1. 1
   In the Haber process ammonia is made from nitrogen and hydrogen: N₂(g) + 3H₂(g) → 2NH₃(g). Average bond enthalpies, in kJ mol⁻¹: N≡N 945, H–H 436, N–H 391. The standard enthalpy of formation of ammonia gas is −46 kJ mol⁻¹.

   (a)

   What is the enthalpy change for the reaction as written, calculated from the bond enthalpies?

   [1 mark]

   • A+1080 kJ
   • B+93 kJ
   • C−93 kJ
   • D−47 kJ

   (b)

   Which equation represents the standard enthalpy of formation of ammonia?

   [1 mark]

   • AN₂(g) + 3H₂(g) → 2NH₃(g)
   • B½N₂(g) + 1½H₂(g) → NH₃(g)
   • CN(g) + 3H(g) → NH₃(g)
   • D½N₂(g) + 1½H₂(g) → NH₃(l)

   (c)

   Compare the value from bond enthalpies with the value obtained from the enthalpy of formation, and explain which is more accurate.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Data for the Born–Haber cycle of potassium chloride, KCl, in kJ mol⁻¹: enthalpy of atomisation of potassium +89; first ionisation energy of potassium +419; enthalpy of atomisation of chlorine (½Cl₂(g) → Cl(g)) +121; first electron affinity of chlorine −349; standard enthalpy of formation of KCl(s) −437. Lattice enthalpy is defined as the enthalpy change for KCl(s) → K⁺(g) + Cl⁻(g).

   (a)

   Which equation represents the first electron affinity of chlorine?

   [1 mark]

   • ACl(g) + e⁻ → Cl⁻(g)
   • B½Cl₂(g) + e⁻ → Cl⁻(g)
   • CCl⁻(g) → Cl(g) + e⁻
   • DCl₂(g) + 2e⁻ → 2Cl⁻(g)

   (b)

   What is the lattice enthalpy of potassium chloride?

   [1 mark]

   • A+1415 kJ mol⁻¹
   • B−157 kJ mol⁻¹
   • C−717 kJ mol⁻¹
   • D+717 kJ mol⁻¹

   (c)

   Explain why the first electron affinity of chlorine is exothermic, whereas the second electron affinity of oxygen is endothermic.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Propane and propene are both obtained from crude oil. Standard enthalpies of combustion, ΔHc⦵, in kJ mol⁻¹: propene C₃H₆(g) −2058; propane C₃H₈(g) −2219; hydrogen H₂(g) −286; carbon (graphite) −394. Propene can be hydrogenated: C₃H₆(g) + H₂(g) → C₃H₈(g).

   (a)

   Calculate the standard enthalpy change for the hydrogenation of propene using the enthalpy of combustion data.

   [3 marks]

   (b)

   Deduce the equation for the standard enthalpy of formation of propane, explain why it cannot be measured directly, and determine its value.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Data for magnesium chloride, in kJ mol⁻¹: enthalpy of atomisation of magnesium +148; first ionisation energy of magnesium +738; second ionisation energy of magnesium +1451; enthalpy of atomisation of chlorine (½Cl₂(g) → Cl(g)) +121; first electron affinity of chlorine −349; lattice enthalpy of MgCl₂(s) +2526. A hypothetical compound MgCl, containing Mg⁺ ions, has an estimated lattice enthalpy of +753. Lattice enthalpy is defined as the endothermic change from solid to gaseous ions.

   (a)

   Determine the standard enthalpy of formation of MgCl₂(s), outlining each step of the Born–Haber cycle, and explain why the second ionisation energy of magnesium is larger than the first.

   [6 marks]

   (b)

   Using the estimated lattice enthalpy of MgCl, evaluate why magnesium forms MgCl₂ rather than MgCl when it reacts with chlorine.

   [6 marks]

   Total for question 4: 12 marks

End of questions