15.E: Oxidation-Reduction Reaction (Exercises)

Exercises (Oxidation-Reduction Reactions)

1. Is this reaction a redox reaction? Explain your answer. 2K(s) + Br₂(ℓ) → 2KBr(s)
    
2. Is this reaction a redox reaction? Explain your answer. 2NaCl(aq) + Pb(NO₃)₂(aq) → 2NaNO₃(aq) + PbCl₂(s)
    
3. Which substance loses electrons and which substance gains electrons in this reaction? 2Mg(s) + O₂(g) → 2MgO
    
4. Which substance loses electrons and which substance gains electrons in this reaction? 16Fe(s) + 3S₈(s) → 8Fe₂S₃(s)
    
5. Which substance is oxidized and which substance is reduced in this reaction? 2Li(s) + O₂(g) → Li₂O₂(s)
    
6. Which substance is oxidized and which substance is reduced in this reaction? 2Fe(s) + 3I₂(s) → 2FeI₃(s)
    
7. What are two different definitions of oxidation?

8. What are two different definitions of reduction?
    
9. Assign oxidation numbers to the atoms in each substance.
   1. P₄
   2. SO₃
   3. SO₃²⁻
   4. Ca₃(PO₃)₂
       
10. Assign oxidation numbers to the atoms in each substance.
    1. PCl₅
    2. (NH₄)₂Se
    3. Ag
    4. Li₂O₂
        
11. Assign oxidation numbers to the atoms in each substance.
    1. NO
    2. NO₂
    3. CrCl₂
    4. CrCl₃
        
12. Assign oxidation numbers to the atoms in each substance.
    1. NaH
    2. N₂O₃
    3. NO₂⁻
    4. CuNO₃
        
13. Assign oxidation numbers to the atoms in each substance.
    1. CH₂O
    2. NH₃
    3. Rb₂SO₄
    4. Zn(C₂H₃O₂)₂
        
14. Assign oxidation numbers to the atoms in each substance.
    1. C₆H₆
    2. B(OH)₃
    3. Li₂S
    4. Au
        
15. Identify what is being oxidized and reduced in this redox reaction by assigning oxidation numbers to the atoms. 2NO + Cl₂ → 2NOCl
     
16. Identify what is being oxidized and reduced in this redox reaction by assigning oxidation numbers to the atoms. Sr + SO₃ → SrSO₃
     
17. Identify what is being oxidized and reduced in this redox reaction by assigning oxidation numbers to the atoms. 2KrF₂ + 2H₂O → 2Kr + 4HF + O₂
     
18. Identify what is being oxidized and reduced in this redox reaction by assigning oxidation numbers to the atoms. SO₃ + SCl₂ → SOCl₂ + SO₂
     
19. Identify what is being oxidized and reduced in this redox reaction by assigning oxidation numbers to the atoms. 2Rb + MgCl₂ → 2RbCl + Mg
     
20. Identify what is being oxidized and reduced in this redox reaction by assigning oxidation numbers to the atoms. 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O

Answers

1. yes because oxidation numbers are changing

3. lose: Mg; gain: O

5. oxidized: Li; reduced: O

7. increase in oxidation number; loss of electrons

9.

1. P: 0
2. S: +6; O: −2
3. S: +4; O: −2
4. Ca: +2; P: +3; O: −2

11.

1. N: +2; O: −2
2. N: +4; O: −2
3. Cr: +2; Cl: −1
4. Cr: +3; Cl: −1

13.

 

1. C: 0; H: +1; O: −2
2. N: −3; H: +1
3. Rb: +1; S: +6; O: −2
4. Zn: +2; C: 0; H: +1; O: −2

15. oxidized: N; reduced: Cl

17. oxidized: O; reduced: Kr

19.oxidized: Rb; reduced: Mg

Exercises (Balancing Redox Reactions)

1. Balance these redox reactions by inspection.
   1. Na + F₂ → NaF
   2. Al₂O₃ + H₂ → Al + H₂O
       
2. Balance these redox reactions by inspection.
   1. Fe₂S₃ + O₂ → Fe₂O₃ + S
   2. Cu₂O + H₂ → Cu + H₂O
       
3. Balance these redox reactions by inspection.
   1. CH₄ + O₂ → CO₂ + H₂O
   2. P₂O₅ + Cl₂ → PCl₃ + O₂
       
4. Balance these redox reactions by inspection.
   1. PbCl₂ + FeCl₃ → PbCl₄ + FeCl₂
   2. SO₂ + F₂ → SF₄ + OF₂
       
5. Balance these redox reactions by the half reaction method.
   1. Ca + H⁺ → Ca²⁺ + H₂
   2. Sn²⁺ → Sn + Sn⁴⁺ (Hint: both half reactions will start with the same reactant.)
       
