Bond enthalpiesAQA A-Level Chemistry: Subtopic test

1. 1
   Hydrogen reacts with chlorine in the gas phase: H₂(g) + Cl₂(g) → 2HCl(g). The mean bond enthalpies, in kJ mol⁻¹, are: H–H 436, Cl–Cl 243 and H–Cl 432.

   (a)

   Which statement about the bonds in H₂ and Cl₂ is correct?

   [1 mark]

   • ABreaking the H–H and Cl–Cl bonds is endothermic because energy must be supplied to separate the bonded atoms
   • BBreaking the H–H and Cl–Cl bonds is exothermic because energy is released when atoms separate
   • CBreaking the H–H bond is exothermic but breaking the Cl–Cl bond is endothermic
   • DBreaking the bonds releases energy because the products are more stable than the atoms

   (b)

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

   [1 mark]

   • A+185 kJ mol⁻¹
   • B−679 kJ mol⁻¹
   • C−185 kJ mol⁻¹
   • D+1543 kJ mol⁻¹

   (c)

   Explain, in terms of bonds, why this reaction is exothermic.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ethene reacts with hydrogen in the gas phase in the presence of a nickel catalyst: C₂H₄(g) + H₂(g) → C₂H₆(g). The mean bond enthalpies, in kJ mol⁻¹, are: C=C 612, C–C 347, C–H 413 and H–H 436.

   (a)

   Which bonds are broken when one mole of ethene reacts with one mole of hydrogen?

   [1 mark]

   • AOne C–C bond and one H–H bond
   • BOne C=C bond and one H–H bond
   • COne C=C bond and two C–H bonds
   • DTwo C–H bonds only

   (b)

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

   [1 mark]

   • A+125 kJ mol⁻¹
   • B−1173 kJ mol⁻¹
   • C−1048 kJ mol⁻¹
   • D−125 kJ mol⁻¹

   (c)

   Define the term mean bond enthalpy.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Methane burns completely in oxygen with all species in the gas phase: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g). The mean bond enthalpies, in kJ mol⁻¹, are: C–H 413, O=O 496, C=O 805 and O–H 463. The standard enthalpies of formation, in kJ mol⁻¹, are: CH₄(g) −74.8, CO₂(g) −393.5 and H₂O(g) −241.8.

   (a)

   Use the mean bond enthalpies to calculate the enthalpy change for the combustion of methane.

   [3 marks]

   (b)

   Calculate the enthalpy change for this reaction using the enthalpies of formation. Explain why this value differs from the value calculated using mean bond enthalpies.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the Haber process, nitrogen and hydrogen react in the gas phase: N₂(g) + 3H₂(g) → 2NH₃(g). The mean bond enthalpies, in kJ mol⁻¹, are: N≡N 945, H–H 436 and N–H 391. The standard enthalpy of formation of NH₃(g) is −46.1 kJ mol⁻¹.

   (a)

   Use the mean bond enthalpies to calculate the enthalpy change for this reaction, and explain why the reaction is exothermic.

   [6 marks]

   (b)

   Evaluate the use of mean bond enthalpies, rather than Hess's law with enthalpies of formation, to find the enthalpy change for this reaction.

   [6 marks]

   Total for question 4: 12 marks

End of questions