(i) NO(g)+Br2(g)⇌NOBr2(g) (ii) NOBr2(g)+NO(g)⟶2NOBr(g) Rate law equation =k[NOBr2][NO] But NOBr2 is intermediate and must not appear in the rate law equation from 1st step KC=
[NOBr2]
[NO][Br2]
∴[NOBr2]=KC[NO][Br2] ∴ Rate law equation =k⋅KC[NO]2[Br2] hence order of reaction is 2 w.r.t. NO.