1. 1
   Margarine is made by adding hydrogen across C=C bonds in vegetable oils. The simplest model of this reaction is the hydrogenation of ethene gas: C₂H₄(g) + H₂(g) → C₂H₆(g). Average bond enthalpies, in kJ mol⁻¹: C=C 614, C–C 346, C–H 414, H–H 436. The experimentally measured enthalpy change for this reaction is −137 kJ mol⁻¹.

   (a)

   Which statement about the energy changes when bonds are broken and formed is correct?

   [1 mark]

   • ABreaking bonds releases energy and forming bonds absorbs energy.
   • BBreaking bonds and forming bonds both absorb energy.
   • CBreaking bonds and forming bonds both release energy.
   • DBreaking bonds absorbs energy and forming bonds releases energy.

   (b)

   What is the enthalpy change for the hydrogenation of ethene, in kJ mol⁻¹, calculated from the average bond enthalpies?

   [1 mark]

   • A−124
   • B−560
   • C+124
   • D+704

   (c)

   Suggest why the value calculated from bond enthalpies differs from the experimental value, and state which value is more reliable for this reaction.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Sulfur trioxide, used to make sulfuric acid, is produced in two stages. Stage 1: S(s) + O₂(g) → SO₂(g), ΔH₁ = −297 kJ. Stage 2: 2SO₂(g) + O₂(g) → 2SO₃(g), ΔH₂ = −198 kJ. Gaseous sulfur trioxide can be condensed: SO₃(g) → SO₃(l), ΔH₃ = −45 kJ.

   (a)

   Which statement is Hess's law?

   [1 mark]

   • AThe enthalpy change for a reaction depends on the number of steps used to carry it out.
   • BThe enthalpy change for a reaction is independent of the pathway between the initial and final states.
   • CThe enthalpy change for a reaction equals the sum of the activation energies of each step.
   • DEnergy is conserved in a reaction only if it takes place in a single step.

   (b)

   What is the enthalpy change for 2S(s) + 3O₂(g) → 2SO₃(g)?

   [1 mark]

   • A−495 kJ
   • B−396 kJ
   • C−792 kJ
   • D−693 kJ

   (c)

   Determine the enthalpy change for S(s) + 1½O₂(g) → SO₃(l).

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student uses bond enthalpies to predict the enthalpy change of H₂ + X₂ → 2HX for three halogens and compares them with experimental values measured with every substance in its standard state at 298 K (chlorine is a gas, bromine a liquid, iodine a solid; HCl, HBr and HI are all gases). Bond enthalpies, in kJ mol⁻¹: H–H 436, Cl–Cl 242, Br–Br 193, I–I 151, H–Cl 431, H–Br 366, H–I 299. Experimental enthalpy changes, in kJ mol⁻¹: HCl −185, HBr −73, HI +53. The student's calculated values are −184 for HCl and −11 for HI.

   (a)

   Calculate the enthalpy change for the formation of 2HBr from bond enthalpies and suggest why it differs from the experimental value.

   [3 marks]

   (b)

   The student concludes that all the differences between the calculated and experimental values arise because bond enthalpies are average values. Evaluate this conclusion.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Hydrazine, N₂H₄, is a rocket fuel. It can react with oxygen or with hydrogen peroxide: Reaction 1: N₂H₄(g) + O₂(g) → N₂(g) + 2H₂O(g). Reaction 2: N₂H₄(g) + 2H₂O₂(g) → N₂(g) + 4H₂O(g). Structures: H₂N–NH₂ and H–O–O–H. Bond enthalpies, in kJ mol⁻¹: N–N 158, N–H 391, N≡N 945, O=O 498, O–O 146, O–H 463. Molar masses, in g mol⁻¹: N₂H₄ 32.06, O₂ 32.00, H₂O₂ 34.02. Experimental data: N₂H₄(l) + O₂(g) → N₂(g) + 2H₂O(l), ΔH = −622 kJ; N₂H₄(l) → N₂H₄(g), ΔH = +45 kJ; H₂O(l) → H₂O(g), ΔH = +44 kJ.

   (a)

   Determine the enthalpy changes of Reactions 1 and 2 from bond enthalpies, and deduce which oxidiser releases more energy per gram of propellant (hydrazine plus oxidiser).

   [6 marks]

   (b)

   Use Hess's law and the experimental data to determine the enthalpy change of Reaction 1, and evaluate how well the bond-enthalpy value from (a) agrees with it.

   [6 marks]

   Total for question 4: 12 marks

End of questions