Chapter 20 Oxidation-Reduction Reactions 20.1 The Meaning of Oxidation and Reduction 20.2 Oxidation Numbers 20.3 Describing Redox Equations Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Why do you need to wash the bottom of your car? CHEMISTRY & YOU Why do you need to wash the bottom of your car? Remember: During winter in cold climates, salt is often spread on roads to lower the freezing point of water and thereby prevent the buildup of slippery ice. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? What happens to a substance that undergoes oxidation? What happens to a substance that undergoes reduction? Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? The combustion of gasoline in an automobile engine and the burning of wood in a fireplace are reactions that require oxygen as they release energy. The reactions that break down food in your body and release energy use oxygen from the air you breathe. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Oxygen and Redox When methane (CH4), the main component of natural gas, burns in air, it oxidizes and forms oxides of carbon and hydrogen. One oxide of carbon is carbon dioxide, CO2. CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Oxygen and Redox Not all oxidation processes involve burning. When elemental iron turns to rust, it slowly oxidizes to compounds such as iron(III) oxide (Fe2O3). 4Fe(s) + 3O2(g) → 2Fe2O3(s) Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Oxygen and Redox Not all oxidation processes involve burning. Common liquid household bleach contains sodium hypochlorite (NaClO), a substance that releases oxygen, which oxidizes stains to a colorless form. Hydrogen peroxide (H2O2) also releases oxygen when it decomposes. It is both a bleach and a mild antiseptic that kills bacteria by oxidizing them. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Oxygen and Redox A process called reduction is the opposite of oxidation. Originally, reduction meant the loss of oxygen from a compound. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Oxygen and Redox The reduction of iron ore to metallic iron involves the removal of oxygen from iron(III) oxide. The reduction is accomplished by heating the ore with carbon, usually in the form of coke. 2Fe2O3(s) +3C(s) → 4Fe(s) + 3CO2(g) Iron(III) oxide Carbon Iron Carbon dioxide Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Oxygen and Redox The reduction of iron also includes an oxidation process. 2Fe2O3(s) +3C(s) → 4Fe(s) + 3CO2(g) Iron(III) oxide Carbon Iron Carbon dioxide As iron(III) oxide is reduced to iron by losing oxygen, carbon oxidizes to carbon dioxide by gaining oxygen. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Oxygen and Redox The reduction of iron also includes an oxidation process. 2Fe2O3(s) +3C(s) → 4Fe(s) + 3CO2(g) Iron(III) oxide Carbon Iron Carbon dioxide As iron(III) oxide is reduced to iron by losing oxygen, carbon oxidizes to carbon dioxide by gaining oxygen. Oxidation and reduction always occur simultaneously. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Oxygen and Redox A substance that undergoes oxidation gains oxygen. A substance that undergoes reduction loses oxygen. No oxidation occurs without reduction, and no reduction occurs without oxidation. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Oxygen and Redox A substance that undergoes oxidation gains oxygen. A substance that undergoes reduction loses oxygen. Reactions that involve the processes of oxidation and reduction are called oxidation-reduction reactions. Oxidation-reduction reactions are also known as redox reactions. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Electron Shift in Redox Reactions The modern concepts of oxidation and reduction have been extended to include many reactions that do not even involve oxygen. Redox reactions are currently understood to involve any shift of electrons between reactants. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Electron Shift in Redox Reactions Oxidation is now defined to mean complete or partial loss of electrons or gain of oxygen. Reduction is now defined to mean complete or partial gain of electrons or loss of oxygen. Oxidation Loss of electrons Gain of oxygen Reduction Gain of electrons Loss of oxygen Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Redox Reactions That Form Ions During a reaction between a metal and a nonmetal, electrons are transferred from atoms of the metal to atoms of the nonmetal. Mg(s) + S(s) MgS(s) heat When magnesium metal is heated with the non-metal sulfur, the ionic compound magnesium sulfide is produced. