1 cm3) 0.1 M solutions of the following cations: Mg2+~ Ca2+~ Sr2+~ Ba2+ 1.0 M solution of OH- ions 0.5 M solutions of S042- and S032- ions 0.05 M solution of C032- ions distilled watef
Procedure 1. Set up four rows of four test-tubes each. 2. For each row~ label the first test-tube Mg2+~ the second test-tube Ca2+~ the third test-tube Sr2+ and the fourth test-tube Ba2+. 3. Add 1 cm3 of the appropriate cation solution to each test-tube~ using a teat pipette with a 1 cm3 mark.
4. Label the first row of test-tubes OH-~ the second row S042-~ the third 2 row S032-~ and the fourth row C03 -. 5. Add the solution of OH-, drop by drop, with shaking~ to each cation solution in the first row, until the first sign of a precipitate appears. 6. Record the number of drops of OH- solution used in a copy of Results Table 4. 7. Repeat Steps 5 and 6 with the remaining anions and cations. 8. If a precipitate appears suddenly~ during the addition of a drop~ then you should classify the precipitate as slight (5) or heavy (h). 9. If no precipitate appears after forty drops~ then write 140+1 and regard the salt as soluble.
30 Results Table 4
Number of drops of anion solution added to give a precipitate Cation solution OH- S04 2- S03 2- C03 2-
Mg2 +
Ca2+
Sr2 +
Ba2 +
(Specimen results on page 50 )
Question For Group II, what are the trends in solubility of the salts listed below?
(a) hydroxides; (c) sulphites; (b) sulphates; (d) carbonates. (Answers on page 50
The exercises which follow extend your knowledge of the solubility of the s-block compounds. You will need to refer to Table 4. Note that solubilities can be quoted in a variety of different units, but always specify either an amount or a mass of solute dissolved in a given quantity of either solvent or solution.
Table 4 Solubilities of Group II compounds in water at 298 K
Singly-charged anions Doubly-charged anions
Solubility Solubility Imol per 100 g Imol per 100 g Compound of water Compound of water
1 4 MgC12 5.6 x 10- MgC03 1.3 x 10- 1 10 -4 CaC12 5.4 x 10- CaC03 0.13 x 1 10-4 SrC12 3.5 x 10- SrC03 0.07 x 1 X 10-4 BaC12 1 .5 10- BaC03 0.09 x
1 10-4 Mg(N03)2 4.9 x 10- MgS04 3600 x 1 10-4 Ca(N03)2 6.2 x 10- CaS04 11 x
1 4 Sr(NO;3)2 1 .6 x 10- SrS04 0.62 x 10- 1 10-4 Ba(N03)2 0.4 x 10- BaS04 0.009 x 4 Mg(OH)2 0.020 x 10-3 MgCr04 8500 x 10- 3 4 '\ Ca(OH)2 1.5 x 10- CaCr04 870 x 10- 4 Sr(OH)2 3.4 x 10-3 SrCr04 5.9 x 10-
3 10-4 Ba(OH)2 15 x 10- BaCr04 0.011 x
31 Exercise 24 (a) Identify the group trends in solubility for each type of salt listed in Table 4. (b) Do the solubilities given above for the carbonates, sulphates and hydroxides agree with your findings in Experiment 3? (c) Do singly- or doubly-charged anions give the more insoluble compounds? (Answers on page 51 )
Exercise 25 The X-ray photograph below shows the human stomach (black region) filled with a suspension of barium sulphate (called barium meal). The stomach appears black because X-rays are strongly absorbed by barium compounds. (This technique is often used to detect stomach ulcers.)
(a) If barium compounds are (generally speaking) very poisonous, why can barium sulphate be swallowed as barium meal without harming the patient? (b) Barium chloride solution is often used to confirm the presence of sulphate ions by adding it to the suspected sulphate solution, previously acidified with dilute hydro- chloric acid. (i) Why is barium chloride solution used, and what would you observe in the test-tube if it is mixed as described above? (ii) Write an ionic equation for the reaction between barium ions and sulphate ions. (iii) What is the purpose of the dilute hydrochloric acid? (c) Barium hydroxide solution is occasionally used instead of limewater, Ca(OH)2(aq), for detecting small quantities of carbon dioxide. It is also used instead of sodium hydroxide in titrations where it is important to exclude dissolved carbonate ions. (i) Suggest one reason why barium hydroxide might be preferred in these uses. (ii) State any disadvantage(s) of using barium hydroxide solution. (iii) Write an equation for the reaction between barium hydroxide solution and carbon dioxide. (Answers on page 51 )
