18.14 确定费率的步骤

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  确定比率步骤

  

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  Ever been stuck in a long line?
  ::被困在长线里过吗?

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  Airline travel can be very frustrating. You usually have to get to the airport two hours before your flight leaves. You stand in line to check your baggage and get your boarding pass. Then you stand in line for your security screen.  Finally, you wait in line to board the plane. Since there are only so many ticket agents, not everybody can be waited on immediately. The same with the security screen – only so many body scanners are available. And getting on the plane involves going one-by-one down a very narrow aisle to get to your seat. All these limits to slow you down.
  ::航空旅行可能非常令人沮丧。 您通常必须在航班起飞前两小时到达机场。 您通常必须在排队中检查行李并领取登机证。 然后排队等待您的安全屏幕。 最后, 您在排队中等待登机。 由于只有这么多的票票务员, 不是每个人都可以立即等待。 同样, 安全屏幕上也是这样 — — 只有这么多的人体扫描机。 登机需要一对一地从一个非常狭窄的过道上登机才能到达您的座位。 所有这些限制都会拖慢您的进度 。

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  Rate-Determining Step
  ::确定比率步骤

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  The determination of a reaction mechanism can only be made in the laboratory. When a reaction occurs in a sequence of elementary steps, the overall is governed by whichever one of those steps is the slowest.  The rate-determining step is the slowest step in the sequence of steps in a reaction mechanism. To get an idea of how one step is rate determining, imagine driving on a one-lane road where it is not possible to pass another vehicle. The rate of flow of traffic on such a road would be dictated by whatever car is traveling at the lowest speed.  The decomposition of hydrogen peroxide is discussed below and illustrates how reaction mechanisms can be determined through experimental studies.
  ::只能由实验室确定反应机制。当反应发生在一系列基本步骤中时,总反应由其中最慢的一个步骤来决定。速度确定步骤是反应机制步骤顺序中最慢的一个步骤。为了了解一个步骤是如何确定比率的,想象一下在单行公路上驾驶不可能通过另一车辆的情况。这条公路上的交通流量将由以最低速度行驶的任何车辆决定。下面将讨论过过氢过氧化物的分解,并说明如何通过实验研究来确定反应机制。

   

   

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  Decomposition of Hydrogen Peroxide
  ::过氧化氢分解

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  Recall that a is a substance , which increases the rate of a without being consumed.  Catalysts lower the overall for a reaction by providing an alternative mechanism for the reaction to follow. One such catalyst for the decomposition of hydrogen peroxide is iodide (I ⁻ ).
  ::回顾一种物质可以提高一种不消费的浓度,通过提供一种替代反应机制来降低反应的总体水平,使过氧化氢分解的催化剂之一是碘化物(I-)。

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  $$\begin{array}{r}2{\text{H}}_2{\text{O}}_2(aq)\stackrel{{\text{I}}^{-}}{⟶}2{\text{H}}_2\text{O}(l)+{\text{O}}_2(g)\end{array}$$

  
  ::2H2O2(aq) I-2H2O(l)+O2(g)

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  By , the rate of reaction is found to be first-order with respect to both H ₂ O ₂ and I ⁻ and second-order overall.
  ::根据H2O2和I的反应率为第一级,总体为第二级。

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  $$\begin{array}{r}\text{rate}=k[{\text{H}}_2{\text{O}}_2][{\text{I}}^{-}]\end{array}$$

  
  ::速率=k[H2O2][I-]

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  The reaction cannot occur in one step corresponding to the overall balanced equation . If it did, the reaction would be second-order with respect to H ₂ O ₂ since the coefficient of the H ₂ O ₂ in the balanced equation is a 2. A reaction mechanism can be constructed which accounts for the rate law and for the detection of the IO ⁻ ion as an intermediate . It consists of two bimolecular elementary steps.
  ::这种反应不可能在与总体平衡等式相对应的一步上发生,如果发生,对H2O2的反应将是次等的,因为平衡等式中H2O2的系数是2。 可以建立一种反应机制,它考虑到比率法和作为中间体的IO-离子的检测,由两个分子基本步骤组成。

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  Step 1: $\begin{array}{r}{\text{H}}_2{\text{O}}_2(aq)+{\text{I}}^{-}(aq)\to {\text{H}}_2\text{O}(l)+{\text{IO}}^{-}(aq)\end{array}$
  ::第1步:H2O2(aq)+I-(aq)+H2O(l)+IO-(aq)

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  Step 2:  $\begin{array}{r}{\text{H}}_2{\text{O}}_2(aq)+{\text{IO}}^{-}(aq)\to {\text{H}}_2\text{O}(l)+{\text{O}}_2+{\text{I}}^{-}(aq)\end{array}$
  ::第2步:H2O2(aq)+IO-(aq)+H2O(l)+O2+I-(aq)

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  If step 1 is the rate-determining step, then the rate law for that step will be the rate law for the overall reaction.
  ::如果第1步是确定费率的步骤,那么该步骤的费率法将是总体反应的费率法。

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  $$\begin{array}{r}\text{rate}=k[{\text{H}}_2{\text{O}}_2][{\text{I}}^{-}]\end{array}$$

  
  ::速率=k[H2O2][I-]

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  The rate law for the slow step of the proposed mechanism agrees with the overall experimentally determined rate law. The IO ⁻ is present as an intermediate in the reaction. The iodide ion catalyst also appears in the mechanism. It is consumed in the first elementary step and then is regenerated in the second step. That is the requirement for a catalyst – that is that it is not used up in the reaction.
  ::拟议机制缓慢步骤的速率法与总体实验确定的速率法一致。 IO - 是反应的中间体。碘离子催化剂也在机制中出现。它先在初级阶段消耗,然后在第二步再生。 这就是催化剂的要求 — — 也就是说,它并没有在反应中被使用。

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  Summary
  ::摘要

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  • The rate-determining step in a reaction is defined.
    ::对反应中的确定速度步骤作了界定。
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  • The process for determining the rate-determining step is described.
    ::报告说明了确定确定确定比率步骤的过程。

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  Review
  ::回顾

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  1. What is the rate-limiting step in a reaction?
     ::反应中限制费率的步骤是什么?
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  2. Can we determine the rate-limiting step simply by looking at the reaction mechanism?
     ::我们能否仅仅通过研究反应机制来确定限制费率的步骤?
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  3. In the hydrogen peroxide example above, why was step 1 determined to be the rate-limiting step?
     ::在以上关于过氧化氢的例子中,为什么第1步被确定为限制费率的步骤?