1. Chapter 20 Oxidation-Reduction Reactions 20.1 The Meaning of Oxidation
and Reduction
20.2 Oxidation Numbers
20.3 Describing Redox
Equations
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2. 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.
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3. What Are Oxidation and Reduction?
What happens to a substance that undergoes oxidation?
What happens to a substance that undergoes reduction?
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4. 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.
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5. 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) O2(g) → CO2(g) H2O(g)
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6. 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) O2(g) → Fe2O3(s)
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7. 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.
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8. 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.
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9. 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
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10. 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.
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11. 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.
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12. 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.
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13. 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.
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14. 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.
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15. 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
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16. 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.
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17. 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 Mg S 2–
Magnesium atom
Sulfur atom
Magnesium ion
Sulfide ion →
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18. What Are Oxidation and Reduction?
Redox Reactions That Form Ions
Mg S Mg 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.
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19. What Are Oxidation and Reduction?
Redox Reactions That Form Ions
Mg S Mg S 2–
Magnesium atom
Sulfur atom
Magnesium ion
Sulfide ion →
The overall process is represented as the two component processes below.
Oxidation: Mg → Mg e– (loss of electrons)
Reduction: S + 2e– → S 2– (gain of electrons)
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20. What Are Oxidation and Reduction?
Redox Reactions That Form Ions
A substance that undergoes oxidation loses electrons.
A substance that undergoes reduction gains electrons.
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21. 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.
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22. 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
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23. 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)
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24. 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.
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25. 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.
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26. 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.
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27. 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)
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28. 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
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29. 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
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30. 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.
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31. 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
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32. 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.
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33. 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.
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34. Corrosion
Corrosion
How does the presence of salts and acids accelerate the corrosion of metals?
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35. 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.
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36. 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.
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37. Corrosion
The presence of salts and acids accelerates corrosion by producing conductive solutions that make electron transfer easier.
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38. 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?
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39. 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.
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40. 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.
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41. 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.
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42. 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.
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43. 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.
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44. 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.
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45. 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.
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46. 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.
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47. 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.
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48. Explain why metal trash cans are frequently made from zinc-coated steel.
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49. 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.
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50. 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.
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51. 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
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52. 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
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53. 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.
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54. END OF 20.1
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