1、1Chemical Equilibrium and Kinetics化学平衡和动力学A major objective of chemist is to understand chemical reactions, to know whether under a given set of conditions two substances will react when mixed, to determine whether a given reaction will be exothermic or endothermic, and to predict the extent to whic
2、h a given reaction will proceed before equilibrium is established. 化学家的一个主要目标是理解化学反应，知道在给定的一组条件下两种物质混合时能否发生反应；确定一个给定的反应放热还是吸热；预测一个给定的反应在平衡前进行的程度。An equilibrium state, produced as a consequence of two opposing reactions occurring simultaneously, is a state in which there is no net change as long as t
3、here is no change in conditions. 两个相反的反应同时进行时的状态叫做平衡状态，在平衡状态下，只要条件不发生改变，这个状态就不会有净变化。In this lesson it will be shown how one predict the equilibrium state of chemical systems from thermodynamic data, and conversely how the experimental measurements on equilibrium states provide useful thermodynamic d
4、ata. 在这节课中，将会展示如何根据热力学数据来预测化学体系的平衡状态，以及相反地，平衡状态的实验测量如何提供有用的热力学数据。Thermodynamics alone cannot explain the rate at which equilibrium is established, nor does it provide details of the mechanism by which equilibrium is established. 单纯的热力学无法解释建立平衡的速度，也不能提供平衡建立所遵循的机理信息。Such explanations can be developed
5、from considerations of the quantum theory of molecular structure and from statistical mechanics. 从分子结构的量子理论和统计力学的角度考虑可以使这样的解释得到发展。To appreciate fully the nature of the chemical equilibrium state, it is necessary first to have some acquaintance with the factors which influence reaction rates. 要充分理解化学
6、平衡状态的性质，首先必须熟知影响反应速率的因素。The factors which influence the rates of a chemical reaction are temperature, concentrations of reactants(or partial pressures of gaseous reactants), and presence of a catalyst. 2影响化学反应速率的因素有温度、反应物浓度（气体反应物的分压）和催化剂。In general, for a given reaction the higher the temperature, t
7、he faster the reaction will occur. 一般来说，对于一个给定的反应，温度越高，反应速率越大。The concentrations of reactants or partial pressure of gaseous reactants will affect the rate of reaction, an increase in concentration or partial pressure increases the rate of most reactions. 反应物浓度或气体反应物的分压影响反应的速率，对于大部分反应来说，增大浓度或分压，反应速率
8、增大。Substances which accelerate a chemical reaction but which themselves are not used up in the reaction are called catalysts. 可以加快反应速率但本身不参加反应的物质称为催化剂。Dynamic Equilibrium动态平衡In many cases, direct reactions between two substances appear to cease before all of either starting material is exhausted. 在许
9、多情况下，两种物质间的直接反应在所有原始物料耗尽之前就会停止。Moreover, the products of chemical reactions themselves often react to produce the starting materials. 此外，化学反应自身的产物能够发生反应生成原始物料。For example, nitrogen and hydrogen combine at 500 in the presence of a catalyst to produce ammonia:N2+3H2=2NH3例如，在催化剂存在下，氮和氢在 500能够反应生成氨：N2+3
10、H2=2NH3At the same temperature and in the presence of the same catalyst, pure ammonia decomposes into nitrogen and hydrogen: 2NH3=3H2+ N2在相同温度和相同催化剂存在下，纯净的氨能够分解生成氮和氢：2NH3=3H2+ N2For convenience, these two opposing reactions are denoted in one equation by use of a double arrow: 323NHA为了方便起见，这两个反应能够用双
11、箭头表示在一个方程式中： 23The reaction proceeding toward the right is called the forward reaction; the other is called the reverse reaction. 向右进行的反应称为正反应，另外一个称为逆反应。If either ammonia or a mixture of nitrogen and hydrogen is subjected to the above condition, a mixture of all the three gases will result. 如果将氨或氮气和
12、氢气的混合物放在上述条件下，将都会产生三种气体的混合物。The rate of reaction between the materials which were introduced into the reaction vessel will decrease after the reaction starts; because their concentrations are decreasing, conversely, after the start of the reaction the material being produced will react faster, since
13、 there will be more of it. 在反应开始后，放入反应器内的材料之间的反应速率将会减小；因为它们的浓度在减小，相反，在反应开始后，产物之间的反应将会加快，由于它们的浓度在增加。Thus the faster forward reaction becomes slower, and the slower reverse reaction speeds up. 因此，较快的正反应变慢，较慢的正反应加快。Ultimately the time comes when the rates of the forward and reverse reactions become equ
14、al, and there will be no further net change. 最后，正反应和逆反应速率变得相等，且不再有进一步净变化的时刻终于到来。This situation is called equilibrium. 这种状态称为平衡状态。Equilibrium is a dynamic state because both reactions are still proceeding; but since the two opposing reactions are proceeding at equal rates, no net change is observed.