32 The next exercise concerns the solubilities of some compounds of alkali metals.
Exercise 26 (a) Look up and record the solubilities of the carbonates and hydroxides of Group I elements. (b) What are the trends in solubility of the Group I carbonates and hydroxides? (c) 00 these trends agree with those of the Group II carbonates and hydroxides? (d) Are the hydroxides and carbonates of Group I more or less soluble than those of Group II? (See Table 4.) (e) Does the general statement about divalent anions forming less soluble salts compared with univalent anions apply to the Group I hydroxides and carbonates?
(Answers on page 51
You have seen that for each type of s-block compound, there is a group trend in solubility, but the trends are not always the same. We shall not attempt an explanation here, because several different factors control the solubility of an ionic compound. We discuss some of these factors in Unit P4.
Similarities between lithium and magnesium are not evident in the solubilities of their compounds but we have already noted some similarities in other properties. In the next section, we bring together these similarit~es and discuss the nature of the 'diagonal relationship' between lithium and magnesium.
THE DIAGONAL RELATIONSHIP BETWEEN LITHIUM AND MAGNESIUM
Because of the relative positions of lithium and magnesium in the Periodic Table, their similarities are often referred to as indicating a 'diagonal relationship'. Such a relationship can also be identified between other elements in the first two periods, e.g. beryllium and aluminium, boron and silicon.
Objectives. When you have finished this section, you should be able to:
(1B) list some physical and chemical properties which illustrate the similarities between lithium and magnesium; (19) give an explanation for this diagonal relationship.
In the next exercise, you bring together information from various parts of this Unit.
33 Exercise 27 Copy and complete Table S. which shows both the ~ diagonal relationship between lithium and magnesium. ~ and also the atypical behaviour of lithium in Group I. We have filled in some parts to give you an idea of the amount of detail you need.
(Answers on page 52)
Table 5
Property Li Rest of Group I Mg
Oxide Li20 (monoxide) formation formed by direct combination
Nitride formation
Effect of heat Stable nitrites on nitrates (N02 -) and oxygen produced
Effect on heat MgO formed
on carbonates wi th CO2
Effect of heat Thermally stable on hydroxides ."
Physical state LiHC03 does of hydrogen- not exist as a carbonates solid
Solubility of Sparingly carbonates soluble
Solubility of Soluble hydroxides
The next exercise shows how similarities in atomic properties between lithium and magnesium can provide an explanation for their diagonal relationship.
34 Exercise 2B (a) How does the polarizing power of Li+ compare with that of 8e2+? (b) How does the polarizing power of Mg2+ compare with that of 8e2+? (c) How do you think the polarizing power of Li+ compares with that of Mg2+? (d) How does your answer to (c)"- provide an explanation for similarities such as those observed when the nitrates and carbonates of lithium and magnesium are heated? (Answers on page 52)
In order to consolidate your knowledge of the chemistry of the s-block elements, we suggest that you attempt one of the questions which follow as a Teacher-marked Exercise. Lo~through your notes and make a rough plan before you start writing.
Teacher-marked The s-block of the Periodic Table contains Group IA, Exercise the alkali metals, and Group IIA, the alkaline earths. Give an account of these two groups of elements and their compounds, paying particular attention to similarities and differences within the groups. OR Illustrate the trends to be found in the chemistry of the Group II elements beryllium to barium by writing a comparative account of the chemistry of their hydrides, oxides, carbonates, chlorides and sulphates.
END-OF-UNIT CHECKLIST
You have now reached the end of this Unit. Read carefully through the following summary of the objectives and check that you have adaque t e notes.
