Knowledge and Understanding of Chemistry
Core ideas
Candidates must connect atomic structure to periodic trends, bonding and reactivity. Know that nuclear charge, shielding and electron–electron repulsion explain trends in atomic radius, ionisation energy and electronegativity, including anomalies such as the lower first ionisation energy from Group 2 to 13 and Group 15 to 16 across a period. Distinguish ionic, covalent and metallic lattices, and connect melting point, solubility and conductivity to the strength and type of particle attraction, including hydrogen bonding and dispersion forces. Quantitative work rests on the mole, molar mass, concentration, stoichiometric ratios and limiting reagents. Be able to write balanced full and net ionic equations, assign oxidation numbers, and apply Le Chatelier’s principle, Kw, pH and Ka. Thermochemistry requires correct sign conventions and Hess’s law; organic chemistry requires functional-group recognition and simple isomerism.
Traps
Common errors are treating concentration as amount, failing to convert units, assuming the most abundant reactant is in excess, forgetting coefficients when calculating enthalpy, and confusing “strong” with “concentrated”. In equilibrium questions, a catalyst does not shift position. In redox, balance atoms and charge. In periodic trends, do not ignore shielding or full subshells.
Worked example
10.0 g of calcium carbonate (M = 100.0 g mol−1) reacts with 50.0 mL of 2.00 mol L−1 HCl. Which is limiting? n(CaCO3) = 0.100 mol; n(HCl) = 0.100 mol. The equation CaCO3 + 2HCl → CaCl2 + CO2 + H2O requires 0.200 mol HCl for 0.100 mol carbonate, so HCl is limiting. Rule of thumb: convert every quantity to moles, divide by the balanced-equation coefficient, and compare; the smallest quotient identifies the limiting reagent.