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AS · Practice questions · Bond energies

Energy in, energy out.

Six original Cambridge-style questions on bond energies: definitions, combustion and addition reactions, working backwards and evaluating the method.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

ΔH = broken − formed.

01
Recall
[2 marks]

Define bond energy and explain the term mean bond energy.

  • The energy needed to break one mole of a given covalent bond in gaseous molecules. ✓
  • A mean bond energy is an average value for that bond taken over many different compounds. ✓
02
Calculation
[2 marks]

Calculate ΔH for H2(g) + Cl2(g) → 2HCl(g). (H–H 436, Cl–Cl 243, H–Cl 432 kJ mol−1)

  • Broken: 436 + 243 = 679 kJ; formed: 2 × 432 = 864 kJ. ✓
  • ΔH = 679 − 864 = −185 kJ mol−1. ✓
03
Calculation
[3 marks]

Calculate ΔH for the combustion of ethene: C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(g). (C=C 610, C–H 410, O=O 496, C=O 805, O–H 460 kJ mol−1)

  • Broken: 610 + 4 × 410 + 3 × 496 = 610 + 1640 + 1488 = 3738 kJ. ✓
  • Formed: 4 × 805 + 4 × 460 = 3220 + 1840 = 5060 kJ. ✓
  • ΔH = 3738 − 5060 = −1322 kJ mol−1. ✓
04
Calculation
[2 marks]

For N2(g) + 3H2(g) → 2NH3(g), calculate ΔH. (N≡N 944, H–H 436, N–H 390 kJ mol−1)

  • Broken: 944 + 3 × 436 = 2252 kJ; formed: 6 × 390 = 2340 kJ. ✓
  • ΔH = 2252 − 2340 = −88 kJ mol−1. ✓
05
Calculation
[2 marks]

For H2(g) + F2(g) → 2HF(g), ΔH = −542 kJ mol−1. The H–H bond energy is 436 and the H–F bond energy is 562 kJ mol−1. Calculate the F–F bond energy.

  • 436 + x − 2 × 562 = −542. ✓
  • x = −542 + 1124 − 436 = 146 kJ mol−1. ✓
06
Explain
[2 marks]

Give two reasons why an enthalpy change calculated from mean bond energies differs from the value found by experiment.

  • Mean bond energies are averages; the actual bond energies in the specific molecules differ. ✓
  • Bond energies refer to gases; some reactants or products (for example water) are liquids or solids under standard conditions. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.