Bond enthalpiesEdexcel International A Level Chemistry: Subtopic test

1. 1
   A gas hob burns methane in air: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g). An engineer estimates the enthalpy change using mean bond enthalpies (kJ mol⁻¹): C–H 413, O=O 498, C=O (in CO₂) 805, O–H 464.

   (a)

   Which statement about bond breaking and bond making is correct?

   [1 mark]

   • ABreaking bonds releases energy and making bonds takes in energy.
   • BBreaking bonds takes in energy (endothermic) and making bonds releases energy (exothermic).
   • CBoth breaking and making bonds release energy.
   • DBoth breaking and making bonds take in energy.

   (b)

   What is the enthalpy change for this reaction, calculated from the mean bond enthalpies, in kJ mol⁻¹?

   [1 mark]

   • A+818
   • B+6114
   • C–2648
   • D–818

   (c)

   The experimental standard enthalpy change of combustion of methane is –890 kJ mol⁻¹. Give two reasons why the value calculated from mean bond enthalpies is different.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Chemists use bond enthalpy data to predict which bonds break most easily and so how quickly a compound reacts at room temperature. Mean bond enthalpies (kJ mol⁻¹): C–Cl 346, C–Br 290, C–I 228, N≡N 945, O=O 498.

   (a)

   According to the bond enthalpy data, which halogenoalkane is hydrolysed most rapidly by water at room temperature?

   [1 mark]

   • A1-chloropropane, because the C–Cl bond is the strongest.
   • B1-bromopropane, because the C–Br bond is intermediate in strength.
   • C1-iodopropane, because the C–I bond has the lowest mean bond enthalpy.
   • DThey react at the same rate, because they all contain a carbon–halogen bond.

   (b)

   Which statement best explains why nitrogen gas is unreactive at room temperature?

   [1 mark]

   • AThe N≡N bond has a very high bond enthalpy, so a lot of energy is needed to break it.
   • BN₂ molecules are strongly polar.
   • CThe N≡N bond enthalpy is lower than that of the O=O bond.
   • DThe N≡N bond is easily broken but the atoms do not react.

   (c)

   The compound BrCH₂CH₂Cl contains a C–Br bond and a C–Cl bond. State which bond breaks first and explain why. Predict, with a reason, whether iodoethane reacts faster or slower than bromoethane.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In the Haber process nitrogen and hydrogen react to form ammonia: N₂(g) + 3H₂(g) → 2NH₃(g). A student uses mean bond enthalpies (kJ mol⁻¹): N≡N 945, H–H 436, N–H 391.

   (a)

   Define the term mean bond enthalpy and explain why bond enthalpy data tables quote mean values.

   [3 marks]

   (b)

   Calculate the enthalpy change for the reaction using the bond enthalpies, and state whether the reaction is exothermic or endothermic.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A research chemist studies methane, CH₄. She needs a value for the mean C–H bond enthalpy and also wants to test how well mean bond enthalpies predict the enthalpy change of a reaction. All the substances in the reactions are gases.

   (a)

   Calculate the mean C–H bond enthalpy in methane from these data: standard enthalpy change of formation of CH₄(g) = –75 kJ mol⁻¹; standard enthalpy change of atomisation of carbon = +717 kJ mol⁻¹; enthalpy change for H₂(g) → 2H(g) = +436 kJ mol⁻¹.

   [6 marks]

   (b)

   Use mean bond enthalpies (C–H 413, Cl–Cl 243, C–Cl 346, H–Cl 432, all in kJ mol⁻¹) to calculate the enthalpy change for CH₄(g) + Cl₂(g) → CH₃Cl(g) + HCl(g). The value from enthalpy changes of formation is –99 kJ mol⁻¹. Evaluate the bond enthalpy method using the two results.

   [6 marks]

   Total for question 4: 12 marks

End of questions