C (Core Theory)

Examiner notes & key calculations

• Molar Enthalpy Signs: In the ethanol combustion question, several students calculated the correct magnitude but omitted the negative sign for the exothermic reaction, losing a vital mark. Always write ΔH=−720 kJ/mol \Delta H = -720\text{ kJ/mol} ΔH=−720 kJ/mol explicitly.
• State Symbols: Simple errors, such as forgetting to include gaseous state symbols for diatomic molecules (like O2(g) \text{O}_2(g) O2​(g)), cost easy marks in chemical equations.
• Bonding Definitions: When defining covalent bonding, describing the attraction between shared electrons and 'the nucleus' (singular) was rejected. Covalent bonding involves attraction to both nuclei (plural).
• Catalyst Misunderstandings: In the reversible reaction question, a common misconception was that adding a catalyst increases the yield of ethanol. Remember, catalysts increase the rate of both forward and reverse reactions equally, leaving the equilibrium position and yield unchanged.