When the First Step Is Fast · 当第一步很快时
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| pre-equilibrium/priː ˌiːkwɪˈlɪbrɪəm/ | 预平衡 | yù píng héng |
When the slow one comes second
- Sometimes the bottleneck sits in the middle, not the start.
- The slow step then needs an intermediate made earlier.
- But an intermediate cannot appear in the final rate law.
- A clever swap fixes this.
当慢的那步排在第二时
- 有时瓶颈在中间,而不在开头。
- 慢步骤于是需要更早生成的中间体。
- 但中间体不能出现在最终的速率方程里。
- 一个巧妙的替换解决了这个问题。
A fast equilibrium first
- The first step is a fast, reversible equilibrium.
- The second, slow step is the rate-determining one.
- Its rate law contains an intermediate from step one.
先来一个快平衡
- 第一步是一个快速、可逆的平衡。
- 第二步(慢)是决速步骤。
- 它的速率方程含有第一步来的中间体。
The pre-equilibrium method is used when the slow step... · 预平衡方法用于慢步骤...
A fast equilibrium precedes the slow rate-determining step. · 快速平衡位于慢的决速步之前。
In this case the ____ step is not the first one, but the second. · 在这种情况下，____步骤不是第一步，而是第二步。
The rate-determining (slow) step is second here. · 这里的决速步（慢步骤）是第二步。
Replace the intermediate
- Intermediates cannot appear in the final rate law.
- Use the fast equilibrium to express it in terms of reactants.
- Substitute, and the intermediate disappears.
替换中间体
- 中间体不能出现在最终速率方程里。
- 用快平衡把它用反应物表示出来。
- 替换后,中间体就消失了。
An intermediate is allowed to remain in the final rate law. · 允许中间体保留在最终速率方程中。
It must be substituted out using the fast equilibrium. · 必须利用快速平衡将其替换掉。
In fast $A \rightleftharpoons B$, slow $B + C \to D$, the intermediate $B$ is replaced using... · 在快 $A \rightleftharpoons B$、慢 $B + C \to D$ 中，中间体 $B$ 使用...进行替换。
The equilibrium gives $[B]$ in terms of $[A]$, removing the intermediate. · 平衡关系给出了$[B]$关于$[A]$的表达式，从而消除了中间体。
The pre-equilibrium idea
- This is the pre-equilibrium 预平衡 approximation.
- The fast first step stays balanced while the slow step sips away the intermediate.
- The result is a rate law in real reactants only.
预平衡的思想
- 这就是预平衡近似。
- 快的第一步保持平衡,而慢步骤一点点抽走中间体。
- 结果是一个只含真实反应物的速率方程。
When the first step is fast · 当第一步很快时
A fast equilibrium feeds a slow step - so the intermediate must be rewritten using the reactants. · 快速平衡向慢步骤提供物质——因此必须用反应物重新表示中间体。
Using a fast first equilibrium to replace an intermediate is the ____ approximation. · 利用快速第一步平衡替换中间体是____近似法。
This is the pre-equilibrium approximation. · 这就是预平衡近似法。
After substitution, the final rate law contains only reactants (and any catalyst). · 替换后，最终速率方程仅包含反应物（及任何催化剂）。
No intermediates should remain in the final rate law. · 最终速率方程中不应包含中间体。
Fast: $A \rightleftharpoons B$. Slow: $B + C \to D$.
- The slow-step rate is $k[B][C]$, but $B$ is an intermediate.
- The equilibrium gives $[B] \propto [A]$, so $\text{rate} = k'[A][C]$.
快:$A \rightleftharpoons B$。慢:$B + C \to D$。
- 慢步骤速率是 $k[B][C]$,但 $B$ 是中间体。
- 平衡给出 $[B] \propto [A]$,所以 $\text{rate} = k'[A][C]$。
An intermediate must not appear in the final rate law -- always substitute it out using the fast equilibrium. This pre-equilibrium method applies when the first step is a fast, reversible step before the slow one. And the final rate law should contain only reactants (and perhaps a catalyst).
中间体不能出现在最终速率方程里——始终用快平衡把它替换掉。这个预平衡方法适用于第一步是慢步骤之前的快速可逆步骤时。而且最终速率方程应只含反应物(也许还有催化剂)。
When a fast equilibrium precedes the slow step, its rate law holds an intermediate. The pre-equilibrium approximation uses the fast step to rewrite that intermediate in terms of real reactants, then substitutes it out -- leaving a rate law with no intermediates.
当一个快平衡在慢步骤之前时,它的速率方程含有中间体。预平衡近似用快步骤把那个中间体改写成真实反应物,再替换掉——留下一个不含中间体的速率方程。