Replacement Reactions

In contrast to synthesis, combustion, and decomposition reactions, many chemical reactions involve the replacement of an element in a compound. There are two types of replacement reactions: single-replacement reactions and double-replacement reactions.

Single-replacement reactions Now that you’ve seen how atoms and molecules rearrange in synthesis and combustion reactions, look closely at the reaction between lithium and water that is shown in Figure 10-8.

Figure 10-8 The reaction of lithium and water is a single-replacement reaction. Lithium replaces hydrogen in water, and the products of the reaction are aqueous lithium hydroxide and hydrogen gas. Lithium hydroxide exists as lithium and hydroxide ions in solution.

The expanded view of the reaction at the molecular level shows that a lithium atom replaces one of the hydrogen atoms in a water molecule. The following chemical equation describes this activity.

2Li(s) + 2H₂O(l) ® 2LiOH(aq) + H₂(g)

A reaction in which the atoms of one element replace the atoms of another element in a compound is called a single-replacement reaction.

A + BX ® AX + B

The reaction between lithium and water is one type of single-replacement reaction in which a metal replaces a hydrogen in a water molecule. Another type of single-replacement reaction occurs when one metal replaces another metal in a compound dissolved in water. For example, Figure 10-9 shows a single-replacement reaction occurring when a spiral of pure copper wire is placed in aqueous silver nitrate. The shiny crystals that are accumulating on the copper wire are the silver atoms that the copper atoms replaced.

Cu(s) + 2AgNO₃(aq) ® 2Ag(s) + Cu(NO₃)₂(aq)

A metal will not always replace another metal in a compound dissolved in water. This is because metals differ in their reactivities. A metal’s reactivity is its ability to react with another substance. In Figure 10-10 you see an activity series of some metals. This series orders metals by their reactivity with other metals. Single-replacement reactions like the one between copper and aqueous silver nitrate determine a metal’s position on the list. The most active metals, which are those that do replace the metal in a compound, are at the top of the list. The least active metals are at the bottom.

You can use Figure 10-10 to predict whether or not certain reactions will occur. A specific metal can replace any metal listed below it that is in a compound. It cannot replace any metal listed above it. For example, you saw in Figure 10-9 that copper atoms replace silver atoms in a solution of silver nitrate. However, if you place a silver wire in aqueous copper (II) nitrate, the silver atoms will not replace the copper. Silver is listed below copper in the activity series and no reaction occurs. The letters NR (no reaction) are commonly used to indicate that a reaction will not occur.

Ag(s) + Cu(NO₃)₂(aq) ® NR

Figure 10-9 The chemical equation for the single-replacement reaction involving copper and silver nitrate clearly describes the replacement of silver by copper, but the visual evidence of this chemical reaction is a solid precipitate.	Figure 10-10 An activity series, similar to the series shown here for various metals and halogens, is a useful tool for determining whether a chemical reaction will take place and for determining the result of a replacement reaction.

A third type of single-replacement reaction involves the replacement of a nonmetal in a compound by another nonmetal. Halogens are frequently involved in these types of reactions. Like metals, halogens exhibit different activity levels in single-replacement reactions. The reactivities of halogens, determined by single-replacement reactions, are also shown in Figure 10-10. The most active halogen is fluorine, and the least active is iodine. A more reactive halogen replaces a less reactive halogen that is part of a compound dissolved in water. For example, fluorine replaces bromine in water containing dissolved sodium bromide. However, bromine does not replace fluorine in water containing dissolved sodium fluoride.

F₂(g) + 2NaBr(aq) ® 2NaF(aq) + Br₂(l)

Br₂(g) + 2NaF(aq) ® NR

Double-replacement reactions The final type of replacement reaction which involves an exchange of ions between two compounds is called a double-replacement reaction.

AX + BY ® AY + BX

In this generic equation, A and B represent positively charged ions (cations), and X and Y represent negatively charged ions (anions). You can see that the anions have switched places and are now bonded to the other cations in the reaction. In other words, X replaces Y and Y replaces X—a double replacement. More simply, you might say that the positive and negative ions of two

compounds switch places. The reaction between calcium hydroxide and hydrochloric acid is a double-replacement reaction.

Ca(OH)₂(aq) + 2HCl(aq) ® CaCl₂(aq) + 2H₂O(l)

The ionic components of the reaction are Ca²⁺, OH^-, H⁺, and Cl^-. Knowing this, you can now see the two replacements of the reaction. The anions (OH^-and Cl^-) have changed places and are now bonded to the other cations (Ca²⁺and H⁺) in the reaction.

The reaction between sodium hydroxide and copper (II) chloride in solution is also a double-replacement reaction.

2NaOH(aq) + CuCl₂(aq) ® 2NaCl(aq) + Cu(OH)₂(s)

In this case, the anions (OH^- and Cl^-) changed places and are now associated with the other cations (Na⁺and Cu²⁺). The result of this reaction is a solid product, copper (II) hydroxide. A solid produced during a chemical reaction in a solution is called a precipitate.

One of the key characteristics of double-replacement reactions is the type of product that is formed when the reaction takes place. All double-replacement reactions produce either a precipitate, a gas, or water. An example of a double-replacement reaction that forms a gas is that of potassium cyanide and hydrobromic acid.

KCN(aq) + HBr(aq) ® KBr(aq) + HCN(g)

It is important to be able to evaluate the chemistry of double-replacement reactions and predict the products of these reactions. The basic steps to do this are given in Table 10-2.

What are the five classes of chemical reactions? Identify two characteristics of combustion reactions. Compare and contrast single-replacement reactions and double-replacement reactions. Describe the result of a double-replacement reaction.

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