Physical Chemistry Measurements: Kinetics, Thermodynamics, and Electrochemistry
Physical Chemistry Measurements: Kinetics, Thermodynamics, and Electrochemistry
This domain asks you to design a measurement that isolates the variable you claim to be measuring, and to recognise instrumental artefacts.
Core rules
- Kinetics: use the isolation method and pseudo-first-order conditions; measure initial rates or an integrated property (absorbance, conductance, pressure). Thermostat to ±0.1 K and record temperature, because k is exponentially sensitive. Arrhenius analysis plots ln k against 1/T; Eyring analysis gives ΔH‡ and ΔS‡.
- Thermodynamics: distinguish bomb calorimetry (constant volume, ΔU) from solution calorimetry (constant pressure, ΔH). Calibrate the calorimeter electrically and correct for heat losses. Use van’t Hoff plots for equilibrium enthalpies.
- Electrochemistry: use a three-electrode cell with a supporting electrolyte for voltammetry; a two-electrode cell is for potentiometry or conductance. Reference electrodes (Ag/AgCl, SCE) must be calibrated against a known couple. For a reversible one-electron wave at 25 °C, ΔEp ≈ 59 mV and ip ∝ v^1/2.
- Correct for liquid-junction potentials, IR drop, and activity coefficients; report concentrations with the medium specified.
Worked example
An absorbance falls from 0.800 to 0.200 in 30.0 min under pseudo-first-order conditions. A = A0e^(−kt), so ln(0.800/0.200) = kt; ln 4 = 1.386 = k × 30.0 min, giving k = 0.0462 min⁻¹ and t1/2 = ln2/k = 15.0 min. The half-life is independent of starting concentration, which supports first-order behaviour; a plot of ln A versus t should be linear with slope −k.
Traps
Running kinetics without temperature control; using a two-electrode cell for CV; ignoring IR drop at high current; comparing potentials measured against different references; treating concentration as activity in non-dilute media.