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CAIE IGCSE Chemistry Enthalpy Change and Its Sign

Practise defining reaction enthalpy change and using its sign to classify thermal energy transfer, with negative values for exothermic reactions.

Syllabus
2026–2028
Course
Chemistry 0620

Exam points

  • define enthalpy change as the thermal energy transferred during a chemical reaction
  • use a negative enthalpy change for exothermic and a positive value for endothermic
  • connect the sign with whether bond formation releases more or less energy than bond breaking

5.1.4—Transfer of thermal energy during a question 1

[Maximum number: 2]

The Haber process is used to manufacture ammonia.

Question (a)

(a)

The equation for the Haber process is shown.

N2( g)+3H2( g)⇌2NH3( g)ΔH=−92 kJ/mol\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightleftharpoons 2 \mathrm{NH}_{3}(\mathrm{~g}) \quad \Delta H=-92 \mathrm{~kJ} / \mathrm{mol}

The reaction is reversible. The forward reaction is exothermic.

[ 2 ]

Question (i)

(i)

State what is meant by the symbol ΔH\Delta H.

[ 1 ]

Question (ii)

(ii)

ΔH\Delta H for the forward reaction is −92 kJ/mol-92 \mathrm{~kJ} / \mathrm{mol}.

State why this value shows that the forward reaction is exothermic.

[ 1 ]
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