The rate of chemical reaction is the difference in concentration of reactant or product in unit time. It could be the rate of decrease in concentration of any one of the reactants or increase of rate in concentration in case of any one of the products. Reactant (R) Product (P) The rate of disappearance of R is = – R/t (negative sign denotes the decrease in concentration) The rate of appearance of P is = P/t Study of reaction rates is referred to as chemical kinetics. Rates may vary, some reactions are fast like ionic reactions e.g. precipitation of silver chloride on mixing aqueous solutions of silver nitrate and sodium chloride. There are reactions which occur very slow e.g. Rusting of iron in presence of air and moisture. Some reactions have moderate rates like hydrolysis of starch. Units of reaction rate are expressed in concentration upon a time, units are generally mol L-1 sec-1. The reaction rate is influenced by various factors firstly concentration of reactants if reactants concentrations are increased there is an increase in the rate of reaction. The rate of reaction increases with the increase of temperature. Reaction taking place on the surface than the solution, for them as the surface area is increased the rate of reaction increases. In presence of catalyst there is an increase in rate but in presence of inhibitors, the reaction rate is delayed. The average rate of reaction is the rate divided by a time interval of reaction and instantaneous rate of reaction is the rate at a particular instant of time. Rate law – the rate of reaction expressed in terms of concentration of reactants is referred to as rate law. Aa + Bb Cc +Dd (Here a, b, c, d are stoichiometric coefficients of reactants and products) Rate = K [A] x [B] y (x and y are exponents that may or may not be equal to stoichiometric coefficients) Here K = Rate constant The rate law is expressed in above equation is given in terms of molar concentration of reactants raised to some power, that power may or may not be equal to the stoichiometric coefficient of the reacting species in a balanced chemical equation. Order of reaction- It is the sum of powers of the concentration of the reactants in the rate law expression. The half-life of the reaction is a time when one-half of the reactant present initially get consumed. Zero-order First order Second order Units of rate constant (K) mol L-1 sec -1 Sec-1 L mol-1 sec-1