Rate Determining Step
Lajoy Tucker & Dr. Davinder Bhachu
Teachers
Contents
Rate-Determining Step
Introduction
The rate-determining step (RDS) or slow step is the slowest step in a multi-step reaction mechanism and limits how quickly the overall reaction proceeds.
Example
When making a cup of tea, the slowest step is boiling the water. No matter how quickly you can get everything else ready, the rate at which you can make that cup of tea is determined by this slow step.
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Rate Determining Steps Explained
Rate Determining Steps Key Concepts
1. Reaction Mechanism
Many reactions occur in multiple steps, forming a reaction mechanism.
Each step has its own rate, and the slowest step is the rate-determining step because it controls the overall reaction rate.
2. Molecularity
Each step can include one or more species
Unimolecular (one molecule) -
Bimolecular (two molecules) - or
Termolecular (three molecules) -
3. Rate Equation and Reaction Order
The rate equation relates the reaction rate to the concentration of reactants:
Rate =
where:is the rate constant.
and are the orders of reaction, showing how reactant concentrations affect the rate.
The order of reaction indicates how a reactant’s concentration influences the rate:
Zero-order: The rate is independent of that reactant. It will not appear in the rate equation because raising anything to the power of equals .
First-order: The rate is directly proportional to the reactant’s concentration.
Second-order: The rate is proportional to the square of the reactant’s concentration.
4. Identifying the Rate-Determining Step
The reactants and catalysts involved up to and including the rate-determining step appear in the rate equation.
The order of reaction indicates the number of molecules involved in the slow step.
Example
If the rate equation is Rate = , this suggests that two molecules are involved in the slow step, and no other reactants are present in the rate-determining step.
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Application: Using the Rate Equation to Determine the RDS
Example: Reaction Between and
If the rate equation is Rate , this means that two molecules are involved up to and including the rate-determining step, not .
A possible mechanism:
1. Slow Step (Rate-Determining Step):
2. Fast Step:
Overall Reaction:
Since the slow step involves two molecules, this matches the rate equation , confirming that this step is the RDS.
Also, does not appear in the rate equation, suggesting that it is zero-order. This means the reaction rate is unaffected by changes in concentration because , so it does not contribute to the RDS.
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Worked Examples
Identify the rate determining step
Overall reaction:
Rate equation: rate
Mechanism steps:
Step 1:
Step 2:
Answer
The reaction rate is order with respect to . This indicates two molecules involved up to and including the rate determining step.
is not in the rate equation so zero order and not involved in the rate determining step.
Choosing an appropriate mechanism
Rate
Which is the correct mechanism?
Mechanism 1:
Step 1:
Step 2: RDS
Mechanism 2:
Step 1: RDS
Step 2:
Answer
Mechanism 1
The rate equation shows order with respect to and with respect to . This means 2 molecules of and 1 molecule of up to and including the rate determining step.
Note – is an intermediate and so does not appear in the rate equation. The formation of depends on two reactant molecules.
Rate becomes Rate
If it were mechanism 2, the rate equation would be rate .
Write the rate equation based on the nucleophilic substitution mechanism below.
Step 1:
Step 2:
Answer
Rate
One molecule of up to and including the rate determining step, therefore first order with respect to .
Key tips and reminders
The orders in the rate equation correspond to molecularity up to and including the rate determining step.
Reactants and catalysts can appear in the rate equation. Products and intermediates do not appear.
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