© Copyright Pearson Prentice Hall Slide 1 of 47 Balancing Redox Equations When concentrated...

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Balancing Redox Equations

When concentrated hydrochloric acid (HCl) and nitric acid (HNO3) are mixed with gold, oxidation and reduction reactions produce gaseous nitrogen monoxide (NO) and soluble, stable AuCl4– ions. You will learn how to write and balance chemical equations for redox reactions such as this one.

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Balancing Redox Equations >

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Identifying Redox Reactions

Identifying Redox Reactions

How are oxidation numbers used to identify redox reactions?

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Balancing Redox Equations > Identifying Redox Reactions

If the oxidation number of an element in a reacting species changes, then that element has undergone either oxidation or reduction. Therefore, the reaction as a whole must be a redox reaction.

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Balancing Redox Equations > Identifying Redox Reactions

K + H2O KOH + H2 2Zn + HCl ZnCl2 + H2

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Balancing Redox Equations > Identifying Redox Reactions

A redox reaction takes place between nitrogen and oxygen when lightning bolts heat the air.

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Balancing Redox Equations > Identifying Redox Reactions

A color change can signal a redox reaction.

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Practice Problems for Conceptual Problem 20.4

Problem Solving 20.18 Solve Problem 18 with the help of an interactive guided tutorial.

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Balancing Redox Equations > Two Ways to Balance Redox Equations

Two Ways to Balance Redox Equations

How are changes in oxidation numbers used to balance a redox equation?

What is the procedure for balancing a redox equation using half-reactions?

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Balancing Redox Equations > Two Ways to Balance Redox Equations

Using Oxidation-Number Changes

In the oxidation-number-change method, you balance a redox equation by comparing the increases and decreases in oxidation numbers.

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Balancing Redox Equations > Two Ways to Balance Redox Equations

To use the oxidation-number-change method, start with the skeleton equation for the redox reaction.

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Balancing Redox Equations > Two Ways to Balance Redox Equations

Step 1 Assign oxidation numbers to all the atoms in the equation.

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Balancing Redox Equations > Two Ways to Balance Redox Equations

Step 2 Identify which atoms are oxidized and which are reduced.

Step 3 Use one bracketing line to connect the atoms that undergo oxidation and another such line to connect those that undergo reduction.

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Balancing Redox Equations > Two Ways to Balance Redox Equations

In a balanced redox equation, the total increase in oxidation number of the species oxidized must be balanced by the total decrease in the oxidation number of the species reduced.

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Balancing Redox Equations > Two Ways to Balance Redox Equations

Step 4 Make the total increase in oxidation number equal to the total decrease in oxidation number by using appropriate coefficients.

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Balancing Redox Equations > Two Ways to Balance Redox Equations

Step 5 Finally, make sure that the equation is balanced for both atoms and charge.

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Practice Problems for Conceptual Problem 20.5

Problem Solving 20.20 Solve Problem 20 with the help of an interactive guided tutorial.

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Balancing Redox Equations > Two Ways to Balance Redox Equations

Using Half-Reactions

• A half-reaction is an equation showing just the oxidation or just the reduction that takes place in a redox reaction.

• In the half-reaction method, you write and balance the oxidation and reduction half-reactions separately before combining them into a balanced redox equation.

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Balancing Redox Equations > Two Ways to Balance Redox Equations

To balance a redox reaction using half-reactions, write separate half-reactions for the oxidation and the reduction. After you balance atoms in each half-reaction, balance electrons gained in the reduction with electrons lost in the oxidation.

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Balancing Redox Equations > Two Ways to Balance Redox Equations

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Balancing Redox Equations > Two Ways to Balance Redox Equations

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Balancing Redox Equations > Two Ways to Balance Redox Equations

The oxidation of sulfur by nitric acid in aqueous solution is one example of a redox reaction that can be balanced by the half-reaction method.

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Balancing Redox Equations > Two Ways to Balance Redox Equations

Steps 1 and 2

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Balancing Redox Equations > Two Ways to Balance Redox Equations

Step 3a

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Balancing Redox Equations > Two Ways to Balance Redox Equations

Step 3b

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Balancing Redox Equations > Two Ways to Balance Redox Equations

Steps 4 and 5

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Balancing Redox Equations > Two Ways to Balance Redox Equations

Steps 6 and 7

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Practice Problems for Conceptual Problem 20.6

Problem Solving 20.21 Solve Problem 21 with the help of an interactive guided tutorial.

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Balancing Redox Equations > Choosing a Balancing Method

Choosing a Balancing Method

• Balancing by oxidation number change usually works well if the oxidized and reduced species appear only once on each side of the equation.

• Balancing by the half-reaction method works best in redox reactions where the same element is both oxidized and reduced and in reactions that take place in acidic or alkaline solution.

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Section Quiz

-or-Continue to: Launch:

Assess students’ understanding of the concepts in Section

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1. Choose the correct words for the spaces. One method used to balance redox reactions compares changes in ______ numbers. When an equation is balanced, the total number of _________ lost must be equal to the total number gained.

a. oxidation, electrons

b. oxidation, protons

c. atomic, electrons

d. mass, protons

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2. Choose the correct numbers for the spaces. Balance the equation below with whole-number coefficients using the oxidation-number-change method.

NH3 + O2 NO2 + H2O

The least common multiple of the changes in oxidation numbers of atoms is _____, and the coefficient of water in the balanced equation is _____.

a. 14, 6

b. 14, 3

c. 28, 3

d. 7, 6

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3. Balance the equation below with the lowest whole-number coefficients using the half-reaction method.

Ag+(aq) + SO2(g) + H2O(l) Ag(s) + SO42–(g) + H+(aq)

The reduction half-reaction must be multiplied by

a. 1

b. 2

c. 3

d. 4

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