1 Balancing Redox Reactions Chapter 20: Day 2. 2 Review of Terminology for Redox Reactions...

11Balancing Redox Balancing Redox ReactionsReactions

Chapter 20: Day 2Chapter 20: Day 2

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Review of Terminology Review of Terminology for Redox Reactionsfor Redox Reactions

Review of Terminology Review of Terminology for Redox Reactionsfor Redox Reactions

• OXIDATIONOXIDATION—loss of electron(s) by a —loss of electron(s) by a species; increase in oxidation number.species; increase in oxidation number.

• REDUCTIONREDUCTION—gain of electron(s); —gain of electron(s); decrease in oxidation number.decrease in oxidation number.

• OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; —electron acceptor; species is reduced.species is reduced.

• REDUCING AGENTREDUCING AGENT—electron donor; —electron donor; species is oxidized.species is oxidized.

• OXIDATIONOXIDATION—loss of electron(s) by a —loss of electron(s) by a species; increase in oxidation number.species; increase in oxidation number.

• REDUCTIONREDUCTION—gain of electron(s); —gain of electron(s); decrease in oxidation number.decrease in oxidation number.

• OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; —electron acceptor; species is reduced.species is reduced.

• REDUCING AGENTREDUCING AGENT—electron donor; —electron donor; species is oxidized.species is oxidized.

20.3 Describing Redox Equations >20.3 Describing Redox Equations >

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Some fruits, including apples, turn brown when you cut them. What is happening on the surface of the fruit?

CHEMISTRY & YOUCHEMISTRY & YOU

Oxygen in air reacts with chemicals on the surface of

the cut fruit. The oxygen oxidizes the chemicals in the fruit, causing a redox reaction and therefore the

color change.

Why does cut fruit turn brown?



• The element whose oxidation number increases is oxidized

• The element whose oxidation number decreases is reduced

If changes in oxidation number occur, the reaction is a redox reaction.



Identifying Redox Reactions

Use the change in oxidation number to identify whether each reaction is a redox

reaction

Cl2(g) + 2NaBr(aq) → 2NaCl(aq) + Br2(aq)

2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)



Sample Problem 20.5Sample Problem 20.5

a. Assign oxidation numbers.

0 +1 –1 +1 –1 0

Cl2(g) + 2NaBr(aq) → 2NaCl(aq) + Br2(aq)

The chlorine is reduced;

The bromide ion is oxidized;



Solve ASSIGN OXIDATION NUMBERS. 2


• NO change in oxidation number.

• This is not a redox reaction.

+1 –2 +1 +1 +6 –2 +1 +6 –1 +1 –2

2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)

This is an acid-base (neutralization) reaction.



Which of the following are redox reactions?

A. NH3 + HCl → NH4Cl

B. SO3 + H2O → H2SO4

C. NaOH + HCl → NaCl + H2O

D. H2S + NHO3 → H2SO4 + NO2 + H2O

B. SO3 + H2O → H2SO4

D. H2S + NHO3 → H2SO4 + NO2 + H2O

99

TRANSFER REACTIONSTRANSFER REACTIONS

Atom/Group transfer (not)

HCl + H2O ---> Cl- + H3O+

Redox: Electron transfer

Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s)

1010

OXIDATION-REDUCTION REACTIONS

OXIDATION-REDUCTION REACTIONS

Cu(s) + 2 Ag+(aq) ---> Cu2+(aq) + 2 Ag(s)

Why 2?

1111Balancing Balancing EquationsEquations

Need to Balance BOTH mass and CHARGE

Cu + Ag+ --give--> Cu2+ + Ag

Step 1:: Divide into half-reactions:

one for oxidation and the other for reduction.

Ox Cu ---> Cu2+

Red Ag+ ---> Ag


Step 2: Balance each for mass.

Already done in this case.

Step 3:Balance each half-reaction for

charge by adding electrons.

Ox Cu ---> Cu2+ + 2e-

Red Ag+ + e- ---> Ag


Step 4: Multiply each half-reaction by a factor to have the electrons lost equal to number gained

Cu ---> Cu2+ + 2e- 2 Ag+ + 2 e- ---> 2 Ag

Step 5:Add to give the overall equation.

