Chemistry (AS Level)·Chemical energetics · NSSCAS 2.1.2
Hess's law & enthalpy cycles
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Some enthalpy changes can't be measured directly — you can't easily form glucose from carbon and hydrogen. Hess's law lets you find them anyway, by building an energy cycle from changes you CAN measure. Learn the law, build cycles, and calculate the enthalpy changes experiment can't reach.
What you'll learn in this lesson
By the end you should be able to (NSSCAS Chemistry (AS Level) 2.1.2):
- Apply Hess's law to construct simple energy cycles and carry out calculations
- Determine enthalpy changes that cannot be found by direct experiment (e.g. ΔHf from enthalpy changes of combustion)
- Construct and interpret a reaction pathway diagram in terms of ΔH and the activation energy
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