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

1. 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.

   (a)

   Which statement is Hess's law?

   [1 mark]

   • AThe enthalpy change of a reaction is the same whichever route is taken, provided the initial and final conditions are the same.
   • BThe enthalpy change of a reaction depends on the number of steps taken between reactants and products.
   • CThe enthalpy change of a reaction is always equal to the energy of the bonds broken.
   • DThe enthalpy change of a reaction is the sum of the activation energies of each step.

   (b)

   What is the standard enthalpy change of formation of methane, in kJ mol⁻¹, calculated from these data?

   [1 mark]

   • A+76
   • B–1856
   • C–76
   • D–966

   (c)

   Explain why Hess's law is used to find the enthalpy change of formation of methane rather than measuring it directly.

   [2 marks]

   Total for question 1: 4 marks

2. 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.)

   (a)

   Which expression gives the standard enthalpy change of a reaction from standard enthalpy changes of formation?

   [1 mark]

   • AΣ ΔfH(reactants) – Σ ΔfH(products)
   • BΣ ΔfH(products) – Σ ΔfH(reactants)
   • CΣ ΔfH(products) + Σ ΔfH(reactants)
   • DΣ ΔfH(products) only

   (b)

   What is the standard enthalpy change of this reaction, in kJ mol⁻¹?

   [1 mark]

   • A–178
   • B–1029
   • C+2236
   • D+178

   (c)

   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]

   Total for question 2: 4 marks

3. 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.

   (a)

   Describe how the student finds the enthalpy change for reaction 2 in a laboratory experiment.

   [3 marks]

   (b)

   Use the data to calculate the standard enthalpy change of formation of magnesium oxide, showing the relationship between the enthalpy changes.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist at a plastics company needs the enthalpy change of hydrogenation of ethene, C₂H₄(g) + H₂(g) → C₂H₆(g), but cannot measure it directly. The standard enthalpy changes of combustion, forming CO₂(g) and H₂O(l), are: ethene –1411 kJ mol⁻¹, hydrogen –286 kJ mol⁻¹ and ethane –1560 kJ mol⁻¹.

   (a)

   Use Hess's law to calculate the enthalpy change of this hydrogenation, explaining your method.

   [6 marks]

   (b)

   Explain why Hess's law allows chemists to calculate this value, and evaluate the reliability of the answer, given that the combustion values come from separate experiments.

   [6 marks]

   Total for question 4: 12 marks

End of questions