Calculate enthalpy changes from enthalpies of combustion.

In the subtopic Applications of Hess's law, this AQA A-Level Chemistry 7405 learning objective focuses on calculate enthalpy changes from enthalpies of combustion. It belongs to Energetics, so revision should stay anchored to this exact subtopic rather than drifting into a generic GCSE-level chemistry summary. Approved keywords to use include enthalpy, changes, from, enthalpies, calculate. Enthalpy change. means the heat change associated with a chemical reaction at constant pressure Avoid the mistake of students often forget to account for the signs of enthalpy changes when using combustion values, leading to incorrect total enthalpy calculations; instead, to fix this, remember that combustion reactions are exothermic, so their enthalpy changes should be negative. Use the formula ΔH = Σ(ΔH_combustion) for the products - Σ(ΔH_combustion) for the reactants, ensuring to apply the correct signs For exam answers, use Hess's law to calculate enthalpy changes from combustion enthalpies by applying the formula ΔH = ΣΔH°f(products) - ΣΔH°f(reactants)