s we can also determine the rate of chemical reaction. From the kinetic stand point the reactions are classified into two groups: a)homogeneousreactionswhichoccurentirelyinonephaseb)heterogeneousreactions where the transformation takes place on the surface of a catalyst or the walls of a container. 1.2 Rateofreaction The rate of reaction i.e. the velocity of a reaction is the amount of a chemical change occurring per unit time. The rate is generally expressed as the decrease in concentration of a reactant or as the increase in concentration of the product. IfCthe concentration of a reactant at any time t is, the rate is−dC dt or if the concentration of a product be xat any time t, the rate would bedx dt. The time is usually expressed in seconds. The rate will have units of concentration divided by time. The concentrations are taken in gm-moles/litre, hence rate is moles/litre/second.