Cecily Zamora
In a chemical reaction, the rate law is determined by the slowest step, also known as the rate-determining step. This means that if the first step of a reaction is slow and the second step is fast, the overall rate of the reaction will be determined by the first step. The second step, despite being faster, cannot influence the overall rate because the first step limits how quickly reactants can be converted into products. As a result, the rate law for the overall reaction will match the rate law of the first step, with the substances involved in the first step influencing the overall rate law.
| Characteristics | Values |
|---|---|
| Overall rate | Determined by the slower step |
| Rate law | Depends on the concentration of reactants |
| Rate-determining step | The slowest step of a chemical reaction that determines the speed (rate) at which the overall reaction proceeds |
| Substances in the fast step | Do not appear in the overall rate law |
| Substances in the slow step | Appear in the rate law equation |
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What You'll Learn
- The rate law for the overall reaction will match the rate law of the first step
- The overall rate of the reaction depends on the slow step
- The slow step is known as the rate-determining step
- The substances in the fast step do not influence the overall rate law
- The overall rate law of a reaction describes how the rate depends on reactant concentration
The rate law for the overall reaction will match the rate law of the first step
The rate law for a chemical reaction is determined by the slowest step in the reaction, also known as the rate-determining step. This is because the slowest step limits how quickly reactants can be converted into products, controlling the rate of the entire reaction.
Consider a reaction with two steps: A → B (slow) and B → C (fast). The overall rate of this reaction is determined by the slower step (A → B). Therefore, the rate law will focus on the reactant A. Even though B is converted to C quickly, the overall rate of transforming A into C is limited by how quickly A can be converted to B.
Research on reaction kinetics indicates that in multi-step reactions, the rate law for the overall reaction will match the rate law of the slowest step. This slowest step can be compared to the neck of a funnel. The rate at which water flows through a funnel is determined by the width of the neck and not by the rate at which the water is poured. Similarly, the slow step of a reaction determines the rate of the overall reaction.
The rate equation is derived by the slowest step in the reaction. For example, consider the reaction: NO2 + F2 → NO2F + F. The rate equation for this reaction is rate = k1 [NO2] [F2], where k1 is the rate constant for the slowest step. In this case, the rate law expression includes only the reactants involved in the rate-determining step.
In summary, when the first step of a reaction is slow and the second step is fast, the rate law for the overall reaction will match the rate law of the first step. This is because the first step is the slowest step, controlling the rate of the entire reaction. The rate equation is derived from the slowest step, and the rate law expression includes only the reactants involved in this step.
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The overall rate of the reaction depends on the slow step
The rate at which a chemical reaction proceeds is determined by its slowest step, also known as the rate-determining step. This step can be likened to the neck of a funnel; the rate at which water flows through the funnel is determined by the width of the neck, not by how quickly water is poured in.
In a multi-step reaction, the rate-determining step is the one that takes the longest to complete. For example, consider a reaction with two steps: A → B (slow) and B → C (fast). The overall rate of this reaction is determined by the slower first step, A → B, and so the rate law will focus on the reactant A. Even though B is converted to C quickly, the overall rate of transforming A into C is limited by how quickly A can convert to B.
The rate equation is derived from the slowest step in the reaction. Using the example above, the rate equation would be written as rate = k1 [A] where k1 is the rate constant of the first step.
The rate-determining step is not always the first step of a reaction. In some cases, the second step may be the rate-determining step if it is slower than the reverse of the first step. In this case, the overall rate is determined by the rate of the second step, as very few molecules that react in the first step continue to the much slower second step.
It is important to note that not all reactions have a rate-determining step. A reaction will only have a rate-determining step if one step is significantly slower than the other steps.
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The slow step is known as the rate-determining step
In a chemical reaction, the rate-determining step is the slowest step that dictates the overall reaction rate. This is comparable to the neck of a funnel, where the rate of water flow is determined by the width of the neck rather than the pouring speed. For instance, consider a two-step reaction mechanism: A → B (slow) and B → C (fast). The overall rate depends on the slower A → B step, making it the rate-determining step. The rate law, therefore, focuses on the reactants involved in this slower step.
In a multi-step reaction, the slowest step, or the rate-determining step, sets the pace for the entire reaction. This step controls the conversion rate of reactants into products. For example, if the conversion of A to B is slow, and B to C is fast, the overall rate of converting A to C is limited by the A to B conversion. Thus, the rate law for the overall reaction matches the rate law of the first step, including only reactants from the rate-determining step.
