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Electrochemistry

Chemistry Unit 9
25 min read
IAT Advanced
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1. Core Concept

Electrochemistry involves the interconversion of chemical energy and electrical energy. It is governed by electron transfer (Redox) and the migration of ions in solutions.

2. Electrochemical Cells

Galvanic (Voltaic) Cell:

Converts chemical energy (spontaneous) to electrical.

  • Anode (-): Oxidation.
  • Cathode (+): Reduction.

Cell Notation: Anode | Anode Ion || Cathode Ion | Cathode

E°cell = E°cathode - E°anode
Always use Standard Reduction Potentials.

Electrolytic Cell:

Converts electrical energy (non-spontaneous) to chemical using an external power source.

Galvanic vs Electrolytic Cells

Feature Galvanic Cell Electrolytic Cell
Energy Change Chemical → Electrical Electrical → Chemical
Spontaneity Spontaneous (ΔG < 0) Non-spontaneous (ΔG > 0)
Anode Polarity Negative (-) Positive (+)
Cathode Polarity Positive (+) Negative (-)

3. Nernst Equation

Ecell = E°cell - (0.0591 / n) log Q
Nernst Equation (at 25°C). n = moles of e- transferred; Q = [Products]/[Reactants].

At equilibrium, Ecell = 0, so E°cell = (0.0591 / n) log Keq.

4. Conductance of Electrolytic Solutions

κ = (1/R) × (l/A)
Conductivity (κ). (l/A is the Cell Constant, G*).
Λm = (κ × 1000) / M
Molar Conductivity. (Units: S·cm2/mol).

Kohlrausch’s Law: At infinite dilution, Λm° = ν+λ+° + ν-λ-° (Independent migration of ions).

5. Electrolysis (Faraday’s Laws)

m = ZIt = (M / nF)It
Faraday's 1st Law. (F ≈ 96500 C/mol e-).

2nd Law: When the same current passes through different cells, masses deposited are proportional to their equivalent weights: m1/m2 = E1/E2.

6. Batteries (Commercial Cells)

Primary Batteries (Non-rechargeable)

Cannot be recharged as the cell reaction becomes dead after a certain period of use.

Cell Type Anode Cathode Electrolyte Key Feature
Dry Cell (Leclanche) Zinc container Graphite rod surrounded by MnO2 + Carbon Paste of NH4Cl + ZnCl2 Voltage drops during use (~1.5V)
Mercury Cell Zn-Hg Amalgam Paste of HgO and Carbon Paste of KOH and ZnO Constant voltage (~1.35V) throughout life

Secondary Batteries (Rechargeable)

Can be recharged by passing current in the opposite direction.

Cell Type Anode (Discharge) Cathode (Discharge) Electrolyte Key Use
Lead Storage Lead (Pb) Lead grid packed with PbO2 38% H2SO4 solution Automobiles and inverters
Nickel-Cadmium (Ni-Cd) Cadmium (Cd) Metal grid containing NiO(OH) Aqueous KOH Longer life but expensive
Important Reaction: During recharging of a Lead Storage Battery, the reaction is reversed: PbSO4(s) on anode and cathode is converted back to Pb and PbO2 respectively.

7. Fuel Cells

Galvanic cells that convert the energy of combustion of fuels (like H2, CH4, CH3OH) directly into electrical energy.

Hydrogen-Oxygen Fuel Cell:

  • Reaction: 2H2(g) + O2(g) → 2H2O(l)
  • Advantages: High efficiency (approx. 70%) and pollution-free (product is water). Used in the Apollo space program.

8. Corrosion

Destruction of metal by chemical/electrochemical reaction with its environment. Rusting of iron is an electrochemical phenomenon.

  • Anode Spot: Oxidation of Fe: 2Fe(s) → 2Fe2+ + 4e-
  • Cathode Spot: Reduction of O2: O2(g) + 4H+(aq) + 4e- → 2H2O(l)
  • Overall: Fe2+ is further oxidized by atmospheric oxygen to form rust (Fe2O3·xH2O).

Prevention:

  • Barrier Protection: Paints, oils, or coating with non-corroding metals.
  • Sacrificial Protection: Coating with a more active metal (e.g., Galvanization uses Zinc).
  • Cathodic Protection: Connecting iron to a more reactive metal (like Mg or Zn) which acts as the anode.

9. Conceptual Insights

Concentration Cell:

A cell where E°cell = 0. The EMF is generated solely by the concentration difference of the same electrolyte in two half-cells.

Conductivity Trends:

Conductivity (κ) decreases with dilution because fewer ions occupy the same unit volume. However, Molar Conductivity (Λm) increases with dilution as ions interact less.

10. Common Mistakes

  • Nernst Q: Forgetting the stoichiometry in Q. For 2Fe3+ + Sn2+ → 2Fe2+ + Sn4+, Q = [Fe2+]2[Sn4+] / [Fe3+]2[Sn2+].
  • Equivalent Weight: For Al3+, n=3 → E=M/3. For Mg2+, n=2 → E=M/2.

11. IAT Exam Focus Points

Calculations:

  • Nernst Numericals: Be ready to calculate Ecell given molarities (e.g., 0.1M, 0.01M).
  • ΔG° & Keq: Standard relation ΔG° = -nFE° and log Keq = (nE°) / 0.0591.
  • Faraday's Law: Calculate metal deposited using nmetal = (I × t) / (n × 96500).
  • Kohlrausch Law: Finding Λm° for weak electrolytes using the sum/subtraction of strong ones.

12. Practice Mock Test

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End of Chapter

Electrochemistry

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