Rate Equations (A Level only) (AQA A-Level Chemistry): Revision Notes
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
Example 1:
Mechanism:
Step 1:
Step 2:
Rate Equation:
Since only appears in the rate equation, Step 1 is the rate-determining step.
The hydroxide ion () is zero order, meaning it does not affect the rate and is therefore not part of the RDS.
Example 2:
Mechanism:
Step 1:
Step 2:
Rate Equation:
In this case, both and 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.
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.