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

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