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4.1SECTION Single Displacement Reactions

Key Terms In a game of basketball, shown in Figure 4.1, a team can have fi ve players on the court at one time. However, a substitute player can take the place of another player. single displacement reaction Substitution also occurs in chemical reactions. A single displacement reaction is a activity series reaction in which one element takes the place of another element in a compound.

single displacement reaction a chemical reaction in which one element in a compound is replaced (displaced) by another element

Figure 4.1 On a basketball court, one player can be replaced with a different player. Similarly, in a single displacement reaction, one element in a compound is replaced by another element.

Characteristics of Single Displacement Reactions In Chapter 3, you learned that the decomposition reaction used to infl ate air bags produces nitrogen gas and metal. A single displacement reaction then converts sodium, which is a very reactive element, into the stable compound sodium : sodium + iron(III) oxide → sodium oxide + iron + → + 6Na(s) Fe2O3(s) 3Na2O(s) 2Fe(s) In this reaction, the element sodium takes the place of, or displaces, iron from the compound iron(III) oxide. Th e products of a single displacement reaction are an element and a compound that are diff erent from the reactants. Two general forms for single displacement reactions are shown below. A and B represent metals, and X and Y represent non-metals.

Reactions in which a metal displaces another metal: ABXAXB++→ ++→

Reactions in which a non-metal displaces another non-metal: AX+→ Y AY+ X +→ +

162 MHR • Unit 2 Chemical Reactions

162-170_S41_CHE11.indd 162 23/08/10 12:39 PM Types of Single Displacement Reactions Th e general forms you just read show two types of single displacement reactions: • a metal displacing another metal from an ionic compound • a non-metal displacing another non-metal from an ionic compound A single displacement reaction does not always occur between two metals or two non-metals. Th e following single displacement reaction also occurs: • a metal displacing from an or

A Metal Displacing Another Metal from an Ionic Compound Most single displacement reactions involve a metal displacing another metal from an ionic compound. Figure 4.2 shows the reaction that occurs when a piece of copper metal is placed in a of silver nitrate. Th e balanced chemical equation for this reaction is copper + silver nitrate → copper(II) nitrate + silver + → + Cu(s) 2AgNO3(aq) Cu(NO3)2(aq) 2Ag(s) Copper begins as a metallic element and becomes metallic dissolved in a solution. Silver begins as metallic ions dissolved in a solution and becomes a metallic element. Th is pattern of change is regularly seen in single displacement reactions.

Reactants

Cu Ag+

- NO3 H2O

Products Ag

+ - Cu2 NO3 H2O

Figure 4.2 Copper displaces silver from silver nitrate dissolved in water. Solid silver forms, and the solution turns blue due to the formation of copper(II) ions. Infer What happens to the nitrate ions during the reaction of copper and silver nitrate?

Chapter 4 Displacement Reactions • MHR 163

162-170_S41_CHE11.indd 163 23/08/10 12:39 PM When a Reaction Does Not Occur What happens if the metals are reversed? Look at Figure 4.3, which shows silver metal in a copper(II) nitrate solution. As you can see, nothing happens. No reaction takes place. Th e silver does not displace the copper from the solution. So, a single displacement reaction occurs only for certain combinations of metals and ionic compounds.

Reactants

Ag

2+ - Cu NO3 H2O

Figure 4.3 When silver metal is placed in a copper(II) nitrate solution, no reaction happens. The silver cannot displace the copper from the copper(II) nitrate dissolved in water.

Activity Series By performing many experiments, chemists were able to develop lists that show the activity series a ranking of the relative relative reactivity of elements, specifi cally metals and . Th ese lists are called reactivity of metals or activity series. Th e activity series of metals is shown in Table 4.1. halogens in aqueous reactions Table 4.1 Activity Series of Metals Metal Displaces Hydrogen … Reactivity lithium most reactive barium sodium from ccoldo water SuggestedInvestigation magnesium Plan Your Own Investigation aluminum 4-A, Making an Activity zinc Series of Metals chromium iron cadmium cobalt nickel tin lead from acidsa hydrogen copper mercury silver platinum gold least rereactive

164 MHR • Unit 2 Chemical Reactions

162-170_S41_CHE11.indd 164 23/08/10 12:39 PM Using the Activity Series of Metals Th e activity series allows chemists to predict whether a single displacement reaction between a metal and an ionic compound will occur. As you can see in Table 4.1, the elements in the activity series are placed in order from the most reactive to the least reactive. Compare the locations of copper and silver in Table 4.1. Copper is more reactive than silver and is higher than silver in the activity series. As a result, a single displacement reaction will occur when copper metal is placed in an aqueous solution of a silver compound. A reaction will not occur when silver metal is placed in an aqueous solution of a copper compound because silver is less reactive than copper.