(1) write down the electronic configurations of the first four elements in Groups I and II, .using the s, p, d notation; (2) explain, in terms of atomic structure, the trends in Groups I and II, in (a) atomic and ionic radii, (b) ionization energy, (c) electro- negativity; (3) state the oxidation numbers shown by the elements of Groups I and II in their compounds; (4) state and explain the trends in melting-points and boiling-points within Groups I and II; (5) describe and explain the difference in hardness for the s-block metals; (6) illustrate trends in reactivity by reference to the reactions of s-block elements with water, dilute acids, oxygen and nitrogen;
35 (7) identify the types of oxide formed by the s-block elements as basic or amphoteric; (B) state how the various oxides of s-block elements react with water, giving equations for the reactions. (9) illustrate the powerful reducing properties of the s-block elements by referring to their reactions with hydrogen; (10) & (11) state and,explain the group trends in reducing power and metallic character of the s-block elements; (12) state the effect of heat on the nitrates and carbonates of the s-block elements; (13) state and explain the trends in thermal stabilities of the nitrates and carbonates for each group; (14) 8. (15) predict the trends in thermal stability and state the products of thermal decomposition of the s-block hydrogencarbonates, sulphates and hydroxides; (16) state the trends in solubility of the following Group II compounds - (a) hydroxides, (c) sulphates, (e) carbonates; (b) chlorides, (d) sulphites; (17) explain why (a) barium chloride solution is used to detect the presence of sulphate ions; (b) barium hydroxide solution can be used to detect small quantities of carbon dioxide; (18) list some physical and chemical properties which illustrate the similarities between lithium and magnesium; (19), give an explanation for this diagonal relationship.
END-OF-UNIT TEST
To find out how well you have learned the material in this Unit, try the test which follows. Read the notes below before starting:
1. You should spend about 1~ hours on this test. 2. Hand your answers to your teacher for marking.
36 END-OF-UNIT TEST
1. Group II of the Periodic Table contains Mg~ Ca~ Sr~ Ba. (a) Give the common general electronic configuration for the elements. ( 1 ) (b) (i) Which element has the lowest ionization energy? ( 1 ) (ii) Suggest an explanation. ( 2 ) (c) (i) Which element has the least stable carbonate? ( 1) (ii) Suggest an explanation. ( 2) (d) Barium forms two compounds with oxygen. (i) Give the formula of each anion. ( 2 ) (ii) Give the reaction of each with water. ( 3) (e) Which element forms the most soluble hydroxide? ( 1 )
2. This question concerns the alkali metals. (a) Using three sets of axes like the one below~ sketch the trends in first ionization energy~ atomic radius and melting point.
E (1)o, 0.o
Li Na K ( 3 ) (b) How do you explain the trends in (a) above? (i) First ionization energy. (ii) Atomic radius. (iii) Melting point. ( 5 )
3. This question is concerned with the s-block elements. Give a concise explanation for each statement. (a) The reactivity of the alkali metals with water increases with atomic number. ( 3 ) (b) The thermal stability of the alkaline earth metal nitrates increases with atomic number. (3) (c) Sodium is a stronger reducing agent than magnesium. (2) (d) When the residue obtained by burning magnesium in air is warmed with an aqueous solution of sodium hydroxide~ ammonia is evolved. (3)
37 4. The tabl~ shows the atomic and ionic radii of the alkaline earth elements.
Atomic radius Ionic radius Element Inm Inm
Beryllium 0.112 0.030 Magnesium 0.160 0.065 Calcium 0.197 0.094 Strontium 0.215 0.110 Barium 0.221 0.134
(a) Using magnesium as an example, explain carefully what is meant by~ (i) atomic radius, (ii) ionic radius. ( 4 ) (b) The ionic radius is) in each case, smaller than the atomic radius. Explain why this is so. (2) (c) Explain why the atomic radius increases from Be to Ba. (1) (d) The ions K+ and Ca2+ have identical electronic configurations, 2 yet the ionic radius of K+ is larger than that of Ca +. Suggest why it is larger. (1) (e) Describe how you would prepare good specimens of
(i) magnesium sulphate, MgS04'7H20, from magnesite (i.e. native magnesium carbonate which may contain some calcium carbonate impurity); (ii) barium sulphate, BaS04, from a sample of barium carbonate. (6)
5. Strontium oxide is made from the mineral celestine by strongly heating it with carbon to form a sulphide. SrS04(s) + 4C(s) ~ SrS(s) + 4CO(g) The sulphide formed is reacted with sodium hydroxide.
