13.1
反应速率
反应速率是每单位时间反应物或生成物的数量的变化。 因此，可以通过测量与反应物或生成物金额相关的某些属性的时间依赖性来确定反应率。 例如，消耗或产生气体物质的反应速率是通过测量体积或压力的变化而方便地确定的。
随着时间的推移，反应物和产品浓度变化的数学表示是反应的速率表达。 括号表示摩尔浓度，符号 d…
化学反应涉及将反应物化学转化 为产物。随着反应的进行，反应物的浓度 降低而产物的浓度 增加。反应物和产物浓度的这种变化 可以作为时间的函数绘制在图表中。反应进行的速度 称为反应速率。它可衡量反应物消失速率 或产物出现速率，并以 M/s 为单位表示。平均反应速率可以 根据特定时间段内 反应物或产物的浓度变化来计算。摩尔浓度值用括号表示，t 代表一段时间，Δ 符号表示"变化"由于在化学反应过程中反应物被耗尽，因此反应物的浓度变化值 始终为负。因此，根据反应物计算的反应速率 加上负号，从而使总值为正。在整个反应过程中，反应速率是不均匀的。反应开始时 即"时间零点"的速率称为初始反应速率。随着反应物浓度的降低，反应速率降低，或反应变慢。给定时间点的反应速率 或瞬时反应速率 是通过计算在感兴趣时间点 反应曲线（反应物或产物）的切线的斜率 来衡量的。对于所有反应物和产物，在特定时间点的斜率值或瞬时速率 均相等。化学反应的反应速率也反映了 反应物和产物的 实际化学计量系数。因此，对于任何平衡反应，其中 a 摩尔 A 与 b 摩尔 B 反应，生成 c 摩尔 C 和 d 摩尔 D，反应速率可以用以下通式表示和计算：反应物 A 的浓度随时间变化的负 1/a 倍，等于反应物 B 的浓度随时间变化的 负 1/b 倍。这些值与产物 C 的浓度 随时间变化的正 1/c 倍相同，与产物 D 的浓度随时间变化的 正 1/d 倍相同。确定反应速率是研究 化学动力学的基础，这有助于 了解药物、催化剂 或合成反应的速度，从而可更好地进行控制并优化其功能。
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Q1: How is reaction rate defined and measured in chemistry?
Reaction rate measures the speed at which reactants disappear or products appear during a chemical reaction, expressed in molarity-per-second. It is calculated from the change in concentration of reactants or products over a specific time period. The average reaction rate can be determined by dividing the change in molar concentration by the change in time, using the formula: −Δ[concentration]/Δt for reactants or +Δ[concentration]/Δt for products.
Q2: Why is a negative sign used when calculating reaction rate from reactant concentration?
A negative sign is applied to reactant concentration changes because reactant concentrations decrease as a reaction progresses, making the change value negative. By convention, reaction rates are expressed as positive quantities. Multiplying the negative concentration change by −1 converts the rate to a positive value, ensuring consistent representation across all reaction rate calculations.
Q3: What is the difference between initial reaction rate and instantaneous reaction rate?
The initial reaction rate is the rate at time-zero, when the reaction begins. The instantaneous reaction rate is the rate at any specific moment during the reaction, calculated by finding the slope of a tangent line drawn to the concentration-versus-time curve at that point. As reactant concentrations decrease, the instantaneous reaction rate also decreases, showing that reactions slow down over time.
Q4: How does stoichiometry relate to reaction rate expressions?
Reaction rates reflect the stoichiometric coefficients of reactants and products in a balanced equation. For a reaction where a moles of A react with b moles of B to produce c moles of C, the rate expressions −1/a × Δ[A]/Δt, −1/b × Δ[B]/Δt, +1/c × Δ[C]/Δt, and +1/d × Δ[D]/Δt are all equal. This relationship ensures that rate values are consistent regardless of which reactant or product is used to express the rate.
Q5: How can you determine the instantaneous rate from a concentration-time graph?
The instantaneous rate at any time point is determined by calculating the slope of a tangent line drawn to the concentration-versus-time curve at that specific moment. The slope value represents the rate of change in concentration at that instant. This graphical method allows you to find the instantaneous rate at any time, including the initial rate at t = 0 and rates at later time points as the reaction proceeds.
Q6: Why does reaction rate change as a chemical reaction progresses?
Reaction rate is not uniform throughout a reaction because it depends on reactant concentration. As reactants are consumed, their concentrations decrease, which reduces the frequency of molecular collisions and slows the reaction. This is why the initial reaction rate is fastest and the rate decreases over time. Understanding this concentration dependence is essential for studying chemical kinetics and optimizing reaction conditions.
Q7: How can average reaction rate be calculated from experimental data?
Average reaction rate is calculated by measuring molar concentrations at the beginning and end of a time interval, then dividing the change in concentration by the change in time. This method provides a single rate value for the entire period. For more precise rate information at specific moments, the instantaneous rate can be determined using the integrated rate law the dependence of concentration on time, which relates concentration changes to specific time points.