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

1. 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.

   (a)

   Which statement is Hess's law?

   [1 mark]

   • AThe enthalpy change of a reaction is always negative if the reaction occurs spontaneously
   • BThe enthalpy change of a reaction is the same whichever route is taken, provided the initial and final conditions and states are the same
   • CThe enthalpy change of a reaction depends on the number of steps in the route taken
   • DThe enthalpy change of a reaction is the sum of the bond enthalpies of the products

   (b)

   She finds that converting X into Y has ΔH = −120 kJ mol⁻¹ and converting Y into Z has ΔH = +45 kJ mol⁻¹. What is ΔH for converting X directly into Z?

   [1 mark]

   • A−165 kJ mol⁻¹
   • B+75 kJ mol⁻¹
   • C−75 kJ mol⁻¹
   • D+165 kJ mol⁻¹

   (c)

   Define the term standard enthalpy change of formation.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which equation represents the standard enthalpy change of formation of ethanol?

   [1 mark]

   • A2C(g) + 6H(g) + O(g) → C₂H₅OH(l)
   • BC₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l)
   • C2C(s) + 3H₂(g) + O₂(g) → C₂H₅OH(l)
   • D2C(s) + 3H₂(g) + ½O₂(g) → C₂H₅OH(l)

   (b)

   The standard enthalpy change of combustion of hydrogen is −286 kJ mol⁻¹. Which substance has a standard enthalpy change of formation of −286 kJ mol⁻¹?

   [1 mark]

   • AH₂O(l)
   • BH₂O(g)
   • CH₂O₂(l)
   • DH(g)

   (c)

   Use an enthalpy cycle to calculate the standard enthalpy change of formation of ethanol.

   [2 marks]

   Total for question 2: 4 marks

3. 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.

   (a)

   Calculate the standard enthalpy change of reaction.

   [3 marks]

   (b)

   A furnace uses 8.0 kg of iron(III) oxide (M = 160 g mol⁻¹). Calculate the energy transferred and the mass of iron formed (A = 55.8), stating whether the energy is released or absorbed.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Calcium carbonate decomposes only at very high temperatures, so the enthalpy change for CaCO₃(s) → CaO(s) + CO₂(g) is found indirectly. A student measures the enthalpy change when each solid reacts with an excess of dilute hydrochloric acid: Reaction 1, CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g), and Reaction 2, CaO(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l). In Reaction 2, 1.40 g of calcium oxide (M = 56.1 g mol⁻¹) was added to 50.0 cm³ of 2.00 mol dm⁻³ hydrochloric acid and the temperature rose by 23.2 °C. Assume the solution has density 1.00 g cm⁻³ and specific heat capacity 4.18 J g⁻¹ K⁻¹.

   (a)

   Calculate the enthalpy change for Reaction 2, in kJ mol⁻¹, including its sign.

   [6 marks]

   (b)

   For Reaction 1 the enthalpy change was found to be −15 kJ mol⁻¹. Construct an enthalpy cycle, use it with the value of −194 kJ mol⁻¹ for Reaction 2 to find the enthalpy change for the decomposition of calcium carbonate, and explain why this indirect method is used.

   [6 marks]

   Total for question 4: 12 marks

End of questions