Cu + 2 Ag+ ---> Cu2+ + 2Ag

The equation is now balanced for BOTH charge and mass.

1414

Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Use the half-reaction method to balance the following redox equation.

FeCl3 + H2S → FeCl2 + HCl + S

Oxidation: H2S → 2H+ + S + 2e–

Reduction: 2Fe3+ + 2e– → 2Fe2+

2FeCl3 + H2S → 2FeCl2 + 2HCl + S



Balancing Redox Equations

What are two different methods for balancing a redox equation?

Two different methods for balancing redox equations are the

oxidation-number-change method and the half-reaction

method.



In the oxidation-number-change method, you balance

a redox equation by comparing the increases and

decreases in oxidation numbers.

Using Oxidation-Number Changes




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

• Write the numbers above the atoms.

• The oxidation number is stated per atom.

Fe2O3(s) + CO(g) → Fe(s) + CO2(g)

+3 –2 +2 –2 0 +4 –2




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

• Iron is reduced. +3 to 0

• Carbon is oxidized. +2 to +4

Fe2O3(s) + CO(g) → Fe(s) + CO2(g)

+3 –2 +2 –2 0 +4 –2




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

–3 (reduction)

Fe2O3(s) + CO(g) → Fe(s) + CO2(g) +3 –2 +2 –2 0 +4 –2

+2 (oxidation)

• Write the oxidation-number change at the midpoint of each line.




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

2 × (–3) = –6

Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g) +3 –2 +2 –2 0 +4 –2

3 × (+2) = +6

• The oxidation-number increase should be multiplied by 3 and the decrease by 2.




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

Fe2O3(s) + 3CO(g) → 2Fe(s) + 3CO2(g)

• If necessary, finish balancing the equation by inspection.



Balancing Redox Equations by Oxidation-Number Change

Balance this redox equation by using the oxidation-number-change method.

Cl2(g) + K(s) → KCl(s)0 0 +1 –1

2 × (-1) = -2

2 × (+1) = +2

2 2

2323Ready for more complex reactions??

YES!YES!

2424Acid / BaseAcid / Base Redox Reactions Redox Reactions

Some redox reactions have equations that must be balanced by special

techniques.If reactions occur in the “presence of”

acid or base

MnO4- + 5 Fe2+ + 8 H+

---> Mn2+ + 5 Fe3+ + 4 H2O

2525

Step 1:: Divide into half-reactions

Permanganate and iron(II) ions are reacted in an acidic solution

MnO4- + Fe2+ ---> Mn2+ + Fe3+

Ox Fe2+ ---> Fe3+

Red MnO4- ---> Mn2+

Need to balance MASS

2626Step 2: Balance each for mass.

Already done for Iron

Fe2+ ---> Fe3+

Need to have “O” on both sided: Add water

MnO4- ---> Mn2+ + H2O

Never add O2, O atoms, or O2- to balance oxygen.

MnO4- ---> Mn2+ + 4H2O

2727

Step 2: Balance each for mass.

MnO4- ---> Mn2+ + 4H2O

Need to have “H” on both sided.

Told in an acidic solution: need H+

8 H+ + MnO4- ---> Mn2+ + 4H2O

Never add H2 or H atoms to balance hydrogen.

2828Step 3:Balance each half-reaction for

charge by adding electrons.

8 H+ + 5 e- +MnO4- ---> Mn2+ + 4H2O

5Fe2+ ---> 5Fe3+ + 5e-

Multiply each half-reaction by a factor to have the electrons lost

equal to number gained

2929Acid / BaseAcid / Base Redox Reactions Redox Reactions

Step 5: Add to obtain the overall equation

MnO4- + 5 Fe2+ + 8 H+

---> Mn2+ + 5 Fe3+ + 4 H2O

Check by adding charges on both sides and by counting atoms

The equation is now balanced for BOTH charge and mass.

3030

Balancing Balancing EquationsEquations

• Never add O2, O atoms, or O2- to balance oxygen.

• Never add H2 or H atoms to balance hydrogen.

• Be sure to write the correct charges on all the ions.

• Check your work at the end to make sure mass and charge are balanced.

• PRACTICE!