6. Balance these redox reactions by the half reaction method.
   1. Fe³⁺ + Sn²⁺ → Fe + Sn⁴⁺
   2. Pb²⁺ → Pb + Pb⁴⁺ (Hint: both half reactions will start with the same reactant.)
       
7. Balance these redox reactions by the half reaction method.
   1. Na + Hg₂Cl₂ → NaCl + Hg
   2. Al₂O₃ + C → Al + CO₂
       
8. Balance these redox reactions by the half reaction method.
   1. Br⁻ + I₂ → I⁻ + Br₂
   2. CrCl₃ + F₂ → CrF₃ + Cl₂
       
9. Balance these redox reactions that occur in aqueous solution. Use whatever water-derived species is necessary; there may be more than one correct balanced equation.
   1. Cu + NO₃⁻ → Cu²⁺ + NO₂
   2. Fe + MnO₄⁻ → Fe³⁺ + Mn
       
10. Balance these redox reactions that occur in aqueous solution. Use whatever water-derived species is necessary; there may be more than one correct balanced equation.
    1. CrO₃ + Ni²⁺ → Cr³⁺ + Ni³⁺
    2. OsO₄ + C₂H₄ → Os + CO₂
        
11. Balance these redox reactions that occur in aqueous solution. Use whatever water-derived species is necessary; there may be more than one correct balanced equation.
    1. ClO⁻ + Ti²⁺ → Ti⁴⁺ + Cl⁻
    2. BrO₃⁻ + Ag → Ag⁺ + BrO₂
        
12. Balance these redox reactions that occur in aqueous solution. Use whatever water-derived species is necessary; there may be more than one correct balanced equation.
    1. H₂O₂ + NO → N₂O₃ + H₂O
    2. VO₂⁺ + NO → V³⁺ + NO₂
        
13. Explain why this chemical equation is not balanced and balance it if it can be balanced: Cr²⁺ + Cl₂ → Cr³⁺ + 2Cl⁻
     
14. Explain why this equation is not balanced and balance it if it can be balanced: O₂ + 2H₂O + Br₂ → 4OH⁻ + 2Br⁻

Answers

1.  

a. 2Na + F₂ → 2NaF
b. Al₂O₃ + 3H₂ → 2Al + 3H₂O

3.

a. CH₄ + 2O₂ → CO₂ + 2H₂O

b. 2P₂O₅ + 6Cl₂ → 4PCl₃ + 5O₂

5.

a. Ca + 2H⁺ → Ca²⁺ + H₂

b. 2Sn²⁺ → Sn + Sn⁴⁺

7.

a. 2Na + Hg₂Cl₂ → 2NaCl + 2Hg

b. 2Al₂O₃ + 3C → 4Al + 3CO₂

9.

a. 4H⁺ + Cu + 2NO₃⁻ → Cu²⁺ + 2NO₂ + 2H₂O in acidic solution; 2H₂O + Cu + 2NO₃⁻ → Cu²⁺ + 2NO₂ + 4OH⁻ in basic solution

b. 24H⁺ + 3MnO₄⁻ + 7Fe → 7Fe³⁺ + 3Mn + 12H₂O in acidic solution; 12H₂O + 3MnO₄⁻ + 7Fe → 7Fe³⁺ + 3Mn + 24OH⁻ in basic solution

11. 

 

a. 2H⁺ + ClO⁻ + Ti²⁺ → Cl⁻ + H₂O + Ti⁴⁺ in acidic solution; H₂O + ClO⁻ + Ti²⁺ → Cl⁻ + Ti⁴⁺ + 2OH⁻ in basic solution

b. 2H⁺ + BrO₃⁻ + Ag → BrO₂ + H₂O + Ag⁺ in acidic solution; H₂O + BrO₃⁻ + Ag → BrO₂ + Ag⁺ + 2OH⁻ in basic solution

 

13. The charges are not properly balanced. The correct balanced equation is 2Cr²⁺ + Cl₂ → 2Cr³⁺ + 2Cl⁻.

Exercises (Applications of Redox Reactions - Voltaic Cells)

1. Draw the voltaic cell represented by this reaction and label the cathode, the anode, the salt bridge, the oxidation half cell, the reduction half cell, the positive electrode, and the negative electrode. Use Fig. 14.4.1 as a guide. Zn + 2Ag⁺ → Zn²⁺ + 2Ag
    
