634-661_Ch20-866418 5/9/06 12:52 PM Page 634 CHAPTER 20 Redox Reactions Chemistry 3.a, 3.g I&E 1.b, 1.c, 1.d What You’ll Learn ▲ You will examine the processes of oxidation and reduction in electron-trans- fer reactions. ▲ You will discover how oxi- dation numbers of elements in compounds are deter- mined and how they relate to electron transfer. ▲ You will separate redox reactions into their oxida- tion and reduction processes. ▲ You will use two different methods to balance oxida- tion–reduction equations. Why It’s Important Oxidation and reduction reac- tions are among the most prevalent in chemistry. From natural phenomena to com- mercial manufacturing, redox reactions play a major role in your daily life. Visit the Chemistry Web site at chemistrymc.com to find links about redox reactions. When threatened, the bom- bardier beetle sprays chemicals from its abdomen that, when combined, undergo an oxida- tion–reduction reaction. The result is a boiling-hot, foul- smelling "bomb" that allows the beetle to escape predators. 634 Chapter 20 634-661_Ch20-866418 5/9/06 12:53 PM Page 635 DISCOVERY LAB Observing an Oxidation–Reduction Reaction ust is the result of a reaction of iron and oxygen. Iron nails can R also react with substances other than oxygen, as you will find out in this experiment. Safety Precautions Always wear safety goggles and an apron in the laboratory. Procedure 1. Use a piece of steel wool to polish the end of an iron nail. 2. Add about 3 mL 1.0M CuSO4 to a test tube. Place the polished end of the nail into the CuSO4 solution. Let stand and observe for about 10 minutes. Record your observations. Materials test tube Analysis iron nail What is the substance found clinging to the nail? What happened to steel wool or sandpaper the color of the copper(II) sulfate solution? Write the balanced 1M copper(II) sulfate (CuSO4) chemical equation for the reaction you observed. Chemistry 3.a, 3.g Section 20.1 Oxidation and Reduction Chemistry 3.a, 3.g I&E 1.d Objectives In Chapter 10, you learned that a chemical reaction can usually be classified as • Describe the processes of one of five types—synthesis, decomposition, combustion, single-replacement, oxidation and reduction. or double-replacement. In this chapter, you’ll investigate a special characteris- tic of many of these reactions—the ability of elements to gain or lose electrons • Identify oxidizing and when they react with other elements. You experimented with this characteristic reducing agents. when you did the DISCOVERY LAB. • Determine the oxidation number of an element in a Electron Transfer and Redox Reactions compound. One of the defining characteristics of single-replacement and combustion • Interpret redox reactions in reactions is that they always involve the transfer of electrons from one atom terms of change in oxida- to another. So do many, but not all, synthesis and decomposition reactions. tion state. For example, you studied the synthesis reaction in which sodium and chlo- Vocabulary rine react to form the ionic compound sodium chloride. oxidation–reduction ϩ 0 Complete chemical equation: 2Na(s) Cl2(g) 2NaCl(s) reaction ϩ 0 ϩ ϩ Ϫ redox reaction Net ionic equation: 2Na(s) Cl2(g) 2Na 2Cl (ions in crystal) oxidation In this reaction, an electron from each of two sodium atoms is transferred to reduction the Cl molecule to form two ClϪ ions. An example of a combustion reaction oxidizing agent 2 is the burning of magnesium in air. reducing agent ϩ 0 Complete chemical equation: 2Mg(s) O2(g) 2MgO(s) ϩ 0 2ϩ ϩ 2Ϫ Net ionic equation: 2Mg(s) O2(g) 2Mg 2O (ions in crystal) 20.1 Oxidation and Reduction 635 634-661-Ch20-828378 1/16/01 6:13 AM Page 636 0 2Mg ϩ O2 2MgO Each gains 2 electrons 2ϩ 2Ϫ X OO OO OO OO 0 O X X OO O X OO OO OO Each loses 2 electrons 2ϩ 2Ϫ OO X O X X O X OO When magnesium reacts with oxygen, as illustrated in Figure 20-1, each mag- Figure 20-1 nesium atom transfers two electrons to each oxygen atom. What is the result of this electron transfer? The two magnesium atoms become Mg2ϩ ions and The reaction of magnesium and 2Ϫ oxygen involves a transfer of the two oxygen atoms become O ions (oxide ions). If you compare this reac- electrons from magnesium to tion with the reaction of sodium and chlorine, you will see that they are alike oxygen. Therefore, this reaction in that both involve the transfer of electrons between atoms. A reaction in which is an oxidation–reduction reac- electrons are transferred from one atom to another is called an oxidation–reduc- tion. Using the classifications tion reaction. For simplicity, chemists