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

Exam questions on Hess's law and enthalpy cycles

1. A student wants the enthalpy change for a reaction that cannot be measured directly in a calorimeter, so she plans to use Hess's law with other reactions whose enthalpy changes can be measured.
   Define the term standard enthalpy change of formation.2 marks
2. Ethanol, C₂H₅OH, is made on a large scale for use as a biofuel. Its standard enthalpy change of formation cannot be measured directly, but standard enthalpy changes of combustion are available: carbon (graphite) −394 kJ mol⁻¹, hydrogen −286 kJ mol⁻¹ and ethanol(l) −1367 kJ mol⁻¹.
   Use an enthalpy cycle to calculate the standard enthalpy change of formation of ethanol.2 marks
3. Iron is extracted from iron(III) oxide in a blast furnace. One of the key reactions is Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g). Standard enthalpy changes of formation are: Fe₂O₃(s) −824 kJ mol⁻¹, CO(g) −111 kJ mol⁻¹, CO₂(g) −394 kJ mol⁻¹.
   Calculate the standard enthalpy change of reaction.3 marks