RESOURCES FOR “HSC-II CHEMISTRY” ZUEB EXAMINATIONS 2021 1 | Page @copyright ZUEB PREFACE: The ZUEB examination board acknowledges the serious problems encountered by the schools and colleges in smooth execution of the teaching and learning processes due to sudden and prolonged school closures during the covid-19 spread. The board also recognizes the health, psychological and financial issues encountered by students due to the spread of covid-19. Considering all these problems and issues the ZUEB Board has developed these resources based on the condensed syllabus 2021 to facilitate students in learning the content through quality resource materials. The schools and students could download these materials from www.zueb.pk to prepare their students for the high quality and standardized ZUEB examinations 2021. The materials consist of examination syllabus with specific students learning outcomes per topic, Multiple Choice Questions (MCQs) to assess different thinking levels, Constructed Response Questions (CRQs) with possible answers, Extended Response Questions (ERQs) with possible answers and learning materials. ACADEMIC UNIT ZUEB: 2 | Page @copyright ZUEB 1. Extended Response Questions (ERQs) HOW TO ATTEMPT ERQs: Write the answer to each Constructed Response Question/ERQs in the space given below it. Use black pen/pencil to write the responses. Do not use glue or pin on the paper. SECTION C ( LONG ANSWER QUESTIONS) 1. What is a covalent bond? Explain the types of covalent bond and their characteristics? . 3 | Page @copyright ZUEB . 4 | Page @copyright ZUEB ERQ ANSWER CL D S# L 1. Define METALLURGY OF METALS K/ D metallurgy. Metallurgy is the art of extracting metals from their natural existing compounds. A Explain the Except gold, platinum, silver, mercury etc, most of the metals occur in nature in manufacture of the combined state. The compounds of metals which occur in nature are called 99.99% pure “Minerals”. The minerals from which metals can be economically extracted are aluminum known as ‘Ores’. from bauxite ore containing METALLURGY OF ALUMINIUM impurities Aluminium does not occur free in nature, but it is widely distributed in combined state. IMPORTANT ORES OF ALUMINIUM (a) Silicates (i) Kaolin = Al2O3.2SiO2.2H2O (ii) Potash felspar = K2O.Al2O3.6SiO2 (iii) Potash- mica=K2O.3Al2O3.6SiO2.2H2O b) Fluoride (i) Cryolite = Na3AlF6 c) Sulphate (i) Alunite = K2SO4.Al2 (SO4)3.4Al(OH)3 d) Oxides (i) Bauxite = Al2O3.nH2O (ii) Gibbsite = Al2O3.3H2O (iii) Diaspore = Al2O3.H2O (iv) Corundum = Al2O3 EXTRACTION Aluminum is extracted from its bauxite ore (Al2O3.nH2O). The extraction of Aluminum involves two steps: (a) Purification of bauxite to alumina (b) Electrolysis of pure Alumina PURIFICATION OF BAUXITE Bauxite usually contains ferric oxide (Fe2O3) and Silica (SiO2) as Chief impurities. These impurities must be removed because these impurities make the metal brittle and liable to corrosion. The bauxite may be purified by any one of the following methods depending upon the nature of impurities present in it. HALL’S METHOD This method is used for the purification of Bauxite containing Fe2O3 and Silica (SiO2) as impurities. Finally divided bauxite is fused with sodium carbonate (Na2CO3), it dissolves to form sodium aluminate, while the impurities Fe2O3 and SiO2 are left unaffected. Al2O3.nH2O + Na2CO3 2NaAlO2 + CO2 + nH2O The fused mass is extracted with water leaving behind both impurities. Sodium aluminate (NaAlO2) upto 50oC-60oC in the presence of CO2., the precipitates of aluminium hydroxide Al(OH)3 are formed. 2NaAlO2 + 3H2O+CO2 2Al(OH)3 + Na2CO3 o 50-60 C o The precipitates of Al(OH)3 are washed and ignited at about 1500 C to get pure alumina. Heat 2Al(OH)3 Al2O3 + 3H2O 1500oC 5 | Page @copyright ZUEB BAEYER’S METHOD This method is used for the purification of ore containing excess of Fe2O3. Finely divided bauxite is treated with strong caustic soda (45%), the bauxite goes into the solution as sodium aluminate. Al2O3.nH2O + 2NaOH 2NaAlO2 + 2(n)H2O o o By heating sodium aluminate (NaAlO2) upto 50 C – 60 C in the presence of CO2, the precipitates of Al (OH)3 are formed. 