Rate-Determining Step

Question 1

What is the reaction mechanism?

Answer 1

The series of steps that make up an overall reaction.

Question 2

What is the RDS?

Answer 2

The slowest step in the sequence.

Question 3

How can we know whether our prediction of reaction mechanisms is correct?

Answer 3

1. The rate equation only includes reacting species involved in the RDS.
2. The orders in the rate equation match the number of species involved in the RDS.

Question 4

How can we make a reaction mechanism using steps? Example?

Answer 4

1. Look at the rate equation which shows that the slow RDS and so can identify what species have no effect on the reaction rate so must be involved in the fast step as the rate equation only involves species involved in the RDS.
2. Compare the overall equation with the information from step 1.
3. Make the mechanism ensuring the slow step 1 matches the rate equation and the sum of the steps gives the overall equation.

• Rate equation = k[(CH3)3CBr]
• RDS = (CH3)3CBr = (CH3)3C+ + Br-
• FAST = (CH3)3C+ + OH- = (CH3)3COH.
• OVERALL = (CH3)3CBr + OH- = (CH3)3COH + Br-.