SECTION 4.1 Single Displacement Reactions Key Terms single displacement reaction activity series In a game of basketball, shown in Figure 4.1, a team can have five players on the court at one time. However, a substitute player can take the place of another player. Substitution also occurs in chemical reactions. A single displacement reaction is a 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 inflate air bags produces nitrogen gas and sodium metal. A single displacement reaction then converts sodium, which is a very reactive element, into the stable compound sodium oxide: sodium + iron(iii) oxide sodium oxide + iron 6Na(s) + Fe 2 O 3 (s) 3Na 2 O(s) + 2Fe(s) In this reaction, the element sodium takes the place of, or displaces, iron from the compound iron(iii) oxide. The products of a single displacement reaction are an element and a compound that are different 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: A + BX AX + B + + Reactions in which a non-metal displaces another non-metal: AX + Y AY + X + + 162 MHR Unit 2 Chemical Reactions
Types of Single Displacement Reactions The 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. The following single displacement reaction also occurs: a metal displacing hydrogen from an acid or water 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 solution of silver nitrate. The 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 ions dissolved in a solution. Silver begins as metallic ions dissolved in a solution and becomes a metallic element. This pattern of change is regularly seen in single displacement reactions. Reactants Ag+ Cu NO3- Ag Products NO3- Cu2+ 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
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. The 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 NO3- Cu2+ 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 activity series a ranking of the relative reactivity of metals or halogens in aqueous reactions By performing many experiments, chemists were able to develop lists that show the relative reactivity of elements, specifically metals and halogens. These lists are called activity series. The activity series of metals is shown in Table 4.1. Table 4.1 Activity Series of Metals Metal Displaces Hydrogen lithium Reactivity most reactive potassium barium calcium SuggestedInvestigation Plan Your Own Investigation 4-A, Making an Activity Series of Metals sodium from co cold water magnesium aluminum zinc chromium iron cadmium cobalt nickel tin lead a from acids hydrogen copper mercury silver platinum gold 164 MHR Unit 2 Chemical Reactions least re reactive
Using the Activity Series of Metals The 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. The products will be the less active metal (as an element) and an ionic compound composed of the more active metal ion and the anion (negatively charged ion) of the original compound. Applications and the Activity Series of Metals The reactivity of metals affects 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 flakes 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 artificial 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 reaction in which a metal displaces another metal? 2. What is the main characteristic of a single displacement reaction? 3. How is an activity series developed for a group of elements? 4. Refer to Table 4.1. Why are some metal objects coated with a thin layer of platinum or gold to prevent corrosion? 5. In an investigation, a piece of copper wire is added to a solution that contains lead ions. a. Predict whether a reaction will occur. b. Explain your prediction. 6. Titanium is not listed in Table 4.1. a. Would you expect titanium to appear closer to the top or the bottom of the activity series? b. Explain your reasoning. Chapter 4 Displacement Reactions MHR 165
A Metal Displacing Hydrogen from Acid or Water The element hydrogen is just below lead in the activity series of metals. Although hydrogen is not a metal, its single valence electron allows it to form hydrogen ions that have a 1+ charge. Because hydrogen forms cations (positively charged ions) like metals do, it is often 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 acids and water. Displacement from an Acid The reaction of magnesium with hydrochloric acid, HCl(aq), shown in Figure 4.5, is an example of a metal displacing hydrogen from an acid. The balanced chemical equation for this reaction is magnesium + hydrochloric acid magnesium chloride + 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. The ions in hydrochloric acid can be treated as H+ and Cl-. The metal displaces the hydrogen and forms an ionic compound with the chloride ion. The 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+ Mg 2+ CIMg CIH2 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
Displacement from Water Hydrogen can also be displaced from water. However, the hydrogen atoms 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. The balanced chemical equation for this reaction is sodium + water sodium hydroxide + hydrogen 2Na(s) + 2( ) 2NaOH(aq) + H2(g) Reactants Products Na OH- Na+ 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 sodium hydroxide during the reaction. The 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-. The metal displaces the hydrogen ions, which form hydrogen gas, and it bonds with the hydroxide ions to form an ionic compound. Making Alkali Metals Safer The 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 liquid ammonia to reduce contact with water and oxygen. Liquid ammonia must be kept well below its boiling point 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 alkali metal is absorbed into a porous silica gel to form a powder. The 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
