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

1. 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.

   (a)

   Which statement is Hess's law?

   [1 mark]

   • AThe enthalpy change of a reaction depends on the route taken between reactants and products
   • BThe products of a reaction always have a lower enthalpy than the reactants
   • CThe enthalpy change of a reaction is the same whichever route is taken, provided the initial and final conditions are the same
   • DThe enthalpy change of a reaction is the sum of the enthalpies of the bonds in the reactants

   (b)

   What is the enthalpy change for the reaction C(graphite) → C(diamond)?

   [1 mark]

   • A−1.9 kJ mol⁻¹
   • B−788.9 kJ mol⁻¹
   • C+788.9 kJ mol⁻¹
   • D+1.9 kJ mol⁻¹

   (c)

   Explain why the standard enthalpy of combustion of graphite is equal to the standard enthalpy of formation of carbon dioxide.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   What is the standard enthalpy change for the decomposition of calcium carbonate?

   [1 mark]

   • A+179.2 kJ mol⁻¹
   • B+572.7 kJ mol⁻¹
   • C−2236.0 kJ mol⁻¹
   • D−179.2 kJ mol⁻¹

   (b)

   How much heat is absorbed when 5.00 mol of calcium carbonate decomposes?

   [1 mark]

   • A35.8 kJ
   • B896 kJ
   • C8960 kJ
   • D179 kJ

   (c)

   Deduce the standard enthalpy change for the reaction CaO(s) + CO₂(g) → CaCO₃(s). Explain your answer.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Use the data to calculate the standard enthalpy of formation of methane.

   [3 marks]

   (b)

   Explain why the enthalpy of formation of methane cannot be measured directly. Calculate the energy change when 8.00 g of methane is formed from its elements.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A gas supplier sells propane, C₃H₈, and butane, C₄H₁₀, as liquefied fuel gases. The standard enthalpies of formation, in kJ mol⁻¹, are: C₃H₈(g) −104.5, C₄H₁₀(g) −126.5, CO₂(g) −393.5 and H₂O(l) −285.8. The relative formula masses are 44.0 for propane and 58.0 for butane.

   (a)

   Explain how Hess's law is used to calculate the standard enthalpy of combustion of propane from enthalpies of formation, and carry out the calculation.

   [6 marks]

   (b)

   Evaluate which of the two fuels releases more energy per mole and per gram, and decide which is the better choice for a camping stove carried on a long walk.

   [6 marks]

   Total for question 4: 12 marks

End of questions