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Redox Reactions

Section 19.1 Oxidation and Reduction

Section 19.2 Balancing Redox Equations

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Section 19-1

Section 19.1 Oxidation and Reduction

• Describe the processes of oxidation and reduction.

spectator ion: an ion that does not participate in a reaction and is not usually shown in an ionic equation• Identify oxidizing and

reducing agents.

• Determine the oxidation number of an element in a compound.

• Interpret redox reactions in terms of change in oxidation state.

Section 19-1

Section 19.1 Oxidation and Reduction (cont.)

oxidation-reduction reaction

redox reaction

oxidation

Oxidation and reduction are complementary—as an atom is oxidized, another atom is reduced.

reduction

oxidizing agent

reducing agent

Section 19-1

Electron Transfer and Redox Reactions

• An oxidation-reduction reaction, or redox reaction involves the transfer of electrons from one atom to another.

Section 19-1

Electron Transfer and Redox Reactions (cont.)

Section 19-1

Electron Transfer and Redox Reactions (cont.)

• Oxidation is defined as the loss of electrons from atoms of a substance.

Na → Na+ + e–

• Reduction is defined as the gain of electrons by the atoms of a substance.

Cl2 + 2e– → 2Cl–

Section 19-1

Electron Transfer and Redox Reactions (cont.)

• The oxidation number of an atom in an ionic compound is the number of electrons lost or gained by the atom when it forms an ion.

• When an atom or ion is reduced, the numerical value of its oxidation number decreases.

• When an atom or ion is oxidized, its oxidation number increases.

Section 19-1

Electron Transfer and Redox Reactions (cont.)

• Oxidation numbers are tools that scientists use to keep track of the movement of electrons in a redox reaction.

Section 19-1

Oxidizing and Reducing Agents

• The substance that oxidizes another substance by accepting its electrons is called an oxidizing agent.

• The oxidizing agent is the substance that is reduced in a redox reaction.

Section 19-1

Oxidizing and Reducing Agents (cont.)

• The substance that reduces another substance by losing its electrons is the reducing agent.

• The reducing agent is the substance that is oxidized in a redox reaction.

Section 19-1

Redox and Electronegativity

• Redox reactions are not limited to atoms of an element changing to ions.

• Some redox reactions involve changes in molecular substances or polyatomic ions.

N2(g) +3H2(g) → NH3(g)

• N is reduced and H is oxidized.

Section 19-1

Redox and Electronegativity (cont.)

• To determine which was oxidized and which was reduced, you must know which atom is more electronegative.

• Elements with high electronegativity are strong oxidizing agents.

Section 19-1

Redox and Electronegativity (cont.)

Section 19-1

Determining Oxidation Numbers

• To understand all types of redox reactions, the oxidation number of the atoms involved in the reaction must be determined.

Section 19-1

Determining Oxidation Numbers (cont.)

Section 19-1

Oxidation Numbers in Redox Reactions

• Oxidation-reduction reactions are changes in oxidation number.

• Atoms that are reduced have their oxidation number decreased.

• Atoms that are oxidized have their oxidation number increased.

Section 19-1

Oxidation Numbers in Redox Reactions (cont.)

A. A

B. B

C. C

D. D

Section 19-1

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Section 19.1 Assessment

In a redox reaction, the reducing agent is:

A. the substance that is reduced

B. the substance that is oxidized

C. the substance that gains electrons

D. none of the above

A. A

B. B

C. C

D. D

Section 19-1

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Section 19.1 Assessment

In redox reactions, more electronegative elements tend to:

A. be reduced

B. be reducing agents

C. lose electrons

D. not change

End of Section 19-1

Section 19-2

Section 19.2 Balancing Redox Reactions

• Relate changes in oxidation number to the transfer of electrons.

net ionic equation: an ionic equation that includes only the particles that participate in the reaction• Use changes in

oxidation number to balance redox equations.

• Balance net ionic redox equations using the oxidation-number method.

Section 19-2

Section 19.2 Balancing Redox Reactions (cont.)

oxidation-number method

species

half-reaction

Redox equations are balanced when the total increase in oxidation numbers equals the total decrease in oxidation numbers of the atoms involved in the reaction.

Section 19-2

The Oxidation-Number Method

• Chemical equations must be balanced to show the correct quantities of reactants and products.

• The number of electrons transferred from atoms must equal the number of electrons accepted by other atoms.

Section 19-2

The Oxidation-Number Method (cont.)

• The total increase in oxidation numbers must equal the total decrease in oxidation numbers in the reaction.

• This method is called the oxidation number method.

Section 19-2

Balancing Net Ionic Redox Equations

• Sometimes it is preferred to express redox reactions in the simplest possible terms, showing only the oxidation and reduction processes.

• When balancing equations in acidic solution, hydrogen ions (H+) or water molecules can be added to either side of the equation.