Predicting Products of a Single Displacement Reaction To predict the products of a single displacement reaction, look at the activity series of metals. If the single element is higher in the activity series, and therefore more reactive than the element it might replace in the compound, a reaction will occur. Th e products will be the less active metal (as an element) and an ionic compound composed of the more active metal and the anion (negatively charged ion) of the original compound.

Applications and the Activity Series of Metals Th e reactivity of metals aff ects how appropriate they are for various applications. For example, many older homes have water pipes made of steel with an inner coating of zinc. Over time, however, the zinc fl akes off , allowing the water to directly contact the steel. As the steel rusts, the pipe can become clogged and even break. Modern homes may have water pipes made of copper, shown in Figure 4.4, or plastic, both of which are resistant to corrosion. Titanium is a metal that has many applications because it is strong, light in weight, and resistant to corrosion. It is commonly used to make replacement hip and knee joints. It is also used in dentistry for tooth replacement. A titanium implant is placed into the jawbone, and the top of the implant is covered with an artifi cial tooth. Titanium is also used for screws and pins inserted to stabilize broken bones.

Figure 4.4 Copper pipes, which are often used in home plumbing systems, are resistant to corrosion. The activity series of metals shows that copper is much less reactive than iron, which is a chief component of steel pipes.

Learning Check

1. What is the general form of a single displacement 5. In an investigation, a piece of copper wire is added reaction in which a metal displaces another metal? to a solution that contains lead ions. 2. What is the main characteristic of a single a. Predict whether a reaction will occur. displacement reaction? b. Explain your prediction. 3. How is an activity series developed for a group 6. Titanium is not listed in Table 4.1. of elements? a. Would you expect titanium to appear closer to 4. Refer to Table 4.1. Why are some metal objects the top or the bottom of the activity series? coated with a thin layer of platinum or gold to b. Explain your reasoning. prevent corrosion?

Chapter 4 Displacement Reactions • MHR 165

162-170_S41_CHE11.indd 165 23/08/10 12:39 PM A Metal Displacing Hydrogen from Acid or Water Th e element hydrogen is just below lead in the activity series of metals. Although hydrogen is not a metal, its single valence allows it to form hydrogen ions that have a 1+ charge. Because hydrogen forms cations (positively charged ions) like metals do, it is oft en involved in single displacement reactions with metals. When hydrogen is replaced by a metal, hydrogen gas, H2(g), is produced. Compounds from which hydrogen can be displaced include and water.

Displacement from an Acid Th e reaction of magnesium with , HCl(aq), shown in Figure 4.5, is an example of a metal displacing hydrogen from an acid. Th e balanced chemical equation for this reaction is magnesium + hydrochloric acid → magnesium + hydrogen + → + Mg(s) 2HCl(aq) MgCl2(aq) H2(g) When an acid is a reactant in a single displacement reaction, you can think of the acid in terms of the ions it forms when it is dissolved in water. Th e ions in hydrochloric acid can be treated as H+ and Cl-. Th e metal displaces the hydrogen and forms an ionic compound with the chloride ion. Th e location of hydrogen in the activity series of metals shows the relative reactivity of hydrogen in acids. Every metal above hydrogen in the activity series can displace hydrogen from an acid, as noted in Table 4.1, but the metals below hydrogen cannot.

Reactants Products

+ H Mg2+

H2O

- H2O CI CI- Mg H2

Figure 4.5 When magnesium displaces hydrogen from hydrochloric acid, bubbles of hydrogen gas, H2(g), form. Identify Give an example of another metal that can displace hydrogen from acids, and give an example of a metal that cannot displace hydrogen from acids.

166 MHR • Unit 2 Chemical Reactions

162-170_S41_CHE11.indd 166 23/08/10 12:39 PM Displacement from Water Hydrogen can also be displaced from water. However, the hydrogen in water are harder to displace than the hydrogen atoms in an acid. As a result, only very active metals can displace hydrogen from water, as shown in Table 4.1. In Figure 4.6, you can see sodium reacting with water in a single displacement reaction. Th e balanced chemical equation for this reaction is sodium + water → sodium + hydrogen +  → + 2Na(s) 2H2O( ) 2NaOH(aq) H2(g)

Reactants Products OH- H2O Na

Na+ H2O

H2

Figure 4.6 Sodium is reactive enough to displace hydrogen from water. The change in colour of the phenolphthalein that was added indicates the presence of hydroxide ions from the formation of during the reaction.

