Question

Which of the following statements are true about reaction mechanisms? (I) A rate law can be written from the molecularity of the slowest elementary step. (II) The final rate law can include intermediates. (III) The rate of the reaction is dependent on the fastest step in the mechanism. (IV) A mechanism can never be proven to be the correct pathway for a reaction. a. I and II b. I and IV c. II and III d. I, II and III

Short answer

b. I and IV

Step-by-step solution

Understanding Statement I

Statement I claims that a rate law can be written from the molecularity of the slowest elementary step. In a reaction mechanism, the slowest step, often called the rate-limiting step, determines the overall rate law. Therefore, statement I is true because the rate law is often consistent with the molecularity of this step.

Analyzing Statement II

Statement II says the final rate law can include intermediates. In reality, intermediates should not appear in the overall rate law. Instead, they are usually substituted with expressions involving reactants or products. Thus, statement II is false.

Evaluating Statement III

Statement III suggests that the reaction rate depends on the fastest step. Normally, the slowest step determines the reaction rate because it acts as a bottleneck. Therefore, statement III is false.

Assessing Statement IV

Statement IV states that a mechanism can never be proven as the correct pathway for a reaction. While experimental data can support a mechanism, it cannot prove that it is the only or entirely correct pathway. Thus, statement IV is true.

Key concepts

Rate Law

The rate law of a reaction is a mathematical expression that relates the rate of the reaction to the concentration of its reactants. It is derived from the reaction mechanism, which is a series of steps that describe the pathway from reactants to products.

In many cases, the rate law is determined by the slowest step, known as the rate-limiting step. This step is critical because it acts like a bottleneck, controlling the speed of the entire process. The molecularity of this step — which is the number of molecules involved — helps define the rate law.

For example, if the slowest step is bimolecular (involves two molecules colliding), the rate law for that step might look like:
\[ ext{Rate} = k[A][B] \]
Where \([A]\) and \([B]\) are the concentrations of the reactants involved, and \(k\) is the rate constant. Understanding the rate law helps predict how changing the concentration of reactants will affect the reaction rate.

Elementary Step

An elementary step is an individual reaction in a sequence of reactions that constitute a reaction mechanism. Each step describes a single event, such as a molecule colliding with another molecule or a molecule breaking apart.

Elementary steps are important because they provide insight into how a reaction progresses on a molecular level. Unlike the overall reaction equation, an elementary step can show the precise molecular interactions that convert reactants into products.

These steps can be characterized by their molecularity:

• Unimolecular step: Involves a single molecule decomposing or rearranging.
• Bimolecular step: Involves a collision between two molecules.
• Termolecular step: Involves the simultaneous collision of three molecules, which is rare due to low probability.

The sum of elementary steps gives the net chemical change represented by the overall reaction equation. Understanding each step helps chemists propose accurate reaction mechanisms and predict the behavior of chemical systems.

Reaction Intermediates

Reaction intermediates are species formed during the reaction mechanism but do not appear in the overall balanced equation. They are produced and consumed in the steps of a reaction, almost acting as temporary compounds.

Intermediates can provide a lot of information about a reaction's mechanism, although they should not appear in the overall rate law. Instead, their concentration in the rate law expression is often replaced by reactants or products concentrations using alternative equations derived from the mechanism.

Detecting intermediates often requires advanced techniques like spectroscopy or rapid sampling methods since these species are usually very unstable and short-lived. Understanding intermediates is crucial for chemists to map out the entire mechanism pathway effectively.

Rate-Limiting Step

The rate-limiting step is the slowest step in a reaction mechanism and determines the overall reaction rate. It's an essential concept because, regardless of how fast other steps occur, the rate-limiting step controls the pace at which the reaction can proceed.

Consider a series of dominos standing in a line: if one domino (representing an elementary step) is slower to fall than the others, it dictates the rate at which the entire line can collapse.

Understanding the rate-limiting step helps chemists optimize reactions by identifying conditions to speed up this particular step, using catalysts, or changing reaction conditions to make it more favorable.

When deriving the rate law, the focus is given to the molecularity and reactants associated with this step. By improving our understanding of the rate-limiting steps, chemists can improve industrial processes or develop new strategies for drug synthesis.