3131

ReductionReduction of VO of VO22++ with Zn with Zn


Balance the following in acid solution—

VO2+ + Zn ---> VO2+ + Zn2+

Step 1:Write the half-reactions

Ox Zn ---> Zn2+

Red VO2+ ---> VO2+

Step 2:Balance each for mass.

VO2+ ---> VO2+ 2 H+ +

Add HAdd H22O on O-deficient side and add HO on O-deficient side and add H++

on other side for H-balance.on other side for H-balance.

+ H2O


Step 3: Add electrons to half reaction.

Zn ---> Zn2+ + 2e-

e- + 2 H+ + VO2+ ---> VO2+ + H2O

Step 4:Multiply by an appropriate factor.

2e- + 4 H+ + 2 VO2+ ---> 2 VO2+ + 2 H2O

Zn ---> Zn2+ + 2e-

Balancing EquationsBalancing EquationsStep 5:Add balanced half-reactions

Zn + 4 H+ + 2 VO2+

---> Zn2+ + 2 VO2+ + 2 H2O

Check by adding charges on both sides and by counting atoms



Balancing Redox Equations by Half-Reactions

Balance this redox equation using the half-reaction

method.

KMnO4(aq) + HCl(l) → MnCl2(aq) + Cl2(g) + KCl(aq)



Step 1:Write the equation in ionic form.


K+(aq) + MnO4–(aq) + H+(aq) + Cl–(aq) →

Mn2+(a) + 2Cl–(a) + Cl2(g) + H2O + K+(a) + Cl–(a)



Step 2: Write half-reactions. Determine the oxidation and reduction process.

Oxidation half-reaction:

Cl– → Cl2

Reduction half-reaction:

MnO4– → Mn2+

–1 0

+7 +2



Step 3: Balance the atoms in each half-reaction.

Oxidation:

2Cl–(aq) → Cl2(g) (atoms balanced)

Reduction:

MnO4–(aq) + 8H+(aq) →

Mn2+(aq) + 4H2O(l) (atoms balanced)

• The solution is acidic, so use H2O and H+ to balance the oxygen and hydrogen.



Step 4: Balance the charges by adding electrons.

Oxidation:

2Cl–(aq) → Cl2(g) + 2e– (charges balanced)

Reduction:

MnO4–(aq) + 8H+(aq) + 5e– →

Mn2+(aq) + 4H2O(l) (charges balanced)



Step 5: Make the numbers of electrons equal.

Oxidation: 10Cl–(aq) → 5Cl2(g) + 10e–

Reduction:

2MnO4–(aq) + 16H+(aq) + 10e– →

2Mn2+(aq) + 8H2O(l)

• Multiply the oxidation half-reaction by 5 and the reduction half-reaction by 2.



Step 6: Add the half-reactions. Then, subtract the terms that appear on both sides.

10Cl–(aq) + 2MnO4–(aq) + 16H+(aq) + 10e– →

5Cl2(g) + 10e– + 2Mn2+(aq) + 8H2O(l)

Step 7: Add the spectator ions, making sure the charges and atoms are balanced.

10Cl– + 2MnO4– + 2K+ + 16H+ + 6Cl– →

5Cl2 + 2Mn2+ + 4Cl– + 8H2O + 2K+ + 2Cl–



Combine the spectator and nonspectator Cl– on each side.

16Cl–(a) + 2MnO4–(a) + 2K+(a) + 16H+(a) →

5Cl2(g) + 2Mn2+(a) + 6Cl–(a) + 8H2O(l) + 2K+(a)



Show the balanced equation for the substances given in the question (rather than for ions).

2KMnO4(aq) + 16HCl(aq) →

2MnCl2(aq) + 5Cl2(g) + 8H2O(l) + 2KCl(aq)



Use the half-reaction method to balance the following redox equation.

FeCl3 + H2S → FeCl2 + HCl + S



Glossary TermsGlossary Terms

• oxidation-number-change method: a method of balancing a redox equation by comparing the increases and decreases in oxidation numbers

• half-reaction: an equation showing either the oxidation or the reduction that takes place in a redox reaction

• half-reaction method: a method of balancing a redox equation by balancing the oxidation and reduction half-reactions separately before combining them into a balanced redox equation



Redox equations can be balanced by two methods, the oxidation-number-change method and balancing the oxidation and reduction half-reactions.

BIG IDEABIG IDEA

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