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Redox Reactions That Form Ions Two electrons are transferred from a magnesium atom to a sulfur atom. The magnesium atoms are made more stable by the loss of electrons. The sulfur atoms become more stable due to the gain of electrons. Mg + S Mg2+ + S 2– Magnesium atom Sulfur atom Magnesium ion Sulfide ion → Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Redox Reactions That Form Ions Mg + S Mg2+ + S 2– Magnesium atom Sulfur atom Magnesium ion Sulfide ion → Because it loses electrons, the magnesium atom is said to be oxidized to a magnesium ion. The sulfur atom gains two electrons and is reduced to a sulfide ion. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Redox Reactions That Form Ions Mg + S Mg2+ + S 2– Magnesium atom Sulfur atom Magnesium ion Sulfide ion → The overall process is represented as the two component processes below. Oxidation: Mg → Mg2+ + 2e– (loss of electrons) Reduction: S + 2e– → S 2– (gain of electrons) Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Redox Reactions That Form Ions A substance that undergoes oxidation loses electrons. A substance that undergoes reduction gains electrons. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Redox Reactions That Form Ions The substance that loses electrons is the reducing agent. The substance that accepts electrons is the oxidizing agent. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Redox Reactions That Form Ions Another way to identify oxidizing and reducing agents is to remember that the species that is reduced is the oxidizing agent, and the species oxidized is the reducing agent. Mg(s) + S(s) → MgS(s) Magnesium (reducing agent) Sulfur (oxidizing agent) Magnesium sulfide oxidized reduced Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Identifying Oxidized and Reduced Reactants Sample Problem 20.1 Identifying Oxidized and Reduced Reactants Silver nitrate reacts with copper to form copper nitrate and silver. From the equation below, determine what is oxidized and what is reduced. Identify the oxidizing agent and the reducing agent. 2AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + 2Ag(s) Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Analyze Identify the relevant concepts. Sample Problem 20.1 Analyze Identify the relevant concepts. 1 Identify the ions in the reaction. Then trace how the electrons were transferred. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Solve Apply concepts to this situation. Sample Problem 20.1 Solve Apply concepts to this situation. 2 Rewrite the equation in ionic form so it will be easier to analyze the reaction. 2Ag+ + 2NO3– + Cu → Cu2+ + 2NO3– + 2Ag In this reaction, two electrons are lost from a copper atom (Cu) when it becomes a Cu2+ ion. These electrons are gained by two silver ions (Ag+), which become neutral silver atoms. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Solve Apply concepts to this situation. Sample Problem 20.1 Solve Apply concepts to this situation. 2 The species that loses electrons is oxidized and is the reducing agent. The species that gains electrons is reduced and is the oxidizing agent. Oxidation: Cu → Cu2+ + 2e– (loss of electrons) Reduction: 2Ag+ + 2e– → 2Ag (gain of electrons) The Cu is the reducing agent. The Ag+ is the oxidizing agent. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Redox With Covalent Compounds Some reactions involve covalent compounds, that is, compounds in which complete electron transfer does not occur. 2H2(g) + O2(g) → H2O(l) Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Redox With Covalent Compounds In each reactant hydrogen molecule, the bonding electrons are shared equally between the hydrogen atoms. In water, however, the bonding electrons are pulled toward oxygen because it is much more electronegative than hydrogen. Hydrogen is oxidized because it undergoes a partial loss of electrons. H—H O—O H—O H electrons shared equally shift of bonding electrons away from hydrogen and toward oxygen Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Redox With Covalent Compounds In oxygen, the bonding electrons are shared equally between oxygen atoms in the reactant oxygen molecule. However, when oxygen bonds to hydrogen in the water molecule, there is a shift of electrons toward oxygen. Oxygen is thus reduced because it undergoes a partial gain of electrons. H—H O—O H—O H electrons shared equally shift of bonding electrons away from hydrogen and toward oxygen Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Redox With Covalent Compounds 2H2(g) + O2(g) → H2O(l) Hydrogen is the reducing agent because it is oxidized. Oxygen is the oxidizing agent because it is reduced. This redox reaction is highly exothermic—that is, it releases a great deal of energy. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

