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Chapter 16: Oxidation-Reduction Reactions

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Chapter 16: Oxidation-Reduction Reactions CHAPTER Oxidation-ReductionOxidation-Reduction 16 ReactionsReactions Chapter Preview Sections Why Do Things Rust? 16.1 The Nature of Oxidation- Reduction Reactions hen iron corrodes, iron metal reacts with MiniLab 16.1 Corrosion of Iron oxygen from the air and water to form ChemLab Copper Atoms and Ions: W iron(III) oxide—rust. Rust is the result of Oxidation and Reduction an oxidation-reduction reaction in which iron metal loses electrons to oxygen. Given time, and 16.2 Applications of Oxidation- Reduction Reactions oxygen from the air and water, all of the drums MiniLab 16.2 Testing for Alcohol in this photo will rust away completely. by Redox 552 Start-up Activities What I Already Know Observing an Oxidation– Review the following concepts Reduction Reaction before studying this chapter. Rust is the result of a reaction of iron and oxygen. Iron Chapter 3: patterns of valence nails can also react with substances other than oxygen, electrons as you will find out in this experiment. Chapter 5: predicting oxidation Safety Precautions number from the periodic table Chapter 6: types of reactions Always wear safety goggles and an apron in the laboratory. Reading Chemistry Materials Look through the Section Previews for this chapter, jotting down some •test tube key ideas. As you read through the • iron nail chapter, make an outline using the •steel wool or sandpaper key ideas you wrote down. For each •1M copper(II) sulfate (CuSO ) 4 topic, review any new vocabulary Procedure words. 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 Preview this chapter’s content and stand and observe for about 10 minutes. Record your activities at chemistryca.com observations. Analysis What is the substance found clinging to the nail? What happened to the color of the copper(II) sulfate solution? Write the balanced chemical equation for the reaction you observed. 553 16.1SECTION The Nature of Oxidation- Reduction Reactions xygen undergoes many reactions when it encounters other substances. SECTION PREVIEW One of these reactions is responsible for the browning of fruits. Anoth- Oer forms the rust that eats away at the metal parts of bikes and cars. In Objectives both of these cases, a type of reaction called oxidation is taking place. You can probably guess how this reaction got its name; oxygen is a reactant. But ✓ Analyze the charac- you will learn that not teristics of an oxidation-reduction all oxidation reactions reaction. involve oxygen. And oxidation reactions ✓ Distinguish between oxidation are never lonely reactions and reduc- because they always tion reactions by have partners— definition. reduction reactions. ✓ Identify the sub- You will see what the stances that are oxi- characteristics of dized and those that these reactions are are reduced in a redox and why they always reaction. take place together. ✓ Distinguish oxidiz- ing and reducing agents in redox reactions. What is oxidation-reduction? Review Vocabulary Oxygen is the most abundant element in Earth’s crust. It is very reactive Buffer: solution that and can combine with almost every other element. An element that bonds resists changes in pH to oxygen to form a new compound, called an oxide, usually loses electrons when moderate because oxygen is more electronegative. You will recall that an electro- amounts of acids or negative element has a strong attraction for electrons. Because of this bases are added. strong attraction, oxygen is able to pull electrons away from other atoms. The reactions in which elements combine with oxygen to form oxides were New Vocabulary among the first to be studied by early chemists, who grouped them togeth- oxidation-reduction er and called them oxidation reactions. Later, chemists realized that some reaction other nonmetal elements can combine with substances in the same way as oxidation oxygen and that these reactions are similar to oxidation reactions. Modern reduction chemists use the term oxidation to refer to any chemical reaction in which oxidizing agent an element or compound loses electrons to another substance. reducing agent A common oxidation reaction occurs when iron metal loses electrons to oxygen. Each year in the United States, corrosion of metals—especially the iron in steel—costs billions of dollars as automobiles, ships, and bridges and other structures are slowly eaten away. Figure 16.1 shows some of this damage and how it can be prevented. 