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 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) 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 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. 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 ions 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 halogens. 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 potassium barium calcium sodium from coldco 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 ion 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 electron 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 acids and water. Displacement from an Acid Th e reaction of magnesium with hydrochloric acid, 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 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. 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.