Hess's law · 盖斯定律
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| Hess's law/ˈhesɪz lɔː/ | 盖斯定律 | gài sī dìng lǜ |
| enthalpy change/enˈθælpi tʃeɪndʒ/ | 焓变 | hán biàn |
| energy cycle/ˈenədʒi ˈsaɪkl/ | 能量循环 | néng liàng xún huán |
| bond energy/bɒnd ˈenədʒi/ | 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.
把能量加起来
- 盖斯定律(Hess's law):无论你走哪条路线,总焓变都相同。
- 这是因为能量守恒。
- 它让我们找到不能直接测量的焓变——包括从键能。
Hess's law states that the total enthalpy change for a reaction: · 盖斯定律陈述一个反应的总焓变:
Because energy is conserved, the overall ΔH depends only on the start and end states, not the path. · 因为能量守恒,总 ΔH 只取决于起始和结束状态,而不是路径。
Hess's law works because energy is conserved. · 盖斯定律之所以成立,是因为能量守恒。
Conservation of energy means the total enthalpy change is fixed by the start and end states alone. · 能量守恒意味着总焓变仅由起始和结束状态确定。
The law stating that enthalpy change is independent of the route taken is ______ law. · 陈述焓变与所走路线无关的定律是______定律。
Enthalpy is a state function. · 焓是一个状态函数。
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
能量循环
- 用一个直接步骤和一条间接路线连接反应物和产物。
- 因为两条路线有相同的总和:
盖斯定律:直接和间接路线给出相同的总和
Hess's law cycle · 盖斯定律循环
Enthalpy change is the same whichever route you take — so an unknown ΔH can be found by an alternative path. · 无论你走哪条路线,焓变都相同——所以一个未知的 ΔH 能通过一条替代路径找到。
In an energy cycle, the direct route and the indirect route: · 在一个能量循环中,直接路线和间接路线:
Both routes start and end at the same states, so their total ΔH must be equal (e.g. ΔHr = ΔH₁ + ΔH₂). · 两条路线在相同的状态开始和结束,所以它们的总 ΔH 必须相等(例如 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.
为什么它有用
- 它找到一个你不能直接测量的焓变(慢反应、副反应)。
- 它让你从题目中给出的键能、生成或燃烧数据计算 $\Delta H_r$。
A key use of Hess's law is to: · 盖斯定律的一个关键用途是:
Hess's law lets us calculate ΔH for reactions that are slow or have side reactions, using other known data. · 盖斯定律让我们用其他已知数据计算慢反应或有副反应的反应的 ΔH。
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)
键能计算
- 断裂键吸收能量;形成键放出能量。
- 所以:ΔH = Σ(键断裂)− Σ(键形成)。
求 H₂ + Cl₂ → 2 HCl 的 ΔH(键能:H–H 436,Cl–Cl 242,H–Cl 431 kJ/mol)。
- 键断裂:1 H–H + 1 Cl–Cl = 436 + 242 = +678
- 键形成:2 H–Cl = 2 × 431 = −862
- ΔH = 678 − 862 = −184 kJ/mol(放热——放出的能量比吸收的多)
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
你掌握了
- 盖斯定律:$\Delta H$ 沿任何路线相同(能量守恒)
- 画一个能量循环:直接路线 = 间接路线,所以 $\Delta H_r = \Delta H_1 + \Delta H_2$
- 键能:$\Delta H$ = 键断裂 − 键形成(例如 H₂ + Cl₂ → 2 HCl 给出 −184 kJ/mol)
- 用一个循环从给定数据找到一个不可测量的 $\Delta H$