The rate and mechanism of chemical reactions are studied in chemical kinetics. The elementary reactions are single step reactions having no mechanism. The order of reaction and molecularity are same for elementary reactions. The rate of forward reaction aA + bB rarr cC + dD is given as : rate = =((dx)/(dt))=-(1)/(a) (d[A])/(dt) =-(1)/(b) (d[B])/(dt) =(1)/(c)(d[C])/(dt)=(1)/(d) (d[D])/(dt) or rate =K [A]^(a) [B]^(b). In case of reversible reactions net rate expression can be written as : rate = K_(1)[A]^(a)[B]^(b) – K_(2)[C]^(c)[D]^(d). At equilibrium, rate= 0. The constants K, K_(1), K_(2), are rate constants of respective reaction. In case of reactions governed by two or more steps reactions mechanism, the rate is given by the solwest step of mechanism. The rate of formation of SO_(3) in the following reaction : 2SO_(2) +O_(2) rarr 2SO_(3)" is "10 g sec^(-1). The rate of disapperance of O_(2) will be :