2NaAlO2 + 3H2O+CO2 2Al (OH)3 + Na2CO3 o These precipitates of Al(OH)3 are ignited at about 1500 C to get pure alumina. Heat 2Al(OH)3 Al2O3 + 3H2O 1500oC SERPEK’S METHOD This method is used for the purification of bauxite containing excess of SiO. Powdered bauxite is mixed with carbon and heated upto 1800oC in the current of nitrogen produce aluminium nitride. o 1800 C Al2O3.nH2O +3C + N2 2AlN + 3CO +nH2O SiO2 + 2C Si+2CO Aluminium nitride on hydrolysis with hot water produce the precipitates of aluminium hydroxides Al(OH)3. AlN + 3H2O Al(OH)3 + NH3 These precipitates now is filtered, dried and ignited to get pure alumina. Heat 2AI(OH)3 Al2O3 + 3H2O 1500oC alumina ELECTROLYSIS OF PURE ALUMINA (HALL’S PROCESS) The pure and dry alumina from either of the above process now is electrolyzed. The electrolysis is carried out in a steel tank which is 8ft in length and 6ft in width lined with carbon (graphite) and serves as cathode where as carbon rods hanging in the molten mass used as anode. The electrolyte consists of alumina dissolved in fused cryolite (Na3AlF6) and o fluorspar (BaF2). Cryolite lowers the melting point of alumina that is about 950 C and fluorspar increase the fluidity of the melt, so that the liberated aluminum may sink at the bottom of electrolytic cell when electric current is passed through this mixture, then aluminum is obtained at cathode in liquid state, so, it sinks to bottom from where it is drawn periodically through the temping hole. 6 | Page @copyright ZUEB REFINING OF ALUMINIUM BY HOOP’S ELECTROLYTIC METHOD The aluminum obtained by electrolysis of alumina is 99% pure which is further refined by hoop’s electrolytic method. The electrolysis is carried out in an iron box which is lined with a carbon at the bottom. It contains three layers of fused mass. The lower layer consists of an alloy of impure aluminum with copper. This layer serves as Anode. The middle layer consist of a solution of Na3AlF6 and BaF2. The upper layer consist of pure aluminum serves as cathode. On Electrolysis Al+3 ions from the middle layer migrate to the upper layer where they are reduced to aluminium. Equal number of Al+3 ions are produced in the lower layer. These Al+3 ions migrate to the middle layer. Therefore, in this way 99.99% pure aluminum is obtained. Na3AlF6 3NaF + AlF3 + - AlF3 Al3 + 3F At cathode Al3++3e- Al + e At anode Al Al3 +3 (goes to molten cryolite) OVERALL REACTION Al3+ + Al Al + Al3+ a g e 7 | Page @copyright ZUEB 2. What are Hydrides K/ M binary Those compounds of hydrogen made up of any two elements are called A compounds of binary compounds of hydrogen. These binary compounds of hydrogen hydrogen? are called hydrides. Explain all Classification of hydrides types of Hydrides can be divided into the following groups in terms of the hydrides differences in their structures bonding and their properties. 1.Ionic hydrides or Saline hydrides 2.Covalent hydrides 3.Complex hydrides 4.Polymeric hydrides 5.Metallic hydrides 6.Borderline hydrides Ionic or Saline hydrides (Salt like hydrides) Those hydrides which are formed by the combination of alkali metals (Group IA) and alkaline earth metals (Group IIA) except of Be and Mg with hydrogen are called ionic hydrides or saline hydrides. These are the ionic compounds in which hydrogen is present as H-ion of - 2+ - general formula MH and M H2 respectively. These hydrides possess salt like characters. Therefore, they are also called saline hydrides. Preparation Ionic hydrides are prepared by passing hydrogen gas over hot alkali metals and alkaline earth metals. 600oC 2Li + H2 2LiH 200oC 2Na + H2 2NaH Properties 1. These are colorless, non-volatile and salt like. 2. They have high melting points. 3. These hydrides are thermally very stable and don’t dissociate easily. 4. They act as dehydrating agent. 5. They react with water or with acids produce hydrogen + Na H + H2O NaOH +H2 + - Na H + HCl NaCl + H2 ++ 2(-) Ca H2 +2H2O Ca(OH)2 +2H2 Covalent hydrides Hydrides of p-block elements are called covalent hydrides. The bonding in these hydrides is of electron sharing type. Due to their volatile nature they are also known as volatile hydrides. Preparation Covalent hydrides are prepared by direct or indirect combination of p- block elements with hydrogen. 8 | Page @copyright ZUEB H2 + Cl2 2HCl 2H + O2 2H2O H2 + S H2S CaC2 + 2H2O Ca (OH)2 + C2H2 Al4C3 + 6H2O 2Al2O3 + 3CH4 Mg3N2+6H2O 3Mg(OH)2+2NH3 Preparation 1. These hydrides are generally gases or liquid at room temperature. 2. They have low melting and boiling points. 3. These are volatile in nature. 4. They do not conduct electricity 5. Hydrides of Group III A and IVA are neutral, Group VA are basic and Group VIA and VIIA are acidic in nature. Complex hydrides Those hydrides which are formed by the combination of hydrides of alkali metals (Group IA) with Boron-family (Group IIIA) are called complex hydrides. These hydrides have complex nature possessing all the three types of chemical bonds. The general formula of complex hydrides is ABH4 where ‘A’ is univalent metal and ‘B’ is trivalent posture ion of Group IIIA. Preparation Complex hydrides are prepared by combining ionic hydrides of alkali metals and covalent hydrides of Group IIIA. +1 +3 -1 LiH + AlH3 Li Al H4 +1 +3 -1 NaH + BH3 Na B H4 Properties of Complex hydrides 1. These are salt like solids and stable upto 300oC 2. They are soluble in water 3. They are used as catalyst.