Th e products of this reaction include diatomic hydrogen gas and sodium hydroxide. When predicting the products of a single displacement reaction that involves water, think of water as being composed of hydrogen and hydroxide ions, H+ and OH-. Th e metal displaces the hydrogen ions, which form hydrogen gas, and it bonds with the hydroxide ions to form an ionic compound.

Making Metals Safer Th e reactivity of sodium and other alkali metals makes these metals useful in a wide range of chemical reactions, including the generation of hydrogen for fuel cells through single displacement reactions. However, the reactivity of alkali metals also makes them dangerous. Alkali metals react easily with water and oxygen in the air, which means that handling them can be hazardous. In several industrial processes, alkali metals are used in ammonia to reduce contact with water and oxygen. Liquid ammonia must be kept well below its of -33°C to prevent the release of toxic ammonia gas. A new technique provides the reactivity of alkali metals without the danger of using very cold ammonia. An is absorbed into a porous silica gel to form a powder. Th e powder can still undergo reactions, but because the particles of the alkali metal are so small and are surrounded by silica, the reactions can take place safely without the need for cold ammonia. As well as making the reaction conditions safer, the powder forms more stable products than those formed using an alkali metal in ammonia.

Chapter 4 Displacement Reactions • MHR 167

162-170_S41_CHE11.indd 167 23/08/10 12:40 PM A Non-metal Displacing Another Non-metal from an Ionic Compound Non-metals also undergo single displacement reactions. When one of the reactants is a diatomic and the other reactant is an ionic compound that contains a halogen, the halogen in the compound may be replaced. For example, when chlorine gas is bubbled through an aqueous solution of sodium bromide, the following reaction occurs: chlorine + sodium bromide → + bromine + → +  Cl2(g) 2NaBr(aq) 2NaCl(aq) Br2( ) Th e non-metal chlorine displaces the non-metal bromine from the compound. Th is reaction is used to produce bromine for use in agriculture, fi re retardants, and petroleum additives.

Halogen Activity Series Similar to metals, halogens can be arranged in an activity series, as shown in Table 4.2. Th e reactivity of the halogens decreases as you move from top to bottom within the group. Th erefore, the activity series of halogens mirrors the arrangement of the halogens in the periodic table.

Table 4.2 Activity Series of Halogens Halogen Reactivity fl uorine most reactive chlorine bromine iodine least reactive

Activity 4.1 Predicting Trends in the Reactivity of Halogens

The halogens, found in Group 17 in the periodic table, share Procedure some common characteristics. For example, the atoms of Construct a line graph for each property. Plot the halogens all the halogens have seven in their outermost along the x-axis. Label the axes and units as necessary. energy level. As with most groups in the periodic table, the halogens exhibit trends, such as their ability to react with Questions other substances. 1. Identify any periodic trends in your graphs. Relate Properties of Halogens these trends to the activity series of halogens shown in Table 4.2. Atomic Ionization Radius Energy 2. does not appear in the activity series of Halogen (picometres) (kJ/mol) halogens, shown in Table 4.2, because it is a very rare fl uorine 72 1681 3.98 element. Predict the location of astatine in the activity series. Provide a brief explanation of your reasoning. chlorine 100 1251 3.16 bromine 114 1140 2.96 iodine 133 1008 2.66 astatine 140 920 2.20

Predicting Products of Single Displacement Reactions When predicting the products of a single displacement reaction involving halogens, you can use the activity series of halogens in the same way that you use the activity series of metals. If the uncombined halogen is higher in the activity series than the halogen in the compound that it might replace, then a reaction will occur. From the activity series, you can see that fl uorine can replace any other halogen, but iodine can replace none.

168 MHR • Unit 2 Chemical Reactions

162-170_S41_CHE11.indd 168 23/08/10 12:40 PM Sample Problem

Predicting Products in Single Displacement Reactions Problem Using the activity series of metals and halogens, write a balanced chemical equation for each single displacement reaction. If you predict that no reaction will occur, write “NR.” +  → + →  + → a. Ca(s) H2O( ) b. Fe(s) CrSO4(aq) c. Br2( ) NaI(aq)

What Is Required? If a single displacement reaction will occur, the chemical formulas of the products are required.

