Electrochemistry Class 12 Notes

• $\text{Anode}:$
• where oxidation occurs (loses electrons); in a galvanic cell it is negative, in an electrolytic cell positive.
• $\text{Cathode}:$
• where reduction occurs (gains electrons); in a galvanic cell positive, in an electrolytic cell negative.
• The salt bridge completes the circuit and balances charge, maintaining electrical neutrality.

• $\text{At equilibrium}:$
• $Q=K$
• and
• $E_{\text{cell}}=0$
• , giving
• $\log K=\frac{n{E}_{\text{cell}}^{\circ }}{0.059}$
• .
• $\text{EMF and Gibbs energy}:$
• $\Delta G^{\circ }=-nF{E}_{\text{cell}}^{\circ }$
• .

Conductance measures how easily charge flows through a solution. It depends on the nature and concentration of the electrolyte. Key terms: conductivity, molar conductivity and its concentration dependence.

• $\text{Conductivity}:$
• $\kappa =\frac{1}{\rho }$
• (S m⁻¹), follows Kohlrausch's law.
• $\text{Molar conductivity}:$
• ${\Lambda }_m=\frac{\kappa \times 1000}{c}$
• (S cm² mol⁻¹).
• Strong electrolytes: Λₘ decreases slowly with √c (linear, Debye-Hückel).
• Weak electrolytes: Λₘ rises sharply at low concentration and can't be extrapolated to Λ⁰ₘ directly — use Kohlrausch's law.

• $\text{Primary cells}:$
• cannot be recharged — e.g. dry cell (Leclanché), mercury cell.
• $\text{Secondary cells}:$
• can be recharged — e.g. lead-acid battery, nickel-cadmium, lithium-ion.
• Lead-acid battery discharge:
• $\text{Pb}+{\text{PbO}}_2+2{\text{H}}_2{\text{SO}}_4\to 2{\text{PbSO}}_4+2{\text{H}}_2\text{O}$
• .

Example: For the cell ${\text{Zn|Zn}}^{2+}(0.1M)||{\text{Cu}}^{2+}(0.5M)|\text{Cu}$ with $E_{\text{cell}}^{\circ }=1.10V$ and $n=2$, find the cell EMF.

Solution: $E_{\text{cell}}=1.10-\frac{0.059}{2}\log \frac{[{\text{Zn}}^{2+}]}{[{\text{Cu}}^{2+}]}=1.10-0.0295\log 0.2\approx 1.12V$.

Exercises (NCERT Q1–Q4)

• 1) Define electrochemical cell and differentiate between galvanic and electrolytic cells.
• 2) Write the Nernst equation and calculate the emf of the following cell at 298 K: Fe(s) | Fe²⁺ (0.001 M) || H⁺ (0.01 M) | H₂ (1 bar) | Pt(s). Given: E°Fe²⁺/Fe = -0.44 V.
• 3) State Kohlrausch's law of independent migration of ions. How is it used to determine the limiting molar conductivity of a weak electrolyte?
• 4) Calculate the quantity of electricity (in coulombs) required to reduce 1 mol of Cr₂O₇²⁻ to Cr³⁺. How much electricity is needed to reduce 0.05 mol of Cr₂O₇²⁻?

Answers at a glance

1. 1
   Q1 → An electrochemical cell converts chemical energy to electrical energy or vice versa. Galvanic cells produce electricity spontaneously; electrolytic cells require external electricity to drive non-spontaneous reactions.
2. 2
   Q2 → EMF of the cell = 0.41 V
3. 3
   Q3 → Kohlrausch's law: Λ°m = Σνiλ°i. Used for weak electrolytes by summing ionic contributions from strong electrolytes.
4. 4
   Q4 → For 1 mol Cr₂O₇²⁻: 5.79 × 10⁵ C; for 0.05 mol: 2.89 × 10⁴ C

MCQ Questions

1 Mark Questions

1. 1
   Define electrochemical cell.
2. 2
   What is the Nernst equation for a cell reaction?
3. 3
   State Faraday’s first law of electrolysis.

2 Mark Questions

1. 1
   Calculate the standard cell potential of the Daniel cell given E°(Cu2+/Cu) = +0.34 V and E°(Zn2+/Zn) = –0.76 V.
2. 2
   How is the molar conductivity of an electrolyte related to its conductivity?

3 Mark Questions

1. 1
   Derive the relationship between ΔG° and E°cell for an electrochemical cell.
2. 2
   Explain how the limiting molar conductivity of a weak electrolyte can be determined using Kohlrausch’s law of independent migration of ions.

5 Mark Questions

1. 1
   The conductivity of 0.001028 mol L−1 acetic acid is 4.95 × 10−5 S cm−1. Calculate its dissociation constant if Λ°(acetic acid) = 390.5 S cm2 mol−1.
2. 2
   For the reaction 2Ag+(aq) + Cu(s) → 2Ag(s) + Cu2+(aq), the standard cell potential is found to be 0.46 V at 298 K. Calculate the equilibrium constant of the reaction. (R = 8.314 J K−1 mol−1, F = 96487 C mol−1)

Assignments walk the chapter's topics in order; the tests follow the current CBSE blueprint (MCQ/VSA, 2M, 3M and 5M questions) with answer keys. They will appear here as direct download links.