Enthalpy Change – IGCSE Chemistry Definition

Enthalpy change is the formal measure of the energy change in a chemical reaction. On the 0620 syllabus (Supplement), it connects the concepts of bond energy, energy level diagrams, and the classification of reactions as exothermic or endothermic. Understanding the sign convention is essential for interpreting and performing calculations on Paper 4.

The 0620 definition

The enthalpy change (represented by the symbol delta H) is the overall energy change in a chemical reaction at constant pressure.

• Negative delta H: exothermic reaction (energy released to surroundings)
• Positive delta H: endothermic reaction (energy absorbed from surroundings)

Sign convention

Type of reaction	Energy transfer	Sign of delta H	Temperature of surroundings
Exothermic	Energy released	Negative (-)	Increases
Endothermic	Energy absorbed	Positive (+)	Decreases

Calculating enthalpy change from bond energies

Delta H = sum of bond energies broken - sum of bond energies formed

Example

For H₂ + Cl₂ → 2HCl:

Bonds broken: H-H (436 kJ/mol) + Cl-Cl (242 kJ/mol) = 678 kJ

Bonds formed: 2 x H-Cl (2 x 431 kJ/mol) = 862 kJ

Delta H = 678 - 862 = -184 kJ/mol

The negative sign confirms this is exothermic.

Enthalpy change on energy level diagrams

On an energy level diagram, delta H is shown as the vertical arrow between the energy levels of reactants and products:

• Exothermic: arrow points downward (products lower than reactants)
• Endothermic: arrow points upward (products higher than reactants)

The length of the arrow corresponds to the magnitude of delta H.

Units

Enthalpy change is measured in kJ/mol (kilojoules per mole). The “/mol” refers to the equation as written — if you double the equation, the enthalpy change doubles.

Worked exam question

The enthalpy change for the reaction N₂ + 3H₂ → 2NH₃ is -92 kJ/mol. (a) Is this reaction exothermic or endothermic? Explain. (2) (b) On an energy level diagram, where would the products be drawn relative to the reactants? (1)

Mark scheme

(a) Exothermic [1]; because delta H is negative / energy is released [1]

(b) Products drawn lower / at a lower energy level than the reactants [1]

Common exam mistakes

• Forgetting the sign. A delta H value without its sign (+ or -) is incomplete. The sign determines whether the reaction is exothermic or endothermic.
• Confusing the subtraction order in bond energy calculations. It must be bonds broken minus bonds formed. Reversing this gives the wrong sign.
• Ignoring units. Enthalpy change must be given in kJ/mol, not just kJ. State the unit in your answer.