Edexcel International A-Level Chemistry: Frequent mistakes

Pearson Edexcel Principal Examiner Feedback for International A-level Chemistry identifies recurring errors in concentration, enthalpy, definitions, bonding and calculations. Precision in terminology is essential.

Conceptual understanding

Concentration when mixing

Students fail to account for concentration changes when equal volumes of solutions are mixed. Concentrations decrease by half; many don’t recognise this.

Fix: Equal volumes → total volume doubles → concentration halves (for each solution).

Excess reactant

Misunderstanding why certain reactants must be in excess (e.g. potassium iodide). Excess ensures reaction goes to completion.

Fix: One reactant in excess so the other is limiting. Ensures full conversion.

Molecular shape explanation

Students describe shape correctly but fail to relate it to bonding electron pairs or repulsion minimisation. Explanation incomplete.

Fix: “Tetrahedral because 4 bonding pairs; repel equally; minimise repulsion.” Link shape to electron pairs.

d sub-shells

Lack of precision: saying “full d orbital” instead of specifying all orbitals in the sub-shell are full. Confusing 3d vs. 4d.

Fix: 3d¹⁰ = all five 3d orbitals full. Specify sub-shell. Chromium, copper exceptions.

Equation balancing

Difficulty balancing chemical equations, even when correct products are identified.

Fix: Count atoms each side. Balance one element at a time. Check at end.

Calculations

Enthalpy calculations

Using incorrect values (e.g. mass of ethanol instead of mass of water) in q = mcΔT. Wrong mass loses marks.

Fix: Use mass of solution heated (usually water). Check which substance’s temperature changes.

Temperature

Adding 273 to temperature difference rather than using it correctly. ΔT in K = ΔT in °C (difference same). Don’t add 273 to ΔT.

Fix: ΔT = T₂ − T₁. Same numerically in K or °C. Use K for T in pV = nRT.

Arithmetic and bond counts

Simple arithmetic errors. Wrong bond counts (e.g. C–O single instead of C=O double). Incorrect bond enthalpy application.

Fix: Count bonds carefully. C=O is double. Check arithmetic.

Minus signs and significant figures

Omitting minus signs in final answers (e.g. ΔH). Incorrect significant figures.

Fix: Exothermic: ΔH negative. Check sf in question. Usually 3 sf.

Bond enthalpy direction

Reversing sums: subtracting bond breaking from bond making instead of correct order. ΔH = Σ(bond breaking) − Σ(bond making).

Fix: ΔH = bonds broken − bonds formed. Breaking = endothermic (+). Forming = exothermic (−).

Definitions

Incomplete definitions

Vague or generic definitions lacking required terminology. E.g. mean bond enthalpy: omitting “gaseous” and “one mole”.

Fix: Mean bond enthalpy: enthalpy change when one mole of bonds (gaseous) is broken. Include all key terms.

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Based on Pearson Edexcel International A-level Chemistry Principal Examiner Feedback (WCH12, WCH15).