SrS(s) + 2NaOH(aq) ~ Sr(OH)2(S) + Na2S(aq) Sodium sulphide is removed with water, and thermal decomposition of the strontium hydroxide is used to produce the oxide. (a) Suggest ONE safety precaution which should be used when heating celestine with carbon in the laboratory. Give a reason. (2) (b) Outline the practical procedure you would use to remove sodium sulphide and obtain pure, dry strontium oxide. (2) (c) Write a balanced equation for the thermal decomposition of strontium hydroxide. (1) (d) (i) State the electronic configuration of strontium (atomic number = 38). (ii) Draw a diagram to show the electronic structure of strontium oxide, showing the outRr shell of electrons only. (3)
38 (e) (i) Using a sample of strontium oxide, what procedure would you use to obtain the flame colour of strontium? (ii) What is the flame colour of strontium? (iii) What is the origin of flame colours? ( 4 )
6. ( a ) For lithium and beryllium give, in each case, one property in which the chemistry of the elements or its compounds ~ differs from that of the other elements in its group. !;:i1\ (4) ( b) For either lithium or beryllium, explain briefly the reason for this difference. ( 3 )
7. Although lithium and magnesium are in different groups of the Periodic Table, they show many chemical similarities. (a) How do you account for this? (b) Give ONE property of the metals or their compounds to illustrate [3) the similarity. (2) (Total 70 marks)
39
APPENDIX
SOME INDUSTRIAL CHEMISTRY OF THE s-BLOCK ELEMENTS
In this appendix, we select some important industrial processes for you to study in outline. We also direct you to consider very briefly some of the uses of s-block elements and their compounds.
Objectives. When you have finished this section, you should be able to:
(20) discuss briefly the principles underlying the extraction of the s-block elements; (21) describe in outline the extraction of sodium by the Downs process; (22) describe in outline the extraction of magnesium from sea-water; (23) describe in outline the manufacture of sodium hydroxide by electro- lysis; (24) state some uses of the s-block elements and their compounds.
The s-block elements are, of course, too reactive to be found 'native', i.e. in an uncombined state. Their high reactivity also means that their com- pounds are generally very stable, and extraction of the metals from them requires a substantial input of energy. This energy input is usually made by means of electrolysis.
The Downs process for the extraction of sodium
Read about the extraction of sodium in a suitable text-book. Look for an account of the Downs process and ignore any reference to o the Castner process, which has long been obsolete. Use what you find CO to help you with the following exercises.
Exercise 29 (a) The standard enthalpy change of formation of sodium chloride is -411 kJ mol-~. What does this tell you about the extraction of sodium?
(b) Why is it necessary to USB sodium chloride in its molten state rather than in aqueous solution for the extraction of sodium by electrolysis? (c) How is the sodium chloride obtained? (Answers on page 52 )
41 Exercise 30 Make a copy of Fig. 7 and label the parts A to G as you answer the questions. o
Fig.7. The Downs cell for the extraction of sodium.
(a) What does the malt (A) consist of? (b) What is the purpose of the calcium chloride? (c) Why is calcium not discharged at the cathode? (d) Band C are the electrodes. Whic~ is which? (e) What are the products at the anode and cathode (0 and E)? (f) Why is the anode made of graphite rather than a metal? (g) What is F and what is its purpose? (h) Why does the steel tank have a refractory lining (G)? (Answers on page 52 )
The extraction of calcium and magnesium by electrolysis is similar in many ways to the extraction of sodium, but suitable electrolytes are not so readily available.
The extraction of magnesium from sea-water
Read about the extraction of magnesium from sea-water in your text- book(s). We suggest that you study only an electrolytic method and o that you concentrate on the Dow process for obtaining the electrolyte because this is the method used in the United Kingdom. (The Dow W process uses hydrochloric acid and hydrogen chloride gas.)
The next exercise is about the preparation of the electrolyte and the one following is about the electrolysis.
42 Exercise 31 (a) Write an equation for the thermal decomposition of the mineral Dolomite, MgC03"CaC03• (b) How does the addition of calcined (i.e. strongly heated) Dolomite to sea-water cause the precipi- tation of magnesium hydroxide? (cJ The magnesium hydroxide is then converted to hydrated magnesium chloride by adding hydrochloric acid and crystallizing. How are the crystals dehydrated? (Answers on page 52 J
Exercise 32 Make a copy of Fig. B and label the parts A to F as you answer the questions.
