Keith Mack
The inclusion of intermediates in rate laws is a topic of discussion in chemistry. Some sources state that intermediates, which are transient substances formed and consumed during a reaction, are not included in the overall rate law expression. However, others suggest that while they may not be present in the overall chemical equation, their concentrations can be significant in specific steps of the reaction, particularly in the rate-determining step, which is the slowest step influencing the overall reaction rate. In such cases, intermediates can appear in the rate law for that particular step. This understanding of intermediates helps chemists accurately predict reaction rates.
| Characteristics | Values |
|---|---|
| Can intermediates be part of rate law? | No, intermediates are transient and form when reactants are forming products. They are not reactants and are produced and then used up by the reaction. |
| Can rate laws include intermediates? | Yes, but only when writing the rate law for a specific reaction (e.g. step 1 or step 2). |
| Can the concentration of an intermediate appear in a rate law? | Yes, when a mechanism includes intermediates that play a role in determining the rate, their concentrations can appear in the rate law for that specific step. |
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What You'll Learn
- Intermediates are generally not included in the final rate law
- Intermediates are transient and form when reactants are forming products
- Intermediates can be included in the rate law for a specific reaction step
- The rate law should only include reactants
- Intermediates can be included in the rate law if they are in the slow step
Intermediates are generally not included in the final rate law
In chemistry, a rate law expresses the relationship between the rate of a reaction and the concentrations of reactants involved in that reaction. While rate laws can incorporate various species, including reactants and, in some cases, intermediates, intermediates are generally not included in the final rate law.
Intermediates are transient species that form when reactants are forming products. They are not reactants and are short-lived, making them difficult to detect. As such, they are typically not included in the overall rate law expression for a complete reaction. Instead, the rate law should only include reactants.
However, it is important to note that intermediates can play a critical role in individual steps of the reaction mechanism. In such cases, their concentrations can appear in the rate law for specific steps where they are involved in the rate-determining step. For example, if a reaction mechanism involves an intermediate, A, transforming into a product, B, the concentration of A could be significant in expressing how fast products are formed and may appear in the rate law for that particular step.
Additionally, when a reaction is not in equilibrium, the rate law only includes reactants in the slow step, which is typically the rate-determining step. In such cases, intermediates are not included in the rate law.
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Intermediates are transient and form when reactants are forming products
In chemistry, a reaction intermediate, or simply an intermediate, is a molecular entity that arises within the sequence of a stepwise chemical reaction. They are transient species within a multi-step reaction mechanism that are produced in an elementary step and consumed in a subsequent step to generate the final reaction product. In other words, intermediates are formed from reactants and/or preceding intermediates but are then consumed in a later step.
For example, in the reaction of ethylene with HBr, the carbocation formed in the first step is a reaction intermediate. This carbocation is distinct from the reactants and is not a transition state or the final product. It exists only transiently during the multistep reaction and is formed in the first step by the reaction of ethylene with H+. In the second step, it further reacts with Br− to yield the final product, bromoethane.
Another example is the glycolysis step reaction in which glucose (GLC) is the initial reactant and is converted through a series of steps to form the final product, pyruvate (PYR). During this process, glucose-6-phosphate is formed as an intermediate. However, it exists for a short time before being consumed in the subsequent reaction and, therefore, does not appear in the overall balanced equation.
Intermediates are typically high-energy, unstable, and short-lived, which makes them difficult to isolate. They are crucial in a variety of biological settings, such as the enzyme reaction intermediate of metallo-β-lactamase, which bacteria employ to gain resistance to antibiotics like penicillin.
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Intermediates can be included in the rate law for a specific reaction step
In the context of chemical kinetics, a rate law expresses the relationship between the rate of a reaction and the concentrations of the reactants involved. While intermediates are generally not included in the overall rate law expression for a complete reaction, they can play a critical role in specific steps of the reaction mechanism.
Intermediates are transient substances formed and consumed during the course of a reaction mechanism. They are not present in the overall chemical equation but can be significant in specific steps, particularly the rate-determining step, which is the slowest step influencing the overall reaction rate. When intermediates are involved in determining the rate, their concentrations can be included in the rate law for that specific step.
For example, consider a reaction mechanism where an intermediate, A, transforms into a product, B. If step 2 includes intermediate A and is the rate-determining step, the concentration of A could be essential in expressing how quickly products are formed. Therefore, the concentration of A may be included in the rate law for that specific step.
However, when writing the overall rate law, intermediates are typically not included, and only the reactants and products that are part of the overall reaction are considered. For instance, in the reaction A + B → C (step 1) followed by C → D (step 2), the overall reaction is A + B → D. Here, C is the intermediate, and its concentration would not be included in the rate law for the overall reaction.
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The rate law should only include reactants
The rate law, or rate equation, is a mathematical expression that describes the relationship between the rate of a chemical reaction and the concentration of its reactants. It is important to note that rate laws are determined experimentally and cannot be predicted by reaction stoichiometry. The rate law takes the form:
> rate = k [A]^m [B]^n
Where k is the rate constant, and [A] and [B] represent the molar concentrations of reactants. The exponents m and n are the reaction orders and are typically positive integers, though they can be fractions, negative, or zero.
The rate constant k is independent of the reactant concentrations but does vary with temperature. The reaction orders in a rate law describe the mathematical dependence of the rate on reactant concentrations. For example, if m = 1 and n = 2, the reaction is first order in A and second order in B. The overall reaction order is the sum of the orders for each reactant.
It is worth noting that a number raised to the power of zero is equal to 1, so the concentration term of a reactant may be omitted from the rate law if its exponent is zero. In such cases, the rate of reaction is solely dependent on the concentration of the remaining reactant(s).
While intermediates are formed during the reaction as reactants are transformed into products, they are transient and are not themselves reactants. Therefore, intermediates generally do not appear in the rate law.
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Intermediates can be included in the rate law if they are in the slow step
In the context of chemical kinetics, a rate law describes the mathematical relationship between the rate of a chemical reaction and the concentrations of the reactants. The rate law is derived from the rate equation, which is determined by the slowest step in a reaction mechanism, also known as the rate-determining step.
Now, let's discuss the role of intermediates in the rate law. Intermediates are molecules or species that are formed during the reaction but are not present in the overall reaction equation. They are transient species that participate in the reaction mechanism but are not reactants or products. In some cases, intermediates can influence the rate of the reaction, especially if they are involved in the slow step or the rate-determining step.
The slow step of a chemical reaction is like the neck of a funnel; it determines the rate at which the overall reaction proceeds. If an intermediate is involved in this slow step, it can significantly impact the reaction rate. In such cases, the intermediate can be included in the rate law to account for its influence on the reaction kinetics.
However, it is important to note that not all intermediates are included in the rate law. Intermediates that are formed and consumed rapidly in a fast step of the reaction mechanism may not have a significant effect on the overall reaction rate. In such cases, they are usually not included in the rate law.
To determine whether an intermediate should be included in the rate law, one must analyze the reaction mechanism and identify the slow step. If the intermediate is involved in the slow step and influences the reaction rate, it can be considered for inclusion in the rate law. This ensures that the rate equation accurately represents the kinetics of the reaction, taking into account the role of the intermediate.
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Frequently asked questions
No, intermediates are transient and form when reactants are forming products, so they cannot be included in a rate law.
Intermediates are produced and then used up by the reaction. They are not present in the overall chemical equation, only in specific steps of the reaction.
When a mechanism includes intermediates that play a role in determining the rate, their concentrations can appear in the rate law.
Rate laws express the relationship between the rate of a reaction and the concentrations of reactants involved. They can be determined experimentally and can include various species, including reactants and intermediates.