What Is Given? Reactants: a. calcium and water b. iron and chromium(II) c. bromine and Type of reaction: single displacement

Plan Your Strategy Act on Your Strategy Identify the elements that are involved in the displacement. a. calcium and hydrogen b. iron and chromium c. bromine and iodine Locate the elements in the activity series. Determine whether a a. Calcium is above hydrogen in the activity reaction will occur. series of metals, and it is reactive enough to displace hydrogen from water. Th erefore, a reaction will occur. b. Iron is below chromium in the activity series of metals. Iron is not reactive enough to displace chromium, so no reaction will occur. c. Bromine is above iodine in the activity series of halogens, so it is reactive enough to displace iodine. Th erefore, a reaction will occur. Predict the products that will form. a. and hydrogen gas will form. b. NR c. Sodium bromide and iodine will form. +  → + Write the formulas of the products. Balance each chemical a. Ca(s) 2H2O( ) Ca(OH)2(aq) H2(g) + → equation. b. Fe(s) CrSO4(aq) NR  + → + c. Br2( ) 2NaI(aq) 2NaBr(aq) I2(aq)

Check Your Solution Based on the activity series of metals, calcium can displace hydrogen from water, but iron cannot displace chromium from chromium(II) sulfate. Based on the activity series of halogens, bromine can displace iodine from sodium iodide.

Practice Problems

+ → Using the appropriate activity series, write a balanced 5. Cl2(g) NaI(aq) chemical equation for each single displacement reaction. +  → 6. Ni(s) H2O( ) If you predict that no reaction will occur, write “NR.” 7. Pb(s) + Sn(ClO ) (aq) → + → 3 4 1. Mg(s) CrSO4(aq) +  → 8. K(s) H2O( ) 2. Br () + KF(aq) → 2 9. HCl(aq) + Cd(s) → 3. Zn(s) + H SO (aq) → 2 4 10. Pb(ClO ) (aq) + Al(s) → + → 3 4 4. F2(g) MgI2(aq)

Chapter 4 Displacement Reactions • MHR 169

162-170_S41_CHE11.indd 169 23/08/10 12:40 PM Section 4.1 REVIEW

Section Summary • In a single displacement reaction, one element replaces • An activity series lists elements in order, from most another element in a compound to produce a new reactive to least reactive. element and a new compound. • Th e activity series of metals and the activity series • Th e general form for single displacement reactions in of halogens are used to predict whether a single which a metal displaces another metal, where A and B are displacement reaction will occur. + → + metals, is A BX AX B. Th e general form for single • A single displacement reaction can only occur when an displacement reactions in which a non-metal displaces uncombined element is higher in an activity series than another non-metal, where X and Y are non-metals, is the element it would replace in a compound. AX + Y → X + AY.

Review Questions 1. K/U Describe the displacement reaction that occurs 12. T/I If the liquid in the photograph is water, in an air bag to change sodium into a less harmful could the metal be zinc? If the liquid is an acid, chemical. could the metal be zinc? Explain your reasoning. 2. K/U Explain why “single displacement” is a suitable term to describe the reactions discussed in this section. 3. T/I Look again at Figures 4.2 and 4.3. Describe how the evidence in these fi gures can be used to determine the relative placement of silver and copper in an activity series. 4. A Based on the activity series of metals, explain why large amounts of gold jewellery and coins have survived from ancient civilizations. 5. T/I For each pair of reactants, write a balanced chemical equation if a single displacement reaction will occur. If you predict that no reaction will occur, 13. K/U Which halogens can be replaced by chlorine? write “NR.” 14. C Design a concept map that shows the a. copper and magnesium sulfate relationships among the following terms: b. zinc and iron(II) chloride • single displacement reaction c. magnesium and aluminum sulfate • activity series d. zinc and hydrochloric acid • metals e. copper and zinc nitrate • non-metals f. magnesium and • hydrogen 6. A Aluminum is much more abundant than iron in • acids Earth’s crust. Use Table 4.1 to infer why aluminum was • halogens very expensive and not widely used until the late 1800s. 15. T/I For each pair of reactants, write a balanced 7. K/U Why is hydrogen included in the activity series chemical equation if a single displacement reaction of metals? will occur. If you predict that no reaction will occur, 8. A Describe two methods you could use to produce write “NR.” hydrogen gas by a single displacement reaction. a. iron and hydrobromic acid b. bromine and magnesium iodide 9. C Draw a model of a single displacement reaction c. magnesium and aluminum sulfate in which hydrogen is displaced. d. lithium and water 10. K/U When a metal displaces hydrogen from e. cobalt and water water, what types of substances form? f. bromine and iron(II) chloride 11. C Describe how to use the periodic table to 16. T/I Which halogen is capable of displacing only one compare the relative reactivities of the halogens. halogen and is itself replaced by two halogens?

170 MHR • Unit 2 Chemical Reactions

162-170_S41_CHE11.indd 170 23/08/10 12:40 PM