Chemistry A-Level
Hess's law
What Hess's law means in Chemistry A-Level, as defined for AQA, OCR.
Definitions of Hess's law
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Reference: OCR
Used with standard enthalpy changes of formation to find an enthalpy change of reaction, following the route reactants → elements → products.
The enthalpy change of a reaction is independent of the route taken, as enthalpy is a state function.
Topics that use Hess's law
- Applications of Hess’s law (AQA A-Level)
- Enthalpy changes (OCR A-Level)
Related terms
- ∆H of combustion
- ∆H of formation
- Activation energy
- Approximate estimate
- Average bond enthalpy
- Averaging over gaseous compounds
- Bond breaking and bond making
- Bond enthalpy
Using Hess's law in the exam
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