The rate-determining step is crucial in understanding the overall rate law of a reaction. While the slow step influences the rate law, the subsequent fast step does not. This is because the substances in the fast step quickly reach equilibrium. As a result, only the reactants and intermediates in the slow step appear in the rate law equation. By studying the rate laws and mechanisms, chemists can predict reaction rates under various conditions, which is essential for industries relying on chemical processes.
The rate-determining step is identified as the slowest step within a chemical reaction. It is determined by setting up reaction mechanisms and understanding the overall reaction order. The overall reaction order is the sum of the orders of each individual reactant, and it influences how changes in reactant concentrations affect the reaction rate. For instance, in a zero-order reaction, the rate is independent of any changes in reactant concentrations. On the other hand, a first-order reaction depends on the concentration of a single reactant species.
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The substances in the fast step do not influence the overall rate law
In a chemical reaction, the rate law is determined by the slowest step, also known as the rate-determining step. This is because the slowest step limits how quickly reactants can be converted into products. For instance, consider a reaction mechanism with two steps: A → B (slow) and B → C (fast). The overall reaction rate is determined by the slower step (A → B), and hence the rate law focuses on A.
Each elementary step in a reaction mechanism represents a single event at the molecular level, which can involve the breaking and making of chemical bonds. These steps are inferred from experimental data and the reaction's overall behavior. Each elementary step has its own distinct rate law, which depends only on the species involved in that particular step.
For example, consider the reaction mechanism: NO2 + F2 → NO2F + F. Here, the rate equation is derived from the slowest step, which is elementary step one. The rate equation is: rate = k1[NO2][F2]. In this case, the substances in the fast step, such as NO2F and F, do not appear in the rate law equation.
In summary, the overall rate law of a reaction describes how the rate of the reaction depends on the concentration of reactants. It is derived from the rate-determining step, which is the slowest step in the reaction mechanism. The substances in the fast step do not influence the overall rate law because they quickly reach equilibrium, and the rate is determined by the slower step.
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The overall rate law of a reaction describes how the rate depends on reactant concentration
The rate of a chemical reaction is influenced by the concentrations of the reactants involved. This relationship is described by the rate law, also known as the rate equation. It is a mathematical expression that provides insight into the instantaneous rate of the reaction. The rate equation is typically written as:
> Rate = k[A]^m[B]^n[C]^p
Here, [A], [B], and [C] represent the molar concentrations of the reactants, and k is the rate constant, which is specific to a particular reaction at a given temperature. The exponents m, n, and p are usually positive integers, but they can also be fractions or negative numbers. These values must be determined experimentally by observing how the reaction rate changes as reactant concentrations are varied.
The overall rate law of a reaction, therefore, depends on how the reactant concentrations influence the rate. The rate equation can be of different orders, such as zero, first, second, or even negative orders. For example, if the exponent m is 1, the reaction is first order with respect to reactant A, meaning that a direct relationship exists between the concentration of A and the reaction rate. If m is 0, the reaction is zero order in A, indicating that changes in A's concentration have no impact on the reaction rate.
In multi-step reactions, the overall rate law is determined by the slowest step, also known as the rate-determining step. This step sets the pace for the entire reaction. For instance, consider a reaction with two steps: A → B (slow) and B → C (fast). The overall rate depends on the slower step (A → B), and thus, the rate law focuses on the concentration of reactant A.
In summary, the overall rate law of a reaction is influenced by the reactant concentrations, as described by the rate equation. The rate equation's form, including the exponents and rate constant, provides insight into how changes in reactant concentrations impact the reaction rate. In multi-step reactions, the slowest step determines the overall rate law, as it controls the pace of the reaction.
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Frequently asked questions
A rate law describes how the rate of a chemical reaction depends on the concentration of reactants.
The rate-determining step is the slowest step of a chemical reaction that determines the speed at which the overall reaction proceeds.
The overall rate law of a reaction is derived from the rate-determining step. The reactants and intermediates involved in the slow step appear in the rate law equation, while substances from the fast step do not directly influence the rate law.
In this case, the rate law for the overall reaction will match the rate law of the first step, which is the rate-determining step. The overall reaction rate is determined by the slowest step.
Consider the reaction mechanism: A → B (slow) and B → C (fast). The overall rate is determined by the slower step (A → B), so the rate law focuses on the reactants involved in this step.
