Hess's law
Introduced| English | Meaning |
|---|---|
| Hess's law/ˈhesɪz lɔː/ | The total enthalpy change of a reaction is the same whatever route is taken, because energy is conserved. |
| enthalpy change/enˈθælpi tʃeɪndʒ/ | The heat transferred in a reaction or process at constant pressure. |
| energy cycle/ˈenədʒi ˈsaɪkl/ | A diagram linking alternative reaction routes, used with Hess’s law to find an enthalpy change. |
| bond energy/bɒnd ˈenədʒi/ | The energy required to break one mole of a specified covalent bond in gaseous molecules. |
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:
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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.
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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.
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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:
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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:
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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