For which type of reactions, order and molecularity have the same value?
For an elementary reaction, order is same as molecularity. Elementary reactions are those reactions which occur in a single step.
Suppose there are 2 reactants A and B which form a product C. Then
A+B → C
Rate is determined by,
Rate = k[A]1[B]1
Order of the reaction is 2.
Molecularity of the reaction is also 2 as there are total 2 molecules of reactants that are reacting for the formation of a product C.