15、平衡是动态的，因为反应仍然在进行；但是由于两个相反方向的反应以相同的速率进行，所以没有净变化被发现。4All chemical reactions ultimately proceed toward equilibrium. 所有的化学反应最后都要向平衡状态进行。In a practical sense, however, some reactions go so far in one direction that the reverse reaction cannot be detected, and they are said to go to completion. 然而，实际上，一些反
16、应在一个方向上进行的很快以至于检测不到逆反应，据此可说它们进行完全了。The principles of chemical equilibrium apply even to these, and it will be seen that for many of them, the extent of reaction can be expressed quantitatively. 化学平衡的原理甚至也适用于被认为是进行完全的反应，显然许多这类反应其反应程度可以定量地表达。Equilibrium Constants平衡常数Equilibrium is a state of dynamic b
17、alance between two opposing processes. For a general reaction at a given temperature:A+B=C+D平衡是两个相反过程的动态平衡的一种状态。在给定温度下，对于一个普通的反应：A+B=C+DAt the point of equilibrium, the following ratio must be a constant: CDABK在平衡时，以下比值肯定是一个常数：The constant, K, is called the equilibrium constant of the reaction. 常数 K
18、 称为反应的平衡常数。It has a specific value at a given temperature. 在一个给定温度下，它有一个比值。If the concentration of any of the components in the system at equilibrium is changed, the concentrations of the other components will change in such a manner that the defined ratio remains equal to K as long as the temperatu
19、re does not change. 如果平衡时体系中任意组分的浓度改变，那么只要温度不变，其他组分的浓度5将以保持相等 K 的固定比例的方式变化。The equilibrium constant expression quantitatively defines the equilibrium state. 平衡常数表达式定量地定义了平衡状态。More generally, for the reversible reactionaAbBcCdD更一般地说，对于一个可逆反应the equilibrium constant expression is written as follows:cd
20、abCDABK平衡常数表达式可以写成： cdabBy convention, the concentration terms of the reaction products are always placed in the numerator of the equilibrium constant expression. 按照惯例，反应产物浓度项通常是放在平衡常数表达式的分子上。It should be noted that the exponents of the concentration terms in the equilibrium constant expression are
21、the coefficients of the respective species in the balanced chemical equation. 应该注意的是，平衡常数表达式中浓度项的指数是平衡化学方程式中各个物种的系数。Chemical Kinetics化学动力学When a system is in the equilibrium state, the rate of the forward reaction is identical to the rate of the reverse reaction. 当一个体系达到平衡状态时，正反应的反应速率等于逆反应的反应速率。It i
22、s important to know just how fast a reactant is being used up in a process, or the speed with which a product is being formed. 重要的是，要知道在反应过程中，反应物消耗完的速率，或者产物生成的速率。6It is also important to have detailed information about rates of reactants in order to test theories and mechanisms for various kinds of
23、chemical processes. 同样重要的是，要有关于反应物反应速率的详细信息，以便测试各种化学过程的理论和机制。Experiments show that a number of reaction variables affect reaction rates: 实验表明，许多反应变量都影响反应速率：Temperature. The rates of chemical reactions are temperature-dependent. Therefore, it is common practice when studying the rate in the laborator
24、y to carry out reactions at constant temperature(isothermally), thus eliminating one variable. 温度。化学反应速率随温度变化。因此，常见的做法是在实验室、在恒温下研究反应的反应速率（等温地） ，这样可以消除一个变量。Pressure and Volume. Pressure is important in a kinetic consideration of gas phase reactions. Usually, volume is fixed by running the reaction in
25、 container of fixed dimensions. For solid and liquid state reactions, pressure is usually atmospheric, and the volume of the reacting system is relatively unimportant because there is little change in volume. 压力与体积。压力是气相反应动力学的一个重要的影响因素。通常在固定尺寸的容器中反应时，反应体积是固定的。对于固体和液体反应，压力通常是大气压，反应体系的体积是相对不重要的，由于反应前后
26、它的体积变化很小。Concentration. At any particular temperature, the rates of most chemical reactions are functions of the concentrations of one or more of the components of the system. In practice, it is usually the concentration of the reactants that are used in determining the overall rates of reaction. 浓度
27、。在任何特定的温度下，大部分化学反应的速率是体系中一个或者多个组分的浓度的函数。实际上，一般正是反应物的浓度用以测定反应的总速度。Catalyst. Any substance that affects the rate of a chemical reaction but cannot be identified as a product or reactant is said to be a catalyst. Catalyst may accelerate the rate, but we usually refer to decelerating catalysts as inhibi
28、tors. 催化剂。影响化学反应速率，但既不是产物，也不是反应物的物质称为催化剂。催化剂可以加快反应速率，但是，通常，我们把减慢反应速率的催化剂称为抑制剂。The order of a chemical reaction is given by the number of atomic or molecular species whose concentrations directly determine the reaction rate. 7化学反应级数是浓度直接影响反应速率的原子或分子物种的数目。The rate of hydrolysis of acetate in water is directly proportional to the ethyl acetate concentration, the reaction is said to be first order. 水中醋酸盐的水解率与乙酸乙酯的浓度成正比，所以说，反应是一级反应。
