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 $aA+bB\to cC+dD$& given as: rate = $\left(\frac{dx}{dt}\right)=-\frac{1}{a}\frac{d\left[A\right]}{dt}=-\frac{1}{b}\frac{d\left[B\right]}{dt}=\frac{1}{c}\frac{d\left[C\right]}{dt}=\frac{1}{d}\frac{d\left[D\right]}{dt}$ or rate = $K{\left[A\right]}^a{\left[B\right]}^b$.

In case of reversible reactions net rate expression can be written as: rate =  $K_1{\left[A\right]}^a{\left[B\right]}^b-K_2{\left[C\right]}^c{\left[D\right]}^d$. At equilibrium, rate = 0. The constants $K,K_1,K_2$ are rate constants of respective reactions. In case of reactions governed by two or more steps reaction mechanism, the rate is given by the slowest step of mechanism