Fig.8. The extraction of magnesium by electrolysis
(aJ Why is a little sodium chloride added to the molten magnesium chloride electrolyte (A)? (bJ Label the anode and cathode (B and C). (c) Why is the anode made of graphite rather than a metal? (d) What is D and what is its purpose? (e) What is passed in and out of the cell at E and F and why is this necessary? (f) Suggest two or three reasons why you might expect magnesium to be more expensive than sodium. Base your answer on differences between the two extraction procedures. (Answers on page 53 )
Another important application of electrolysis is the manufacture of sodium hydroxide, which we now consider.
43 The manufacture of sodium hydroxide
R8ad about th8 manufactur8 of sodium hydroxide ih an electrolytic o cell which has a flowing mercury cathode. Ther8 are sev8ral type~ of cell, which go by different names but they are otherwise very CO similar. You may find references to cells named after Castner, Kellner or Solvay; anyone of these will do. We suggest that you do not study the alternative diaphragm cell, which is not as important in the United Kingdom as it is elsewhere, and that you ignore any reference to the Gossage process, which is obsolete.
Exercise 33 Copy Fig. 9 and label the parts A to G as you answer the qU8stions.
J ~I F B c - --=- .- - A- - -I
0 E I~IG L-H --, ------I
Fig.9. A flowing mercury cell for the manufacture of sodium hydroxide.
(a) What is th8 electrolyte (A), and how is it obtained? (b) Why is it necessary to keep the electrolyt8 flowing through th8 upper cell (8 and C)? (c) Why is m8rcury used as th8 cathod8 (0 and E)? (d) Titanium has now r8placed graphite for th8 anod8s (F). What two prop8rti8s must titanium have in order to be suitable? (e) Write equations for the r8actions occurring in each compartment. (Label G, H, I and J.J (f) Why does sodium amalgam react with water in the lower compartment but not in the upp8r one? (Answ8rs on page 53 )
44 Uses of s-block elements and compounds
The final exercise in this section refers to the uses of some o s-block elements and their compounds. Read the relevant sections of your text-booK to help you with those uses which are not already ill familiar to you.
Exercise 34 For each of the following products, name one s-block element or simple compound which is an important constituent. (a) Gunpowder. (b ) Ba th sal ts . (c) Epsom salts. (d) Indigestion remedy. (e) Oven cleaner. (f) Grit for icy roads. (g) Coolant in nuclear reactor. (h) Baking powder. (i) Compound fertilizer providing the three major plant nutrients. (jJ Fertilizer for correcting acidity. (kJ Cement. (1) A very light strong alloy.
(Answers on page 53 )
45
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Ul .cr-iroo.c.c h ro 0(lJ .c4- .cQJ4-0 o .c III a "rl a a H ro I- Ul 1-+'1-31- I- OJ ....J "rl I-QJEUl I-+' Cl CJ D o 3 o+' Ul E (lJ CJ 0 ~r-i "rl I- h a "rl :::J U U CJ CJ h h QJ ~ ~ OJ ~ 4- x ro 1] CJ QJ 4- X III 1] U w ~ ~ W ~ 53 CONTRIBUTORS Project Director Colin Robertson , Inspector for Science , ILEA Writing team The materials were written and revised by practising teachers seconded to the Project for limited periods : Lambros Atteshlis Frank McManus Lesley Bulman Leonard Roselaar Mike Foley Fran Rowe Ann Friend Alec Thompson Lawrence Halstead Steve Wa xman Terence Kelly Production Team Tony Langham i/c production and cover design Vanda Chan Graphics John Sangwin AVA Technician Peter Faldon AVA Technician Dawn Devereux Office and typing Constance Godfrey : Typing Stella Jefferies Typing and layout Videotapes Brian Babb, Producer, Educational Television Centre , ILEA Reader John Stephens , Department of Natural Sciences, South London College Evaluator John Gilbert, Institute for Educational Technology , University of Surrey ILPAC trial schools The following schools and colleges took part in the trials of the Independent Learning Project for Advanced Chemistry . The Inner London Education Authority wishes to thank the teachers in these schools and their students for their help . Abbey Wood School John Roan School Acland Burghley School Ladbroke School Bacon's C.E . School London Nautical School Brooke House School Morpeth School Dunraven School North Westminster Community School Elliott School Quintin Kynaston School Eltham Hill School St . Mark's C.E . School Ensham School Sydenham School Forest Hill School Thomas Calton School Highbury Grove School Walsingham School Hull College of Further Education Woodberry Down School Hydeburn School. Woolverstone Hall