What Are Oxidation and Reduction? Redox With Covalent Compounds In some reactions involving covalent reactants or products, the partial electron shifts are less obvious. Processes Leading to Oxidation and Reduction Oxidation Reduction Complete loss of electrons (ionic reactions) Complete gain of electrons (ionic reactions) Shift of electrons away from an atom in a covalent bond Shift of electrons toward an atom in a covalent bond Gain of oxygen Loss of oxygen Loss of hydrogen by a covalent compound Gain of hydrogen by a covalent compound Increase in oxidation number Decrease in oxidation number Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

The element that donates electrons in a redox reaction is called the reducing agent. Which of the following is always true of the reducing agent? A. It is oxidized. B. It is reduced. C. It is ionic. D. It is covalent. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

The element that donates electrons in a redox reaction is called the reducing agent. Which of the following is always true of the reducing agent? A. It is oxidized. B. It is reduced. C. It is ionic. D. It is covalent. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Corrosion Corrosion How does the presence of salts and acids accelerate the corrosion of metals? Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Iron corrodes by being oxidized by oxygen to ions of iron. Corrosion Iron corrodes by being oxidized by oxygen to ions of iron. Water in the environment accelerates the rate of corrosion. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Iron corrodes by being oxidized by oxygen to ions of iron. Corrosion Iron corrodes by being oxidized by oxygen to ions of iron. Oxygen, the oxidizing agent, is reduced to oxide ions (in compounds such as Fe2O3) or to hydroxide ions. 2Fe(s) + O2(g) + 2H2O(l) → 2Fe(OH)2(s) 4Fe(OH)2(s) + O2(g) + 2H2O(l) → 4Fe(OH)3(s) Corrosion occurs more rapidly in the presence of salts and acids. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Corrosion The presence of salts and acids accelerates corrosion by producing conductive solutions that make electron transfer easier. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

CHEMISTRY & YOU If your car is exposed to salt on the roads in the winter, why is it important to wash the salt off your car? Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

CHEMISTRY & YOU If your car is exposed to salt on the roads in the winter, why is it important to wash the salt off your car? Many parts of your car’s body are made of steel, an iron alloy. The salt can cause the metal to corrode, or rust, faster than it would otherwise because when mixed with water, the salt creates a conductive solution that allows electrons to transfer more easily. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

The corrosion of some metals can be a desirable feature. The copper on this building reacted with water vapor, carbon dioxide, and other substances in the air to form a patina. This patina consists of a pale-green film of basic copper(II) carbonate. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Resistance to Corrosion Not all metals corrode easily. Gold and platinum are called noble metals because they are very resistant to losing their electrons by corrosion. Other metals lose electrons easily but are protected from extensive corrosion by the oxide coating formed on their surface. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Resistance to Corrosion Iron(III) oxide Aluminum oxide Water Oxygen Iron forms a coating when it corrodes, but the coating of Aluminum oxidizes quickly in air to form a coating of very iron oxide that forms is not tightly packed. Water and air can penetrate the coating and attack the iron metal beneath it. tightly packed aluminum oxide particles. This coating protects the aluminum object from further corrosion. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Controlling Corrosion To prevent corrosion, the metal surface may be coated with oil, paint, plastic, or another metal. These coatings exclude air and water from the surface, thus preventing corrosion. If the coating is scratched or worn away, however, the exposed metal will begin to corrode. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Controlling Corrosion To prevent corrosion, the metal surface may be coated with oil, paint, plastic, or another metal. Painting a surface, like the Golden Gate Bridge, protects it from the effects of the environment. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Controlling Corrosion To prevent corrosion, the metal surface may be coated with oil, paint, plastic, or another metal. Chromium metal also serves as a protective coating. Like aluminum, chromium forms a corrosion-resistant oxide film on its surface. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Controlling Corrosion In another method of corrosion control, one metal is “sacrificed,” or allowed to corrode, to save a second metal. To protect an iron object, a piece of magnesium may be placed in electrical contact with the iron. When oxygen and water attack the iron object, the iron atoms lose electrons as the iron begins to be oxidized. However, because magnesium is a better reducing agent than iron, the magnesium immediately transfers electrons to the iron atoms, preventing their oxidation. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Controlling Corrosion In another method of corrosion control, one metal is “sacrificed,” or allowed to corrode, to save a second metal. zinc block Zinc blocks are attached to the steel (iron) hull of this ship. The zinc blocks oxidize (corrode) instead of the iron, preventing the hull from corroding. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Explain why metal trash cans are frequently made from zinc-coated steel. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Explain why metal trash cans are frequently made from zinc-coated steel. A trash can made from uncoated steel would rust if left outside in the rain. The zinc coating corrodes first, protecting the steel underneath. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Key Concepts A substance that undergoes oxidation gains oxygen or loses electrons, while a substance that undergoes reduction loses oxygen or gains electrons. The presence of salts and acids accelerates corrosion by producing conductive solutions that make electron transfer easier. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Glossary Terms oxidation-reduction reaction: a reaction that involves the transfer of electrons between reactants oxidation: a process that involves complete or partial loss of electrons or a gain of oxygen; it results in an increase in the oxidation number of an atom Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Glossary Terms reduction: a process that involves a complete or partial gain of electrons or the loss of oxygen; it results in a decrease in the oxidation number of an atom reducing agent: the substance in a redox reaction that donates electrons; in the reaction, the reducing agent is oxidized oxidizing agent: the substance in a redox reaction that accepts electrons; in the reaction, the oxidizing agent is reduced Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

BIG IDEA Reactions Oxidation-reduction reactions always occur simultaneously in redox reactions. Losing electrons is oxidation. Gaining electrons is reduction. If oxygen is involved in the reaction, then the substance gaining oxygen is oxidized, while the substance losing oxygen is reduced. The species that is reduced is the oxidizing agent, while the oxidized species is the reducing agent. Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

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