554 Chapter 16 Oxidation-Reduction Reactions Figure 16.1 Corrosion of Iron When iron corrodes, iron metal reacts with oxygen to form iron(III) oxide—rust. Corrosion of iron can be prevented by covering the surface of exposed steel with paint or other coatings such as plastic. If the protective coating is damaged or cracked, rust forms quickly. ᮣ ᮤ Steel can be protected from oxidation if it is coated with a more active metal such as zinc. Zinc loses electrons to oxygen more readily than iron does, so the zinc is oxi- dized preferentially, forming a tough protective layer of zinc oxide. The coating of zinc and zinc oxide prevents the formation of rust by keeping oxygen from reaching the iron. Steel that has been coated with zinc is called gal- vanized steel. The bucket on the left has been galvanized. Redox What happens to the zinc in galvanized steel? It reacts with oxygen to form zinc oxide in the following reaction. ϩ ˇ 2ϩ ϩ 2Ϫ 2Zn(s) O2(g) 2Zn (s) 2O (s) Does this type of reaction look familiar? You learned in Chapter 6 that this is classified as a synthesis reaction. You also know that early chemists called it an oxidation reaction because oxygen is a reactant. The formation of zinc oxide falls into another, broader class of reactions characterized by the transfer of electrons from one atom or ion to another. This type of reaction is called an oxidation-reduction reaction, commonly known as a redox reaction. Many important chemical reactions are redox reactions. Forma- tion of rust is one example; combustion of fuels is another. In each redox reaction, one element loses electrons, and another element takes them. How do atoms or ions lose electrons in a redox reaction? If you examine the equation for the reaction between zinc and oxygen more closely, you can see which atoms are gaining electrons and which are losing them. You also can determine where the electrons go during a redox reaction by comparing the oxidation number of each type of atom or ion before and after the reac- tion takes place. Recall from Chapter 5 that the oxidation number of an ion is equal to its charge. All elements, when in their free form, have a charge of zero and are assigned an oxidation number of zero. In the formation of zinc oxide, the zinc atom and the diatomic oxygen molecule that react each has an oxidation number of zero. In the ionic compound formed, each oxide ion has a 2Ϫ charge and an oxidation number of 2Ϫ.Because the com- pound must be neutral, the total positive charge must be 4ϩ;thus, each zinc ion must have a charge and an oxidation number of 2ϩ. 16.1 The Nature of Oxidation-Reduction Reactions 555 Oxidation You have learned that a reaction in which an element loses electrons is called an oxidation reaction. The element that loses the electrons becomes more positively charged; that is, its oxidation number increases. That ele- ment is said to be oxidized during the reaction. Zinc is oxidized during the formation of zinc oxide because metallic zinc atoms each lose two electrons. The oxidation reaction can be written by itself to show how zinc changes during the redox reaction. Here’s what happens to each atom of zinc. Zn ˇ[Zn]2+ϩ 2e – (loss of electrons) reduction: re (L) back Reduction ducere (L) to lead In a reduction What happens to the electrons that are lost by the zinc atom? Electrons reaction, the addi- do not wander around by themselves; they must be transferred to another tion of electrons atom or ion. This is why oxidation reactions never occur alone. They are results in a always paired with reduction reactions. A reduction reaction is one in decrease in oxida- which an element gains one or more electrons. The element that picks up tion number of an the electrons and becomes more negatively charged during the reaction is atom or ion. said to be reduced. Its oxidation number decreases, or is reduced. Because oxidation and reduction reactions occur together, each is referred to as a half-reaction. In every redox reaction, at least one element undergoes reduction while another undergoes oxidation. Just as a successful pass in football requires a quarterback to throw the ball and a receiver to catch it, a redox reaction must have one element that gives up electrons and one that accepts them. The electronic structure of both reactants changes during a redox reaction. Figure 16.2 shows the movement of electrons in the formation of zinc oxide. Oxygen accepts the electrons that zinc loses. Oxygen is reduced Figure 16.2 during the reaction between zinc and oxygen because each oxygen atom Formation of Zinc Oxide gains two electrons. Like the oxidation reaction, the reduction reaction In the formation of zinc can be written by itself. Here’s what happens to each atom of oxygen. oxide, the zinc atom loses two 2– electrons during the reaction, O ϩ 2e – ˇ O (gain of electrons) becoming a zinc ion. Its oxi- dation number increases from zero to 2ϩ. The oxygen atom gains ϩ the two electrons Zn ϩ ⌷ [Zn]2 ϩ 2Ϫ from zinc, becom- ing an oxide ion.
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