Rate determining step

  Rate determining step

• The slowest step during a chemical reaction determines the overall speed of a reaction towards completion is called rate determining step.
• Let us consider the following reaction,

NO2(g)+CO(g)→NO(g)+CO2(g)

• The elementary steps of the reaction are as follows: 

Step 1:   NO2+NO2→NO+NO3 (Rate constant = k1, slow)

Step 2: NO3+CO→NO2+CO2 (Rate constant = k2, fast)

• As the first step is the slowest step in the reaction it will determine the rate of the overall reaction. Therefore Step1 is the rate determining step of the given reaction and hence the rate expression for the given reaction is the product of rate constant and the reactants of this step.

Rate = k1[NO2][NO2]=k1[NO2]2