5.2.8 Rate Determining Step

What is the Rate-Determining Step (RDS)?

In a multi-step reaction mechanism, the rate-determining step (RDS) is the slowest step that controls the overall reaction rate. Understanding this step is crucial because only the reactants involved in the rate-determining step appear in the rate equation with non-zero orders.

Characteristics of the Rate-Determining Step

Slowest Step: The rate-determining step has the slowest rate and limits the speed of the entire reaction.
Rate Equation: The rate equation reflects the species involved in this step. Reactants with zero order do not participate in the RDS.
Reaction Mechanism: Each reaction has a specific sequence of steps, but only those involved in the RDS affect the observed rate.

Determining the Rate-Determining Step

To find the rate-determining step:

Analyze the Rate Equation: Identify the reactants in the rate equation and their orders.
Identify Mechanism Steps: Compare each step in the proposed mechanism with the species in the rate equation.
Match Orders to Species: If a reactant in the overall equation is zero order (not in the rate equation), it does not appear in the rate-determining step.

Examples

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Example 1:

\text{C}_4\text{H}_9\text{Br} + \text{OH}^- \rightarrow \text{C}_4\text{H}_9\text{OH} + \text{Br}^-

Mechanism:

Step 1:

\text{C}_4\text{H}_9\text{Br} \rightarrow \text{C}_4\text{H}_9^+ + \text{Br}^-

Step 2:

\text{C}_4\text{H}_9^+ + \text{OH}^- \rightarrow \text{C}_4\text{H}_9\text{OH}

Rate Equation:

\text{rate} = k[\text{C}_4\text{H}_9\text{Br}]

Since only $\text{C}_4\text{H}_9\text{Br}$ appears in the rate equation, Step 1 is the rate-determining step.

The hydroxide ion ( $\text{OH}^-$ ) is zero order, meaning it does not affect the rate and is therefore not part of the RDS.

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Example 2:

\text{X} + 2\text{Y} \rightarrow \text{XY}_2

Mechanism:

Step 1:

\text{X} + \text{Y} \rightarrow \text{XY}

Step 2:

\text{XY} + \text{Y} \rightarrow \text{XY}_2

Rate Equation:

\text{rate} = k[\text{X}][\text{Y}]

In this case, both $\text{X}$ and $\text{Y}$ appear in the rate equation in a 1:1 ratio, indicating that Step 1 is the rate-determining step.

This step involves one molecule of each reactant, matching the order in the rate equation.

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Key Points for Understanding the Rate-Determining Step

Only Species in Rate Equation Affect Rate: Reactants with zero order are not involved in the RDS.
Matching Mechanism to Rate Equation: The coefficients in the rate equation guide you to the RDS by showing which species and how many of each participate in this step.

Rate Determining Step Simplified Revision Notes for A-Level AQA Chemistry

5.2.8 Rate Determining Step

What is the Rate-Determining Step (RDS)?

Characteristics of the Rate-Determining Step

Determining the Rate-Determining Step

Examples

Key Points for Understanding the Rate-Determining Step

500K+ Students Use These Powerful Tools to Master Rate Determining Step For their A-Level Exams.

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