Energy Change of Reactions (Higher Tier)

This lesson extends your understanding of energy changes by exploring how we quantify energy changes using $\Delta H$ΔH notation, how energy changes relate to bond breaking and bond forming at a deeper level, and how to interpret energy data from experiments. This content is targeted at Higher Tier students on the AQA GCSE Combined Science Trilogy specification (8464).

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Enthalpy Change ($\Delta H$ΔH)

The enthalpy change ($\Delta H$ΔH) of a reaction is the overall energy change measured under standard conditions. It tells you how much energy is transferred to or from the surroundings per mole of reaction.

$\Delta H = H_{\text{products}} - H_{\text{reactants}}$ΔH=Hproducts​−Hreactants​

Value of $\Delta H$ΔH	Meaning
Negative ($\Delta H < 0$ΔH<0)	Exothermic — energy is released to the surroundings
Positive ($\Delta H > 0$ΔH>0)	Endothermic — energy is taken in from the surroundings

Units

$\Delta H$ΔH is measured in kilojoules per mole (kJ/mol).

For example:

• Combustion of methane: $\Delta H = -890 \text{ kJ/mol}$ΔH=−890 kJ/mol (exothermic)
• Thermal decomposition of calcium carbonate: $\Delta H = +178 \text{ kJ/mol}$ΔH=+178 kJ/mol (endothermic)

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