BRAINCAKEThermochemical Cycles and Enthalpy Diagrams
Hess's law states that the enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same. This means enthalpy changes that cannot be measured directly can be calculated from other enthalpy changes.
A thermochemical cycle links the reactants and products by two routes. The direct route is the reaction of interest, and the indirect route goes via intermediate substances, usually the elements (using enthalpies of formation) or the combustion products (using enthalpies of combustion). The total enthalpy change is the same for each route. When an arrow is followed backwards, the sign of its enthalpy change is reversed.
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Key terms in this lesson
- Enthalpy
- A measure of the heat energy stored in a chemical system.
More in Energetics
- Allowing for Heat Loss
- Hess's Law and Thermochemical Cycles
- The Enthalpy Change of Formation
- The Enthalpy Change of Combustion
- Bond Enthalpies
- Using Mean Bond Enthalpies to Calculate Enthalpy Change
- Comparison with Thermochemical Cycles
- Example Question - 1
All 24 lessons in Energetics · All AQA AS-level Chemistry topics