Hess's law
Hess's law
- Hess's law: the total enthalpy change is the same whatever route you take.
- This is because energy is conserved.
- It lets us find enthalpy changes we cannot measure directly.
Practice
Hess's law states that the total enthalpy change for a reaction:
Because energy is conserved, the overall ΔH depends only on the start and end states, not the path.
Practice
Hess's law works because energy is conserved.
Conservation of energy means the total enthalpy change is fixed by the start and end states alone.
Energy cycles
- Link reactants and products by a direct step and an indirect route.
- Because both routes have the same total:
$$\Delta H_r = \Delta H_1 + \Delta H_2$$
Practice
In an energy cycle, the direct route and the indirect route:
Both routes start and end at the same states, so their total ΔH must be equal (e.g. ΔHr = ΔH₁ + ΔH₂).
Why it's useful
- It finds an enthalpy change you cannot measure directly (slow reactions, side reactions).
- It lets you calculate $\Delta H_r$ from bond energy, formation or combustion data given in the question.
Practice
A key use of Hess's law is to:
Hess's law lets us calculate ΔH for reactions that are slow or have side reactions, using other known data.
You've got it
Key idea
- Hess's law: $\Delta H$ is the same by any route (energy is conserved)
- draw an energy cycle: direct route = indirect route, so $\Delta H_r = \Delta H_1 + \Delta H_2$
- use it to find an unmeasurable $\Delta H$, or to calculate from formation/combustion/bond data