13.3
浓度和速率定律
反应速率受反应物浓度的影响。 速率定律 (差分速率定律) 或率方程式是描述化学反应速率与其反应物的浓度之间关系的数学表达式。
例如，在一般反应aA + bB ⟶ 生成物，其中 a 和 b 是化学计量系数，速率定律可写为：
[A] 和 [B] 代表反应物的摩尔浓度，k 是速率常数，它是特定温度下特定反…
反应速率通常取决于 反应物的浓度。对于任何反应，反应速率和反应物浓度之间的关系 可以使用速率定律或速率方程 以数学方式表示。在速率定律中，k 是比例常数，或速率常数，n 是 单个反应物的反应级数，其值通常 是整数。在多反应物反应速率定律中，总反应级数是所有反应物反应级数的总和。对于每种反应物，均通过实验确定 反应速率、速率常数、浓度和反应级数。速率定律表达了 所有这些参数之间的关系。单个反应物的反应级数通常 取值为 0、1 或 2，根据总反应级数，化学反应可分为 零级、一级或二级反应。单一反应物或单分子化学反应，其反应速率在整个反应过程中保持不变，为零级反应。零级反应的反应物的反应阶数为零，根据速率定律，反应物浓度的幂指数 为零。由于任何数值的零次幂均为 1，因此零级反应的反应速率 等于速率常数，与反应物浓度无关。因此，在零级反应中，即使反应物浓度降低，反应速度也不会减慢。反应速率与反应物浓度成正比的 单分子化学反应 是一级反应。一级反应的反应物的反应级数为 1，根据速率定律，反应物浓度的幂指数 是 1。由于任何数值的 1 次幂保持相同，因此一级反应的反应速率 直接取决于反应物的浓度。随着反应物浓度降低，反应速率以线性方式 成比例地降低。反应速率取决于反应物浓度的平方的 单分子化学反应 是二级反应。反应物的反应级数为 2，反应物浓度的幂指数 是 2。因此，二级反应中的反应速率 直接取决于反应物浓度的平方。随着反应物浓度的降低，反应速率呈 二次幂指数下降。
View the full transcript and gain access to JoVE Core videos
Q1: What is a rate law and how does it relate to reactant concentration?
A rate law is a mathematical expression describing the relationship between reaction rate and reactant concentrations. It expresses how the rate depends on the concentration of each reactant raised to a specific power, called the reaction order. The rate law includes the rate constant k, which is specific to a particular reaction at a given temperature, and the reaction orders for each reactant, which are determined experimentally.
Q2: How are reaction orders and the rate constant determined experimentally?
Reaction orders and the rate constant are determined by observing how the reaction rate changes as reactant concentrations vary. A common experimental approach is the method of initial rates, which involves measuring reaction rates using different initial reactant concentrations. Comparing these measured rates allows chemists to determine the reaction orders and subsequently calculate the rate constant to formulate the rate law.
Q3: What happens to the reaction rate in a zero-order reaction as reactant concentration decreases?
In a zero-order reaction, the reaction rate remains constant throughout and is independent of reactant concentration. Since the reactant concentration is raised to the zeroth power in the rate law, any number raised to zero equals one, making the rate equal to the rate constant alone. Therefore, even as the reactant concentration decreases, the reaction rate does not slow down.
Q4: How does reactant concentration affect the rate of a first-order reaction?
In a first-order reaction, the reaction rate is directly and linearly proportional to reactant concentration. The reactant concentration is raised to the first power in the rate law, so the rate depends directly on the concentration value. As the reactant concentration decreases, the reaction rate decreases proportionally in a linear manner.
Q5: Why does a second-order reaction rate decrease exponentially as concentration drops?
In a second-order reaction, the reactant concentration is raised to the second power in the rate law, making the rate dependent on the square of the concentration. This quadratic relationship means small changes in concentration produce larger changes in rate. As reactant concentration decreases, the reaction rate decreases exponentially rather than linearly, following the squared concentration term.
Q6: What is the overall reaction order and how is it calculated?
The overall reaction order is the sum of the individual reaction orders for all reactants in the rate law. For example, in a reaction that is first-order in reactant A and second-order in reactant B, the overall reaction order is three (1 + 2 = 3). The overall order determines the mathematical dependence of the reaction rate on all reactant concentrations combined.
Q7: Can reaction orders be predicted from the stoichiometric coefficients of a reaction?
No, reaction orders cannot be reliably predicted from stoichiometric coefficients. Rate laws are determined by experiment only and must be established through direct observation of how reaction rates change with concentration. Reaction orders can be integers, fractions, negative, or zero, and this experimental determination is essential for accurately describing reaction kinetics.