CAIE A Level Chemistry 9701 · Chemical energetics
A student burned ethanol from a spirit burner to heat water in a copper calorimeter and determined the enthalpy change of combustion of ethanol as −980 kJ mol⁻¹. A data book gives the standard enthalpy change of combustion of ethanol as −1367 kJ mol⁻¹.
Suggest why the student's value is less exothermic than the data-book value. Suggest also why enthalpy changes are quoted under standard conditions.
4 marks, so the mark scheme is looking for about 4 separate creditworthy points. Making the same point twice in different words earns it once.
Suggest means: Apply what you know to an unfamiliar situation. There may be more than one acceptable answer. Reason from the principle you have learned to the new context, and say which principle you are using.
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