Practice Problems Electrochemistry

1. What is the difference between an oxidation-reduction reaction and a half-reaction?

2. What is the function of the salt bridge in an electrochemical cell?

3. What is the criterion for spontaneous chemical change based on cell potentials? Explain.

4. Complete and balance the following half–equations, and indicate whether oxidation or reduction is involved.

(a) ClO₂(g) → ClO₃^–(aq) (acidic solution)

(b) MnO₄^–(aq) → MnO₂(s) (acidic solution)

(c) SbH₃(g) → Sb(s) (basic solution)

5. Balance the following redox equations in acidic solution.

(a) Fe²⁺(aq) + Cr₂O₇^2–(aq) → Fe³⁺(aq) + Cr³⁺(aq)

(b) S₈(s) + O₂(g) → SO₄^2–(aq)

(c) Fe³⁺(aq) + NH₂OH₂⁺(aq) → Fe²⁺(aq) + N₂O(g)

6. Balance the following equations in acidic solution.

(a) Ag(s) + NO₃^–(aq) → Ag⁺(aq) + NO(g)

(b) H₂O₂(aq) + MnO₄^–(aq) → Mn²⁺(aq) + O₂(g)

(c) Cl₂(g) + I^–(aq) → Cl^–(aq) + IO₃^–(aq)

7. Balance the following equations in basic solution.

(a) Fe(OH)₂(s) + O₂(g) → Fe(OH)₃(s)

(b) S₈(s) → S₂O₃^2–(aq) + S^2–(aq)

(c) CrI₃(s) + H₂O₂(aq) → CrO₄^2–(aq) + IO₄^–(aq)

8. Balance the following equations in basic solution.

(a) CrO₄^2–(aq) + AsH₃(g) → Cr(OH)₃(s) + As(s)

(b) CH₃OH(aq) + MnO₄^–(aq) → HCOO^–(aq) + MnO₂(s)

(c) [Fe(CN)₆]^3–(aq) + N₂H₄(aq) → [Fe(CN)₆]^4–(aq) + N₂(g)

9. Write balanced equations for

(a) the reaction of oxalic acid (HOOCCOOH) and permanganate ion in acid solution to produce manganese(II) ion and carbon dioxide gas

(b) the reaction of Cr₂O₇^2– and UO²⁺ to produce UO₂²⁺ and Cr³⁺ in an acidic aqueous environment

(c) the reaction in basic solution of nitrate ion and zinc to produce zinc(II) ion and gaseous ammonia.

10. For the reaction

2 CuI(s) + Cd(s) → Cd²⁺(aq) + 2 I^–(aq) + 2 Cu(s) E°_cell = +0.23 V

given that E°_Cd²⁺/Cd = –0.403 V, determine E° for the half–reaction

2 CuI(s) + 2 e^– → 2 Cu(s) + 2 I^–(aq)

11. E°_cell = 1.47 V for the voltaic cell

V(s) | V²⁺(1 M) || Cu²⁺(1 M) | Cu(s)

Determine the value of E°_V^2+//V.

12. Write equations for the half–reactions and the overall cell reaction, and calculate E°_cell for each of the voltaic cells diagrammed below.

(a) Pt | I₂(s) | I^–(aq) || Cl^–(aq) | Cl₂(g) | Pt

(b) Pt | PbO₂(s) | Pb²⁺(aq), H⁺(aq) || S₂O₈^2–(aq),SO₄^2–(aq) | Pt

13. Predict whether a spontaneous reaction will occur in the forward direction in each of the following. Assume that all reactants and products are in their standard states.

(a) Sn(s) + Co²⁺(aq) → Sn²⁺(aq) + Co(s)

(b) 6 Br^–(aq) + Cr₂O₇^2–(aq) + 14 H⁺(aq) → 2 Cr³⁺(aq) + 7 H₂O(l) + 3 Br₂(l)

14. Rhodium is a rare metal used as a catalyst. The metal does not react with HCl(aq), but it does react with HNO₃(aq), producing Rh³⁺(aq) and NO(g). Copper will displace Rh³⁺ from aqueous solution, but silver will not. Estimate a value of E°_Rh³⁺/Rh.

15. Determine the values of E°_cell and ΔG° for the following reactions.

(a) O₂(g) + 4 I^–(aq) + 4 H⁺(aq) → 2 H₂O(l) + 2 I₂(s)

(b) Cr₂O₇^2–(aq) + 3 Cu(s) + 14 H⁺(aq) → 2 Cr³⁺(aq) + 3 Cu²⁺(aq) + 7 H₂O(l)

16. Determine the values of E°_cell and ΔG° for the following reactions.

(a) Al(s) + 3 Ag⁺(aq) → Al³⁺(aq) + 3 Ag(s)

(b) 4 IO₃^–(aq) + 4 H⁺(aq) → 2 I₂(s) + 2 H₂O(l) + 5 O₂(g)

17. Write the equilibrium constant expression for each of the following reactions, and determine the numerical value of K_eq at 25 °C.

(a) PbO₂(s) + 4 H⁺(aq) + 2 Cl^–(aq) → ← Pb²⁺(aq) + 2 H₂O(l) + Cl₂(g)

(b) 3 O₂(g) + 2 Br^–(aq) → ← 2 BrO₃^–(aq) (basic solution)

18. Write the equilibrium constant expression for each of the following reactions, and determine the numerical value of K_eq at 25 °C.

(a) Ag⁺(aq) + Fe²⁺(aq) → ← Fe³⁺(aq) + Ag(s)

(b) MnO₂(s) + 4 H⁺(aq) + 2 Cl^–(aq) → ← Mn²⁺(aq) + 2 H₂O(l) + Cl₂(g)

(c) 2 OCl^–(aq) → ← 2 Cl^–(aq) + O₂(g) (basic solution)

19. What is the value of E_cell of each of the following reactions when carried out in a voltaic cell?

(a) Fe(s) + 2 Ag⁺(aq, 0.0015 M) → Fe²⁺(aq, 1.33 M) + 2 Ag(s)

(b) 4 VO²⁺(aq, 0.050 M) + O₂(g, 0.25 atm) + 2 H₂O(l) → 4 VO₂⁺(aq, 0.75 M) + 4 H⁺(aq, 0.30 M)

20. What is E_cell for the voltaic cell diagrammed below?

Pt | H₂(g, 1 atm) | 0.0025 M HCl || H⁺ (1 M) | H₂(g, 1 atm) | Pt

21. How many coulombs of electric charge are required to deposit 25.0 g of Cu(s) at the cathode in the electrolysis of CuSO₄(aq)?

22. Balance the following redox equations.

(a) B₂Cl₄(aq) + OH^–(aq) → BO₂^–(aq) + Cl^–(aq) + H₂O(l) + H₂(g)

(b) CH₃CH₂ONO₂(aq) + Sn(s) + H⁺(aq) → CH₃CH₂OH(aq) + NH₂OH(aq) + Sn²⁺(aq) + H₂O(l)

(c) F₅SeOF(aq) + OH^–(aq) → SeO₄^2–(aq) + F^–(aq) + H₂O(l) + O₂(g)

(d) As₂S₃(s) + OH^–(aq) + H₂O₂(aq) → AsO₄^3–(aq) + SO₄^2–(aq) + H₂O(l)

(e) XeF₆(s) + OH^–(aq) → XeO₆^4–(aq) + F^–(aq) + H₂O(l) + Xe(g) + O₂(g)

Answers