Physical Chemistry - Thermodynamics, Kinetics and Quantum Foundations
What the exam tests
Physical Chemistry covers chemical thermodynamics, phase and solution behaviour, chemical kinetics, electrochemistry, quantum chemistry and spectroscopy from a quantitative angle.
Core rules
- First law: ΔU = q + w, with w = −PΔV for expansion work; ΔH = qp at constant pressure.
- Spontaneity: ΔG = ΔH − TΔS; equilibrium at ΔG = 0, and ΔG° = −RT ln K.
- Phase rule: F = C − P + 2; Clausius–Clapeyron gives vapour pressure versus temperature.
- Colligative properties depend on particle count: ΔTb = iKb m, ΔTf = iKf m.
- Kinetics: zero order t½ = [A]₀/2k; first order t½ = 0.693/k; second order t½ = 1/(k[A]₀). Arrhenius: ln(k₂/k₁) = −Ea/R (1/T₂ − 1/T₁).
- Quantum: four quantum numbers n, l, ml, ms; orbital energies and selection rules.
Worked example
k = 1.0 × 10⁻³ s⁻¹ at 300 K and 4.0 × 10⁻³ s⁻¹ at 320 K. Find Ea. ln(4.0) = −Ea/8.314 (1/320 − 1/300). 1.386 = −Ea/8.314 × (−2.083 × 10⁻⁴). Ea = 1.386 × 8.314 / 2.083 × 10⁻⁴ = 5.53 × 10⁴ J/mol ≈ 55.3 kJ/mol. A fourfold rate rise over only 20 K signals a fairly large barrier; check the sign and units before accepting the answer.
Traps
Using °C in Arrhenius or gas equations; sign errors in w and ΔH (exothermic is negative); forgetting the van't Hoff factor i for electrolytes; treating reaction order as a stoichiometric coefficient; reporting Ea in J when kJ is asked.