Hess's law and enthalpy cyclesAQA A-Level Chemistry: Flashcards

Exam questions on Hess's law and enthalpy cycles

1. Carbon exists as two common forms, graphite and diamond. The conversion of graphite into diamond cannot be carried out in a simple laboratory experiment, but both forms burn completely in oxygen to form carbon dioxide. The standard enthalpy of combustion of graphite is −393.5 kJ mol⁻¹ and that of diamond is −395.4 kJ mol⁻¹.
   Explain why the standard enthalpy of combustion of graphite is equal to the standard enthalpy of formation of carbon dioxide.2 marks
2. In a lime kiln, calcium carbonate is decomposed by heating: CaCO₃(s) → CaO(s) + CO₂(g). The standard enthalpies of formation, in kJ mol⁻¹, are: CaCO₃(s) −1207.6, CaO(s) −634.9 and CO₂(g) −393.5.
   Deduce the standard enthalpy change for the reaction CaO(s) + CO₂(g) → CaCO₃(s). Explain your answer.2 marks
3. The enthalpy change for the formation of methane from its elements, C(s) + 2H₂(g) → CH₄(g), cannot be measured directly. The standard enthalpies of combustion, in kJ mol⁻¹, for complete combustion to CO₂(g) and H₂O(l) are: C(graphite) −393.5, H₂(g) −285.8 and CH₄(g) −890.3. The relative atomic masses are C = 12.0 and H = 1.0.
   Use the data to calculate the standard enthalpy of formation of methane.3 marks