章节大纲

  • 常规

    全部折叠 展开全部

  • 限制反应

    Two pancakes on a blue plate with syrup, illustrating a cooking example of limiting reactants.

    Don’t you hate running out of cooking ingredients?
    ::你难道不讨厌烹饪原料耗尽吗?

    Cooking is a great example of everyday chemistry . In order to correctly follow a recipe, a cook needs to make sure that he has plenty of all the necessary ingredients in order to make his dish. Let us suppose that you are deciding to make some pancakes for a large group of people. The recipe on the box indicates that the following ingredients are needed for each batch of pancakes:
    ::烹饪是日常化学的一个很好的例子。 为了正确遵循一种配方, 厨师需要确保他有足够的所有必需的成分来做饭。 让我们假设你决定为一大群人做一些煎饼。 盒子上的配方表明每批煎饼都需要以下成分:

    1 cup of pancake mix
    ::1杯煎煎饼

    3 4  cup milk
    ::34杯杯牛奶

    1 egg
    ::1个鸡蛋

    1 tablespoon vegetable oil
    ::1 面粉植物油

    Now you check the pantry and the refrigerator and see that you have the following ingredients available:
    ::现在你们检查储藏室和冰箱,看你们是否有以下可用成分:

    2 boxes of pancake mix (8 cups)
    ::2箱煎饼混合(8个杯)

    Half gallon of milk (4 cups)
    ::半加仑牛奶(4个杯)

    2 eggs
    ::2个鸡蛋

    Full bottle of vegetable oil (about 3 cups)
    ::全瓶植物油(约3杯)

    The question that you must ask is: How many batches of pancakes can I make? The answer is two. Even though you have enough pancake mix, milk, and oil to make many more batches of pancakes, you are limited by the fact that you only have two eggs. As soon as you have made two batches of pancakes, you will be out of eggs and your “reaction” will be complete.
    ::您必须问的问题是:我可以做多少批煎饼?答案是两批。尽管您有足够的煎饼、牛奶和油来做更多的煎饼,但您却受到限制,因为您只有两个鸡蛋。一旦您做了两批煎饼,您就会出蛋,而您的“反应”就会完全完成。

    Limiting Reactant
    ::限制反应

    For a chemist, the balanced chemical equation is the recipe that must be followed. As you have seen earlier, the Haber process is a reaction in which nitrogen is combined with hydrogen gas to form ammonia. The balanced equation is shown below.
    ::对于化学家来说,平衡的化学方程式是必须遵循的配方。 正如你先前所见,哈伯工艺是一种反应,氮与氢气结合形成氨。平衡的方程式如下所示。

    N 2 ( g ) + 3 H 2 ( g ) → 2 NH 3 ( g )

    ::N2(g)+3H2(g)=2NH3(g)

    We know that the coefficients of the balanced equation tell us the that is required for this reaction to occur. One mole of N 2 will react with three moles of H 2 to form two moles of NH 3 .
    ::我们知道,平衡方程式的系数告诉我们发生这种反应所需的系数。 N2的一毫尔将用三毫尔H2的反应形成NH3的两毫尔。

    Now let us suppose that a chemist were to react three moles of N 2 with six moles of H 2 (see Figure ).
    ::现在让我们假设一名化学家会用六枚甲状腺H2来对三枚N2的摩尔作出反应(见图 )。

    Molecular representation illustrating the limiting and excess reactants in a chemical reaction.

    Reaction in presence of limiting reagent.
    ::在限制试剂的情况下作出反应。

    So what happened in this reaction? The chemist started with 3 moles of N 2 . You may think of this as being 3 times as much as the “recipe” (the balanced equation) requires since the coefficient for the N 2 is a 1. However, the 6 moles of H 2 that the chemist started with is only two times as much as the “recipe” requires, since the coefficient for the H 2 is a 3 and 3 × 2 = 6. So the hydrogen gas will be completely used up while there will be 1 mole of nitrogen gas left over after the reaction is complete. Finally, the reaction will produce 4 moles of NH 3 because that is also two times as much as shown in the balanced equation. The overall reaction that occurred in words:
    ::那么,这个反应中发生了什么?化学家从3个N2的摩尔开始,你可能会认为这比N2的系数(平衡方程式)要求的“回收量”高三倍。 但是,化学家开始的6个H2的摩尔只有“回收量”要求的2倍,因为H2的系数是3和3×2=6。所以氢气将完全用完,而反应完成后将剩下1个氮气的摩尔。最后,该反应将产生4个NH3的摩尔,因为这也是平衡方程式所显示的2倍。

    2  mol N 2 + 6  mol H 2 → 4  mol NH 3

    ::2 mol N2+6 mol H2+4 mol NH3

    All the amounts are doubled from the original balanced equation.
    ::所有金额从原来的平衡方程式上翻一番。

    The limiting reactant (or limiting reagent) is the reactant that determines the amount of product that can be formed in a . The reaction proceeds until the limiting reactant is completely used up. In our example above, the H 2 is the limiting reactant. The excess reactant (or excess reagent) is the reactant that is initially present in a greater amount than will eventually be reacted. In other words, there is always excess reactant left over after the reaction is complete. In the above example, the N 2 is the excess reactant.
    ::限制反应器(或限制试剂)是决定一个反应器中可形成的产品数量的反应器。 反应器一直持续到限制反应器完全耗尽为止。 在上文的例子中,H2是限制反应器。 过剩反应器(或过量试剂)是最初存在的反应器,其数量大于最终的反应量。 换句话说, 反应完成后总是有多余反应器。 在以上的例子中, N2是过量反应器。

     

     

    Summary
    ::摘要

    • The amount of limiting reactant determines how much product will be formed in a chemical reaction.
      ::限制反应器的数量决定了在化学反应中将形成多少产品。

    Review
    ::回顾

    1. In the Haber reaction illustrated above, how do we know that hydrogen is the limiting reactant?
      ::在上述Haber反应中,我们怎么知道氢是限制反应剂?
    2. What if hydrogen were left over?
      ::如果氢被丢弃怎么办?
    3. Which material would be limiting if no hydrogen or nitrogen were left over?
      ::如果不留下氢或氮,哪一种物质会受到限制?