Hess's law and enthalpy cyclesEdexcel International A Level Chemistry: Flashcards

Exam questions on Hess's law and enthalpy cycles

1. A gas company wants the standard enthalpy change of formation of methane, CH₄(g). Carbon and hydrogen do not react to give pure methane in the laboratory, so the value must be found indirectly. The standard enthalpy changes of combustion are: C(graphite) –394 kJ mol⁻¹, H₂(g) –286 kJ mol⁻¹ and CH₄(g) –890 kJ mol⁻¹, with CO₂(g) and H₂O(l) as the products.
   Explain why Hess's law is used to find the enthalpy change of formation of methane rather than measuring it directly.2 marks
2. A cement works heats limestone in a kiln: CaCO₃(s) → CaO(s) + CO₂(g). The standard enthalpy changes of formation are: CaCO₃(s) –1207 kJ mol⁻¹, CaO(s) –635 kJ mol⁻¹ and CO₂(g) –394 kJ mol⁻¹. (Ar: Ca = 40.0, C = 12.0, O = 16.0.)
   Calculate the energy transferred when 250 g of calcium carbonate is decomposed in the kiln, and state whether it is taken in or given out.2 marks
3. A student determines the standard enthalpy change of formation of magnesium oxide, which cannot be measured directly. She measures the enthalpy change for two reactions with excess hydrochloric acid: (1) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g), ΔH₁ = –467 kJ mol⁻¹; and (2) MgO(s) + 2HCl(aq) → MgCl₂(aq) + H₂O(l), ΔH₂ = –146 kJ mol⁻¹. The standard enthalpy change of formation of H₂O(l) is –286 kJ mol⁻¹.
   Describe how the student finds the enthalpy change for reaction 2 in a laboratory experiment.3 marks