Chemistry - Equilibrium, Kinetics and Organic Chemistry
Core concepts
The chemistry paper tests quantitative solution chemistry (concentration, dilution, pH, acid–base titration, buffers), chemical equilibrium (Le Chatelier's principle, Kc, Ksp, weak-acid dissociation), kinetics (rate laws, factors affecting rate, catalysis) and organic chemistry (functional groups, nomenclature, addition/substitution/esterification, polymerisation). Electrochemistry and redox balancing appear regularly.
Traps
- Forgetting that pH = −log[H₃O⁺] applies to dilute aqueous solutions and that strong and weak acids behave differently at the same concentration.
- Dilution errors: moles of solute are conserved, so C₁V₁ = C₂V₂, but pH is not linear in dilution.
- Applying Le Chatelier without checking whether the species is actually present (adding an inert gas at constant volume shifts nothing).
- Confusing rate with equilibrium constant: a catalyst changes rate, not K.
- Organic: mislabelling a functional group (aldehyde vs ketone; ester vs ether) and forgetting Markovnikov's rule in addition.
Worked example
Titration: 20.0 mL of HCl is neutralised by 25.0 mL of 0.10 M NaOH. At equivalence, n(NaOH) = 0.0250 × 0.10 = 2.50 × 10⁻³ mol, so n(HCl) is the same, giving C(HCl) = 2.50 × 10⁻³ / 0.0200 = 0.125 M. Rule of thumb: write the balanced equation first, convert everything to moles, then back to concentration - never work directly in concentrations across a reaction.