The order of the reaction → Y is 3 when the concentration of is small. However, at higher concentrations of , the order changes to 2. The mechanism of the reaction is:
[Assume that the steady-state approximation can be applied to the intermediate ]
1
X→Y
2
X ⇌ X*
X* ⇌ Y
3
X+X+X ⇌ X* + X
X* → Y
4
X + X → X*
X* → Y