often refer to oxidation–reduction reac- given in Chapter 10, this redox tions as redox reactions. reaction also is classified as a combustion reaction. Now consider the single-replacement reaction in which chlorine in an aqueous solution replaces bromine from an aqueous solution of potassium bro- mide, which is shown in Figure 20-2. ϩ 0 ϩ Complete chemical equation: 2KBr(aq) Cl2(aq) 2KCl(aq) Br2(aq) Figure 20-2 Net ionic equation: 2BrϪ(aq) ϩ Cl (aq) 0 Br (aq) ϩ 2ClϪ(aq) Like the reaction shown in 2 2 Figure 20-1, this reaction of Note that chlorine “steals” electrons from bromide ions to become chloride chlorine and bromine in an ions. When the bromide ions lose their extra electrons, the two bromine atoms aqueous solution is also a redox reaction. Here, electrons are form a covalent bond with each other to produce Br2 molecules. The result of transferred from bromide ions this reaction, the characteristic color of elemental bromine in solution, is to chlorine. shown in Figure 20-2. The formation of the covalent bond by sharing of elec- trons also is an oxidation–reduction reaction. Ϫ ϩϩ0 Ϫ 2Br Cl2 Br2 2Cl Ϫ Each gains XX XX XX X X 1 electron X X X X X ϩ X X X XX OO OO XX XX O O O 0 O O O Ϫ Loses electron OO OO OO Ϫ O O XX O X X X OO X X XX Ϫ OO Loses electron O O O X OO 636 Chapter 20 Redox Reactions 634-661-Ch20-828378 06/28/2001 02:24 PM Page 637 How do oxidation and reduction differ? Originally, the word oxidation referred only to reactions in which a substance combined with oxygen, such as the burning of magnesium in air or the burning of natural gas (methane, CH4) in air. Today, oxidation is defined as the loss of electrons from atoms of a sub- stance. Look again at the net ionic equation for the reaction of sodium and chlorine. Sodium is oxidized because it loses an electron. To state this reac- tion more clearly, Biology Na 0 Naϩ ϩ eϪ CONNECTION For oxidation to take place, the electrons lost by the substance that is oxi- hat do the bacteria dized must be accepted by atoms or ions of another substance. In other words, WXenorhabdus luminescens, fireflies, and many deep-sea fish there must be an accompanying process that involves the gain of electrons. have in common? These and Reduction is defined as the gain of electrons by atoms of a substance. other organisms emit light. Following our sodium chloride example further, the reduction reaction that Bioluminescence is the conversion accompanies the oxidation of sodium is the reduction of chlorine. of potential energy in chemical bonds into light during a redox Ϫ 0 Ϫ reaction. Bioluminescent light is a Cl2 2e 2Cl cold light that gives off little heat. Depending on the species, Can oxidation occur without reduction? By our definitions, oxidation and bioluminescence is produced by reduction are complementary processes; oxidation cannot occur unless reduc- different chemicals and by differ- ent means. In fireflies, for exam- tion also occurs. ple, light results from the It is important to recognize and distinguish between oxidation and reduc- oxidation of the molecule tion. The following memory aid may help. luciferin. Scientists are still unraveling LEO GER LEO GER the mystery of bioluminescence. the lion says or, for short, Some luminescent organisms emit light all the time, whereas others This phrase will help you remember that Loss of Electrons is Oxidation, and emit light only when they are dis- Gain of Electrons is Reduction. turbed, such as when churned by ocean waves. Deep-sea fish and Changes in oxidation number You may recall from previous chapters some jellyfish appear to be able to control the light they omit, that the oxidation number of an atom in an ionic compound is the number and one species of mushroom is of electrons lost or gained by the atom when it forms ions. For example, look known to emit light of two dif- at the following equation for the redox reaction of potassium metal with ferent colors. Zoologists have also determined that some light-emit- bromine vapor. ting organisms do not produce ϩ 0 light themselves, but produce Complete chemical equation: 2K(s) Br2(g) 2KBr(s) light by harboring bioluminescent ϩ 0 ϩ ϩ Ϫ bacteria. Net ionic equation: 2K(s) Br2(g) 2K (s) 2Br (s) Organisms appear to use biolu- minescence for different pur- Potassium, a group 1A element that tends to lose one electron in reactions poses. In the ocean depths, because of its low electronegativity, is assigned an oxidation number of ϩ1. bioluminescence probably aids On the other hand, bromine, a group 7A element that tends to gain one elec- vision and recognition.