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CAIE IGCSE Chemistry Enthalpy Change from Bond Energies

CAIE IGCSE Chemistry Enthalpy Change from Bond Energies
Cambridge IGCSE Chemistry 0620 syllabus for exams in 2026, 2027 and 20282026–2028

Practise calculating reaction enthalpy by totalling bonds broken and formed, preserving stoichiometric multipliers and the final positive or negative sign.

How this is tested

  • count every bond from the balanced equation and total the energy required to break them
  • total energy released by bonds formed and calculate broken minus formed
  • retain the sign when classifying the result or rearrange the energy balance for an unknown bond

Question 3(e)

[Maximum number: 3]

The symbol equation for the industrial production of ammonia is shown.

N2( g)+3H2( g)⇌2NH3( g)ΔH=−90 kJ/mol\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H=-90 \mathrm{~kJ} / \mathrm{mol}

The symbol equation for the reaction can be represented as shown in Fig. 3.1.

Fig. 3.1

Fig. 3.1

Table 3.1 shows some bond energies.

Table 3.1

Table 3.1

Use the bond energies in Table 3.1 and ΔH\Delta H to calculate the bond energy of an N-H bond, in kJ/mol.

Use the following steps.
- Calculate the energy needed to break bonds in the reactants. kJ
- Calculate the energy released when bonds form in the products. kJ