Chapter 9 — Electrochemistry Exercise Solutions
Electrochemistry requires correct sign convention. Always identify the anode (oxidation) and cathode (reduction) before calculating cell potential.
Key Concepts Quick Recap
| Term | Definition |
|---|---|
| Galvanic cell | Converts chemical energy to electrical energy (spontaneous redox reaction) |
| Anode | Electrode where oxidation occurs; negative terminal in galvanic cell |
| Cathode | Electrode where reduction occurs; positive terminal in galvanic cell |
| Salt bridge | Maintains electrical neutrality by allowing ion flow between half-cells |
| Standard electrode potential (E°) | Potential measured under standard conditions (1 M, 1 atm, 25 °C) |
| EMF of cell | Ecell = Ecathode − Eanode |
| Gibbs energy relation | ΔG = −nFEcell; ΔG° = −nFE°cell |
| Nernst equation | E = E° − (RT/nF) ln Q = E° − (0.0591/n) log Q at 25 °C |
| Faraday's first law | Mass deposited ∝ charge passed: m = ZIt = ZQ |
| Faraday's second law | Same charge deposits equivalent weights of different substances |
| Molar conductivity (Λm) | Conductance of one mole of electrolyte between electrodes 1 m apart |
| Kohlrausch's law | Λ°m = λ⁺ + λ⁻ (sum of limiting molar conductivities of ions) |
Formula Recap
| Concept | Formula |
|---|---|
| Cell EMF | Ecell = E°cathode − E°anode |
| Nernst equation | E = E° − (0.0591/n) log Q (at 25 °C) |
| Gibbs energy | ΔG = −nFEcell |
| Equilibrium constant | log Keq = nE°cell / 0.0591 |
| Faraday's 1st law | m = (M/nF) × Q = (M/nF) × It |
| Molar conductivity | Λm = (κ × 1000) / C (C in mol/L) |
| Electrochemical equivalent | Z = M / (n × F) |
I. Choose the Best Answer (MCQs)
1. In a galvanic cell, oxidation takes place at: (a) Cathode (b) Both electrodes (c) Anode (d) Neither electrode
2. The standard EMF of Daniell cell is (+0.34 V for Cu²⁺/Cu and −0.76 V for Zn²⁺/Zn): (a) 0.42 V (b) 1.10 V (c) −0.42 V (d) 1.76 V
3. The Nernst equation for a cell reaction with n = 2 at 25 °C is: (a) E = E° − (0.0591) log Q (b) E = E° − (0.0296) log Q (c) E = E° + (0.0591) log Q (d) E = E° − (0.0591/n²) log Q
4. According to Kohlrausch's law, the molar conductivity of acetic acid at infinite dilution is: (a) λ(CH₃COO⁻) − λ(H⁺) (b) λ(H⁺) + λ(CH₃COO⁻) (c) λ(Na⁺) + λ(Cl⁻) (d) λ(CH₃COOH)
5. Which law states that the mass of substance deposited during electrolysis is proportional to the charge passed? (a) Kohlrausch's law (b) Ohm's law (c) Faraday's first law (d) Nernst's law
6. The corrosion of iron (rusting) is an example of: (a) A galvanic cell (b) An electrolytic cell (c) A concentration cell (d) A fuel cell
7. If E°cell > 0 for a reaction, then: (a) ΔG > 0 (non-spontaneous) (b) ΔG < 0 (spontaneous) (c) Keq < 1 (d) The reaction is at equilibrium
II. Short Answer Questions
Worked Examples
Example 1 — Standard cell EMF
(E°Zn²⁺/Zn = −0.76 V, E°Cu²⁺/Cu = +0.34 V)
| **Find E°cell for: Zn(s) | Zn²⁺(aq) | Cu²⁺(aq) | Cu(s)** |
|---|
Example 2 — Nernst equation
Find Ecell for Daniell cell when [Zn²⁺] = 0.1 M and [Cu²⁺] = 0.01 M at 298 K.
Example 3 — Faraday's law (mass deposited)
Find the mass of copper deposited when 2.5 A flows for 30 minutes through CuSO₄ solution. (Cu = 63.5 g/mol, n = 2)
Example 4 — Equilibrium constant from E°cell
Find Keq for the Daniell cell reaction at 25 °C. (E°cell = 1.10 V, n = 2)
Example 5 — Molar conductivity and degree of ionisation
The molar conductivity of 0.01 M acetic acid is 16.6 S cm² mol⁻¹ and Λ°m = 390.5 S cm² mol⁻¹. Find degree of ionisation.
Common Mistakes
- Reversing cathode and anode — remember: anode = oxidation, cathode = reduction.
- Forgetting to subtract E°anode from E°cathode (not add).
- Not converting time to seconds in Faraday's law calculations.
- Using the wrong value of n (number of electrons transferred) in Nernst and Faraday calculations.
- Wrong sign for Gibbs energy: ΔG = −nFE (not +nFE).
Exam Checklist
- Anode and cathode are correctly identified.
- Cell EMF formula: Ecell = Ecathode − Eanode.
- n (number of electrons) is correct for the specific reaction.
- Time is converted to seconds for Faraday's law.
- Nernst equation Q uses concentrations in correct positions ([products]/[reactants]).
- Final answer has correct sign and unit.