Hess's law
| English | Chinese | Pinyin |
|---|---|---|
| Hess's law | 盖斯定律 | gài sī dìng lǜ |
| enthalpy change | 焓变 | hán biàn |
| energy cycle | 能量循环 | néng liàng xún huán |
| bond energy | 键能 | jiàn néng |
Adding up the energy
- Hess's law 盖斯定律: the total enthalpy change 焓变 is the same whatever route you take.
- This is because energy is conserved.
- It lets us find enthalpy changes we cannot measure directly — including from bond energies.
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.
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.
The law stating that enthalpy change is independent of the route taken is ______ law.
Enthalpy is a state function.
Energy cycles 能量循环
- Link reactants and products by a direct step and an indirect route.
- Because both routes have the same total:
Hess's law: the direct and indirect routes give the same total
Hess's law cycle
Enthalpy change is the same whichever route you take — so an unknown ΔH can be found by an alternative path.
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 键能, formation or combustion data given in the question.
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.
Bond energy calculations
- Breaking bonds takes in energy; making bonds gives out energy.
- So: ΔH = Σ(bonds broken) − Σ(bonds made).
Find ΔH for H₂ + Cl₂ → 2 HCl (bond energies: H–H 436, Cl–Cl 242, H–Cl 431 kJ/mol).
- Bonds broken: 1 H–H + 1 Cl–Cl = 436 + 242 = +678
- Bonds made: 2 H–Cl = 2 × 431 = −862
- ΔH = 678 − 862 = −184 kJ/mol (exothermic — more energy out than in)
You've got it
- Hess's law: $\Delta H$ is the same by any route (energy is conserved)
- draw an energy cycle: direct route = indirect route, so $\Delta H_r = \Delta H_1 + \Delta H_2$
- bond energies: $\Delta H$ = bonds broken − bonds made (e.g. H₂ + Cl₂ → 2 HCl gives −184 kJ/mol)
- use a cycle to find an unmeasurable $\Delta H$ from given data