Rates and reaction mechanisms

The rate determining step is the slowest step in a reaction mechanism

The overall reaction rate is determined by the rate determining step

All reactants up to and including the rate determining step will be in the rate equation, not including intermediates

Knowing the rate equation and the rate determining step can also allow suggestions for reaction mechanisms to be made.

Example:

• The mechanism for the reaction NO₂ + CO → NO + CO₂ has 2 steps

• Rate = [NO₂]²

• The rate determining step is the first step and results in the formation of NO

• Since the reaction is second order with respect to NO₂ and the rate determining step is first, the first step must have 2NO₂ reacting

• 2NO₂ → NO

• Balance the equation using a sensible suggestion (this may take a little trial and error)

• First step is: 2NO₂ → 2NO + O₂

• Because the overall reaction only produces one NO, NO must be a reactant in the second step.

• O₂ also isn’t present in the overall equation so must be used in step 2

• CO is present in the overall equation and was not present in step 1 so must be present in step 2

• So the reactants for step 2 are NO + O₂ + CO

• In the overall equation only one NO₂ is reacting, so NO₂ must be a product of step 2

• CO₂ is produced in the overall equation but was not produced in step 1 so must be produced in step 2

• Therefore the second step is: NO + O₂ + CO → NO₂ + CO₂