2. Draw the voltaic cell represented by this reaction and label the cathode, the anode, the salt bridge, the oxidation half cell, the reduction half cell, the positive electrode, and the negative electrode. Use Fig. 14.4.1 as a guide. 3Mg + 2Cr³⁺ → 3Mg²⁺ + 2Cr
    
3. What is the voltage of this half reaction? 2F⁻ → F₂ + 2e⁻
    
4. What is the voltage of this half reaction? Na → Na⁺ + e⁻
    
5. What is the voltage of the voltaic cell in Exercise 1? Consult Table 14.4.1.
    
6. What is the voltage of the voltaic cell in Exercise 2? Consult Table 14.4.1.
    
7. Balance this redox reaction and determine its voltage. Is it spontaneous? Li⁺ + Al → Li + Al³⁺
    
8. Balance this redox reaction and determine its voltage. Is it spontaneous? Pb²⁺ + Ni → Pb + Ni²⁺
    
9. Balance this redox reaction and determine its voltage. Is it spontaneous? Cu²⁺ + Ag + Cl⁻ → Cu + AgCl
    
10. Balance this redox reaction and determine its voltage. Is it spontaneous? Mn²⁺ + Br₂ → MnO₄⁻ + Br⁻
     
11. Which reaction represents the cathode reaction in Exercise 7? The anode reaction?
     
12. Which reaction represents the cathode reaction in Exercise 8? The anode reaction?
     
13. Which reaction represents the cathode reaction in Exercise 9? The anode reaction?
     
14. Which reaction represents the cathode reaction in Exercise 10? The anode reaction?
     
15. A voltaic cell is based on this reaction: Ni + 2Au⁺ → Ni²⁺ + 2Au; If the voltage of the cell is 0.33 V, what is the standard reduction potential of the Au⁺ + e⁻ → Au half reaction?
     
16. A voltaic cell is based on this reaction: 3Pb + 2V³⁺ → 3Pb²⁺ + 2V; If the voltage of the cell is −0.72 V, what is the standard reduction potential of the V³⁺ + 3e⁻ → V half reaction?
     
17. What species is being oxidized and what species is being reduced in a dry cell?
     
18. What species is being oxidized and what species is being reduced in an alkaline battery?
     
19. What species is being oxidized and what species is being reduced in a silver oxide button battery?
     
20. What species is being oxidized and what species is being reduced in a lead storage battery?
     
21. Based on the data in Table 14.4.1, what is the highest voltage battery you can construct?
     
22. Based on the data in Table 14.4.1, what is the lowest voltage battery you can construct? (This may be more challenging to answer than Exercise 21.)

Answers

1.   

3.   −2.87 V

5.   1.56 V

7.   3Li⁺ + Al → 3Li + Al³⁺; −1.39 V; not spontaneous

9.   Cu²⁺ + 2Ag + 2Cl⁻ → Cu + 2AgCl; 0.12 V; spontaneous

11.  cathode reaction: Li⁺ + e⁻ → Li; anode reaction: Al → Al³⁺ + 3e⁻

13.  cathode reaction: Cu²⁺ + 2e⁻ → Cu; anode reaction: Ag + Cl⁻ → AgCl + e⁻

15.  0.08 V

17.  oxidized: Zn; reduced: Mn

19.  oxidized: Zn; reduced: Ag

21.  5.92 V from the reaction of F₂ and Li

Exercises (Electrolysis)

1. Define electrolytic cell.
    
2. How does the operation of an electrolytic cell differ from a voltaic cell?
    
3. List at least three elements that are produced by electrolysis.
    
4. Write the half reactions for the electrolysis of the elements listed in Exercise 3.
    
5. Based on Table 14.4.1, what voltage must be applied to an electrolytic cell to electroplate copper from Cu²⁺?
    
6. Based on Table 14.4.1, what voltage must be applied to an electrolytic cell to electroplate aluminum from Al³⁺?

Answers

1.  an electrochemical cell in which charge is forced through and a nonspontaneous reaction occurs

3   any three of the following: Al, K, Li, Na, Cl₂, or Mg

5.  0.34 V

Additional Exercises

1. Oxidation was once defined as chemically adding oxygen to a substance. Use this reaction to argue that this definition is consistent with the modern definition of oxidation: 2Mg + O₂ → 2MgO
    
2. Reduction was once defined as chemically adding hydrogen to a substance. Use this reaction to argue that this definition is consistent with the modern definition of reduction: C₂H₂ + 2H₂ → C₂H₆
    
3. Assign oxidation numbers to the atoms in each substance.
   1. Kr (krypton)
   2. krypton tetrafluoride (KrF₄)
   3. dioxygen difluoride (O₂F₂)
       