• When balancing equations in basic solution, hydroxide ions (OH–) or water molecules can be added to either side of the equation.

Section 19-2

Balancing Redox Equations Using Half-Reactions

• In chemistry, a species is any kind of chemical unit involved in a process.

• Oxidation-reduction reactions occur whenever a species that can give up electrons comes in contact with another species that can accept them.

Section 19-2

Balancing Redox Equations Using Half-Reactions (cont.)

• A half-reaction is one of the two parts of a redox reaction—the oxidation half of the reduction half.

Section 19-2

Balancing Redox Equations Using Half-Reactions (cont.)

A. A

B. B

C. C

D. D

Section 19-2

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Section 19.2 Assessment

A redox reaction split into two parts is called ____.

A. net reaction

B. oxidation-reaction

C. half-reaction

D. reduction-reaction

A. A

B. B

C. C

D. D

Section 19-2

Section 19.2 Assessment

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In acid solution, what would you use to balance the charge in a redox-reaction?

A. electrons

B. hydrogen ions and water

C. hydroxide ions and water

D. hydrogen ions and hydroxide ions

End of Section 19-2

Resources Menu

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Study Guide

Chapter Assessment

Standardized Test Practice

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Concepts in Motion

Study Guide 1

Section 19.1 Oxidation and Reduction

Key Concepts

• Oxidation-reduction reactions involve the transfer of electrons from one atom to another.

• When an atom or ion is reduced, its oxidation number is lowered. When an atom or ion is oxidized, its oxidation number is raised.

• In oxidation-reduction reactions involving molecular compounds (and polyatomic ions with covalent bonds), the more-electronegative atoms are treated as if they are reduced. The less-electronegative atoms are treated as if they are oxidized.

Study Guide 1

Section 19.1 Oxidation and Reduction (cont.)

Key Concepts

Study Guide 2

Section 19.2 Balancing Redox Equations

Key Concepts

• Redox equations in which the same element appears in several reactants and products can be difficult to balance using the conventional method.

• The oxidation-number method is based on the number of electrons transferred from atoms equaling the number of electrons accepted by other atoms.

• To balance equations for reactions in an acid solution, add enough hydrogen ions and water molecules to balance the equation.

Study Guide 2

Section 19.2 Balancing Redox Equations (cont.)

Key Concepts

• To balance equations for reactions in a basic solution, add enough hydroxide ions and water molecules to balance the equation.

• A half-reaction is one of the two parts of a redox reaction.

A. A

B. B

C. C

D. D

Chapter Assessment 1

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What type of reaction involves the transfer of electrons from one atom to another?

A. synthesis

B. decomposition

C. double replacement

D. redox

A. A

B. B

C. C

D. D

Chapter Assessment 2

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Less electronegative atoms in redox reactions are most often ____.

A. oxidized

B. reduced

C. oxidizing agents

D. neutral

A. A

B. B

C. C

D. D

Chapter Assessment 3

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Any chemical unit involved in a process is called a(n) ____.

A. atom

B. type

C. species

D. ion

A. A

B. B

C. C

D. D

Chapter Assessment 4

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Determine the oxidation number of oxygen in HNO3.

A. +3

B. –3

C. +2

D. –2

A. A

B. B

C. C

D. D

Chapter Assessment 5

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Determine the oxidation number of nitrogen in HNO3.

A. +1

B. +4

C. +5

D. –5

A. A

B. B

C. C

D. D

STP 1

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In the reaction MgI2 + Br2 → MgBr2 + I2, what is the oxidation number of I2?

A. –1

B. +1

C. 0

D. +½

A. A

B. B

C. C

D. D

STP 2

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Which is NOT an oxidizing agent in a redox reaction?

A. substance reduced

B. electron acceptor

C. oxidizer of another substance

D. electron donor

A. A

B. B

C. C

D. D

STP 3

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How does the oxidation number change in sodium in the following equation?

2NaI(aq) + Cl2(aq) → 2NaCl(aq) + I2(aq)

A. It changes from 0 to –1.

B. It changes from –1 to 0.

C. It changes from 2 to –2.

D. no change

A. A

B. B

C. C

D. D

STP 4

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If the temperature in a system is held constant, the pressure of a gas must do what when the volume is increased?

A. no change

B. increase

C. decrease

D. unable to determine

A. A

B. B

C. C

D. D

STP 5

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What is the oxidizing agent in the following equation?

Na2SO4 + 4C → Na2S + 4CO

A. C

B. S

C. O

D. Na

IB Menu

Click on an image to enlarge.

IB 1

IB 2

IB 3

IB 4

IB 5

IB 6

IB 7

IB 8

IB 9

CIM

Figure 19.2 Redox Reaction

Table 19.1 Summary of Redox Reactions

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