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 halogen molecule 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 sodium chloride + bromine Cl 2 (g) + 2NaBr(aq) 2NaCl(aq) + Br 2 (l) The non-metal chlorine displaces the non-metal bromine from the compound. This reaction is used to produce bromine for use in agriculture, fire retardants, and petroleum additives. Halogen Activity Series Similar to metals, halogens can be arranged in an activity series, as shown in Table 4.2. The reactivity of the halogens decreases as you move from top to bottom within the group. Therefore, the activity series of halogens mirrors the arrangement of the halogens in the periodic table. Table 4.2 Activity Series of Halogens Halogen fluorine chlorine bromine iodine Reactivity most reactive least reactive Activity 4.1 Predicting Trends in the Reactivity of Halogens The halogens, found in Group 17 in the periodic table, share some common characteristics. For example, the atoms of all the halogens have seven electrons in their outermost energy level. As with most groups in the periodic table, the halogens exhibit trends, such as their ability to react with other substances. Properties of Halogens Halogen Atomic Radius (picometres) Ionization Energy (kj/mol) Electronegativity fluorine 72 1681 3.98 chlorine 100 1251 3.16 bromine 114 1140 2.96 iodine 133 1008 2.66 astatine 140 920 2.20 Procedure Construct a line graph for each property. Plot the halogens along the x-axis. Label the axes and units as necessary. Questions 1. Identify any periodic trends in your graphs. Relate these trends to the activity series of halogens shown in Table 4.2. 2. Astatine does not appear in the activity series of halogens, shown in Table 4.2, because it is a very rare element. Predict the location of astatine in the activity series. Provide a brief explanation of your reasoning. 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 fluorine can replace any other halogen, but iodine can replace none. 168 MHR Unit 2 Chemical Reactions
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) + H 2 O(l) b. Fe(s) + CrSO 4 (aq) c. Br 2 (l) + 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) sulfate c. bromine and sodium iodide Type of reaction: single displacement Plan Your Strategy Identify the elements that are involved in the displacement. Locate the elements in the activity series. Determine whether a reaction will occur. Predict the products that will form. Write the formulas of the products. Balance each chemical equation. a. calcium and hydrogen b. iron and chromium c. bromine and iodine Act on Your Strategy a. Calcium is above hydrogen in the activity series of metals, and it is reactive enough to displace hydrogen from water. Therefore, 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. Therefore, a reaction will occur. a. Calcium hydroxide and hydrogen gas will form. b. NR c. Sodium bromide and iodine will form. a. Ca(s) + 2H 2 O(l) Ca(OH) 2 (aq) + H 2 (g) b. Fe(s) + CrSO 4 (aq) NR c. Br 2 (l) + 2NaI(aq) 2NaBr(aq) + I 2 (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 chemical equation for each single displacement reaction. If you predict that no reaction will occur, write NR. 1. Mg(s) + CrSO 4 (aq) 2. Br 2 (l) + KF(aq) 3. Zn(s) + H 2 SO 4 (aq) 4. F 2 (g) + MgI 2 (aq) 5. Cl 2 (g) + NaI(aq) 6. Ni(s) + H 2 O(l) 7. Pb(s) + Sn(ClO 3 ) 4 (aq) 8. K(s) + H 2 O(l) 9. HCl(aq) + Cd(s) 10. Pb(ClO 3 ) 4 (aq) + Al(s) Chapter 4 Displacement Reactions MHR 169
Section 4.1 REVIEW Section Summary In a single displacement reaction, one element replaces another element in a compound to produce a new element and a new compound. The general form for single displacement reactions in which a metal displaces another metal, where A and B are metals, is A + BX AX + B. The general form for single displacement reactions in which a non-metal displaces another non-metal, where X and Y are non-metals, is AX + Y X + AY. An activity series lists elements in order, from most reactive to least reactive. The activity series of metals and the activity series of halogens are used to predict whether a single displacement reaction will occur. A single displacement reaction can only occur when an uncombined element is higher in an activity series than the element it would replace in a compound. Review Questions 1. K/U Describe the displacement reaction that occurs in an air bag to change sodium into a less harmful chemical. 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 figures 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, write NR. a. copper and magnesium sulfate b. zinc and iron(ii) chloride c. magnesium and aluminum sulfate d. zinc and hydrochloric acid e. copper and zinc nitrate f. magnesium and sulfuric acid 6. A Aluminum is much more abundant than iron in Earth s crust. Use Table 4.1 to infer why aluminum was very expensive and not widely used until the late 1800s. 7. K/U Why is hydrogen included in the activity series of metals? 8. A Describe two methods you could use to produce hydrogen gas by a single displacement reaction. 9. C Draw a model of a single displacement reaction in which hydrogen is displaced. 10. K/U When a metal displaces hydrogen from water, what types of substances form? 11. C Describe how to use the periodic table to compare the relative reactivities of the halogens. 12. T/I If the liquid in the photograph is water, could the metal be zinc? If the liquid is an acid, could the metal be zinc? Explain your reasoning. 13. K/U Which halogens can be replaced by chlorine? 14. C Design a concept map that shows the relationships among the following terms: single displacement reaction activity series metals non-metals hydrogen acids halogens 15. 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, write NR. a. iron and hydrobromic acid b. bromine and magnesium iodide c. magnesium and aluminum sulfate d. lithium and water e. cobalt and water f. bromine and iron(ii) chloride 16. T/I Which halogen is capable of displacing only one halogen and is itself replaced by two halogens? 170 MHR Unit 2 Chemical Reactions