4. Assign oxidation numbers to the atoms in each substance.
   1. lithium hydride (LiH)
   2. potassium peroxide (K₂O₂)
   3. potassium fluoride (KF)
       
5. N atoms can have a wide range of oxidation numbers. Assign oxidation numbers for the N atom in each compound, all of which are known compounds.
   1. N₂O₅
   2. N₂O₄
   3. NO₂
   4. NO
   5. N₂H₄
   6. NH₃
       
6. Cr atoms can have a wide range of oxidation numbers. Assign oxidation numbers for the Cr atom in each compound, all of which are known compounds.
   1. Na₂CrO₄
   2. Na₂Cr₂O₇
   3. CrF₅
   4. CrCl₃
   5. CrCl₂
       
7. Balance this redox reaction by inspection: S₈ + O₂ → SO₂
    
8. Balance this redox reaction by inspection: C₁₈H₃₈ + O₂ → CO₂ + H₂O
    
9. Balance this redox reaction by the half reaction method by assuming an acidic solution: Cr₂O₇²⁻ + Fe → Cr³⁺ + Fe³⁺
    
10. Balance the redox reaction in Exercise 9 by the half reaction method by assuming a basic solution.
     
11. The uranyl ion (UO₂²⁺) is a fairly stable ion of uranium that requires strong reducers to reduce the oxidation number of uranium further. Balance this redox reaction using the half reaction method by assuming an acidic solution.UO₂²⁺ + HN₃ → U + N₂
     
12. Balance the redox reaction in Exercise 11 by the half reaction method by assuming a basic solution.
     
13. Zinc metal can be dissolved by acid, which contains H⁺ ions. Demonstrate that this is consistent with the fact that this reaction has a spontaneous voltage: Zn + 2H⁺ → Zn²⁺ + H₂
     
14. Copper metal cannot be dissolved by acid, which contains H⁺ ions. Demonstrate that this is consistent with the fact that this reaction has a nonspontaneous voltage: Cu + 2H⁺ → Cu²⁺ + H₂
     
15. A disproportionation reaction occurs when a single reactant is both oxidized and reduced. Balance and determine the voltage of this disproportionation reaction. Use the data in Table 14.4.1 - Standard Reduction Potentials of Half Reactions: Cr²⁺ → Cr + Cr³⁺
     
16. A disproportionation reaction occurs when a single reactant is both oxidized and reduced. Balance and determine the voltage of this disproportionation reaction. Use the data in Table 14.4.1 - Standard Reduction Potentials of Half Reactions: Fe²⁺ → Fe + Fe³⁺
     
17. What would be overall reaction for a fuel cell that uses CH₄ as the fuel?
     
18. What would be overall reaction for a fuel cell that uses gasoline (general formula C₈H₁₈) as the fuel?
     
19. When NaCl undergoes electrolysis, sodium appears at the cathode. Is the definition of cathode the same for an electrolytic cell as it is for a voltaic cell?
     
20. When NaCl undergoes electrolysis, chlorine appears at the anode. Is the definition of anode the same for an electrolytic cell as it is for a voltaic cell?
     
21. An award is being plated with pure gold before it is presented to a recipient. If the area of the award is 55.0 cm² and will be plated with 3.00 µm of Au, what mass of Au will be plated on the award? The density of Au is 19.3 g/cm³.
    ​​​
22. The unit of electrical charge is called the coulomb (C). It takes 96,500 coulombs of charge to reduce 27.0 g of Al from Al³⁺ to Al metal. At 1,040 cm³, how many coulombs of charge were needed to reduce the aluminum in the cap of the Washington monument, assuming the cap is pure Al? The density of Al is 2.70 g/cm³.

Answers

1. As oxygen is added to magnesium, it is being oxidized. In modern terms, the Mg atoms are losing electrons and being oxidized, while the electrons are going to the O atoms.

3. 

1. Kr: 0
2. Kr: +4; F: −1
3. O: +1; F: −1

5. 

1. +5
2. +4
3. +4
4. +2
5. −2
6. −3

7.    S₈ + 8O₂ → 8SO₂

9.   14H⁺ + Cr₂O₇²⁻ + 2Fe → 2Cr³⁺ + 7H₂O + 2Fe³⁺

11.  6HN₃ + UO₂²⁺ → U + 2H₂O + 9N₂ + 2H⁺

13.  The voltage of the reaction is +0.76 V, which implies a spontaneous reaction.

15.  3Cr²⁺ → Cr + 2Cr³⁺; −0.50 V

17.  CH₄ + 2O₂ → CO₂ + 2H₂O

19.  yes because reduction occurs at the cathode

21.  0.318 g

 

1.