Hess's Law · 盖斯定律
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| state function/steɪt ˈfʌŋkʃn/ | 状态函数 | zhuàng tài hán shù |
| Hess's law/ˈhesɪz lɔː/ | 赫斯定律 | hè sī dìng lǜ |
Adding reactions like recipes
- Some reactions are too hard to measure directly.
- But you can build them from steps you already know.
- Add those steps up and their heats add up too.
- The total heat depends only on start and end, not the route.
像食谱一样把反应相加
- 有些反应太难直接测量。
- 但你能用你已经知道的步骤把它搭出来。
- 把那些步骤相加,它们的热量也相加。
- 总热量只取决于起点和终点,而不是路线。
A path-independent quantity
- $\Delta H$ is a state function 状态函数: it depends only on start and end.
- The route taken does not change the total heat.
- So any path from reactants to products gives the same $\Delta H$.
一个与路径无关的量
- $\Delta H$ 是一个状态函数:它只取决于起点和终点。
- 走的路线不改变总热量。
- 所以从反应物到产物的任何路径都给出相同的 $\Delta H$。
$\Delta H$ is a state function, which means it depends only on... · $\Delta H$ 是状态函数，意味着它仅取决于...
A state function ignores the path between start and end. · 状态函数忽略起始和终了状态之间的路径。
Adding known steps
- Hess's law 赫斯定律 adds known reactions to reach a target.
- Sum their $\Delta H$ values to get the target $\Delta H$.
- Choose steps that cancel to leave the reaction you want.
相加已知的步骤
- 赫斯定律把已知反应相加以达到目标。
- 把它们的 $\Delta H$ 相加,得到目标的 $\Delta H$。
- 选择能相消的步骤,留下你想要的反应。
When you reverse a reaction step, its $\Delta H$... · 当你反转反应步骤时，其 $\Delta H$...
The reverse step has the opposite $\Delta H$. · 反转步骤具有相反的 $\Delta H$。
If you double a reaction step, you double its $\Delta H$. · 如果你加倍反应步骤，其 $\Delta H$ 也加倍。
Scaling the amounts scales $\Delta H$ by the same factor. · 按比例缩放物质的量会按相同因子缩放 $\Delta H$。
Steps are chosen so the ____ cancel, leaving the target reaction. · 选择步骤使得 ____ 相互抵消，留下目标反应。
Intermediates on opposite sides cancel when the steps are added. · 当相加步骤时，位于相反侧的中间体会抵消。
Flip and scale
- Reverse a step and flip the sign of its $\Delta H$.
- Multiply a step and multiply its $\Delta H$.
- Then add the adjusted steps together.
翻转与缩放
- 逆转一个步骤,把它的 $\Delta H$ 变号。
- 把一个步骤放大,也把它的 $\Delta H$ 放大。
- 然后把调整后的步骤相加。
Hess's law cycle · 盖斯定律循环
Enthalpy is a state function, so any route from reactants to products gives the same total. · 焓是状态函数，因此从反应物到产物的任何路径给出的总焓变都相同。
Step 1 has $\Delta H = -100$; step 2 has $\Delta H = -50$; their sum is the target.
- Add them: $\Delta H_{target} = -100 + (-50) = -150\ \text{kJ}$.
- The intermediates cancel out.
步骤 1 的 $\Delta H = -100$;步骤 2 的 $\Delta H = -50$;它们之和就是目标。
- 相加:$\Delta H_{target} = -100 + (-50) = -150\ \text{kJ}$。
- 中间体相消了。
Two steps have $\Delta H = -120\ \text{kJ}$ and $+30\ \text{kJ}$. Their sum (in kJ)? · 两个步骤分别有 $\Delta H = -120\ \text{kJ}$ 和 $+30\ \text{kJ}$。它们的和（单位：kJ）是多少？
$-120 + 30 = -90\ \text{kJ}$.
Hess's law works because enthalpy is a state function. · 盖斯定律成立是因为焓是状态函数。
Path-independence is exactly why the step $\Delta H$ values add. · 路径无关性正是步骤 $\Delta H$ 值可以相加的原因。
When you reverse a step, flip the sign of its $\Delta H$; when you scale it, multiply $\Delta H$ too. Line the steps up so the intermediates cancel. And Hess's law works only because $\Delta H$ is a state function -- the path does not matter.
当你逆转一个步骤时,把它的 $\Delta H$ 变号;当你缩放它时,也把 $\Delta H$ 相乘。把步骤排好,使中间体相消。而且赫斯定律成立,正是因为 $\Delta H$ 是状态函数——路径无关紧要。
Because $\Delta H$ is a state function (path-independent), Hess's law lets you add known reactions to find an unknown $\Delta H$. Reverse a step and flip its sign; scale a step and scale its $\Delta H$; then sum so the intermediates cancel.
因为 $\Delta H$ 是状态函数(与路径无关),赫斯定律让你把已知反应相加来求未知的 $\Delta H$。逆转一步就把它变号;缩放一步就缩放它的 $\Delta H$;再求和使中间体相消。