HKDSE Chemistry - Compulsory Concepts and Elective Applications
HKDSE Chemistry: what the subject tests
HKDSE Chemistry (Category A Senior Secondary Elective Subject) assesses the compulsory curriculum - Planet Earth, Microscopic World I and II, Metals, Acids and Bases, Redox, Chemical Cells and Electrolysis, Chemical Reactions and Energy, Rate of Reaction, Chemical Equilibrium, Carbon Compounds and Patterns in the Chemical World - plus one elective: Industrial Chemistry, Materials Chemistry or Analytical Chemistry. Paper 1 tests compulsory content; Paper 2 tests the elective. Marks depend on correct equations, units, and particle-level or energy-level explanations, not just final numbers.
Core rules and concepts
- Mole and stoichiometry: use n = m/M and n = cV; convert cm³ to dm³ by dividing by 1000. Balance equations before mole ratios.
- Structure and bonding: ionic lattices conduct when molten or aqueous; simple molecular substances have weak intermolecular forces; giant covalent and metallic structures explain high melting points and conductivity.
- Energetics: bond breaking absorbs energy, bond forming releases energy; ΔH is negative for exothermic reactions. Calorimetry uses q = mcΔT.
- Kinetics and equilibrium: rate depends on concentration, temperature, surface area and catalyst; equilibrium is dynamic in a closed system. Changing temperature changes Kc; changing concentration or pressure does not.
- Acids, bases and redox: Brønsted–Lowry acids are proton donors; pH = −log[H⁺]. Oxidation is loss of electrons and increase in oxidation number; reduction is gain.
- Organic chemistry: identify functional groups, homologous series, isomerism and reaction types such as substitution, addition, oxidation and esterification.
Worked example
20.0 cm³ of 0.100 M HCl is exactly neutralised by 25.0 cm³ of NaOH. Find [NaOH].
HCl + NaOH → NaCl + H₂O, so the mole ratio is 1:1. n(HCl) = cV = 0.100 × 0.0200 = 2.00 × 10⁻³ mol. Therefore n(NaOH) = 2.00 × 10⁻³ mol. [NaOH] = n/V = 2.00 × 10⁻³ ÷ 0.0250 = 0.0800 M. The answer is 0.0800 M, not 0.100 M, because the NaOH volume is larger.
Traps that cost marks
- Forgetting to convert cm³ to dm³ before using cV.
- Treating the excess reagent as the limiting reagent.
- Omitting state symbols in ionic equations or forgetting to cancel spectator ions.
- Giving the wrong sign for ΔH or confusing bond breaking with bond forming.
- Reversing anode and cathode in cells or electrolysis; oxidation occurs at the anode.
- Assuming a catalyst changes the equilibrium position; it only speeds up attainment.
- Naming organic compounds without checking the longest carbon chain and functional-group priority.