A level Chemistry · Edexcel 9CH0 · Topic 8B
Hess's law
Key points
- Hess's law: the enthalpy change for a reaction is independent of the route taken, as long as start and end conditions are the same.
- Draw an enthalpy cycle linking reactants and products through a common set of elements or combustion products.
- Using formation data: .
- Using combustion data: .
- Multiply each value by the number of moles in the balanced equation, and remember elements have = 0.
- Typical use: find for a reaction that is too slow or impossible to measure directly, e.g. CuSO₄ + 5H₂O → CuSO₄·5H₂O via two measurable dissolving reactions.
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Many enthalpy changes cannot be measured directly: the reaction is too slow, gives a mixture of products, or never happens cleanly at all. Hess's law gets round this by letting you calculate the enthalpy change from other reactions you can…
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