Order of Reaction

KINETICS Kinetics is the study of the rate at which processes occur. It is useful in providing information that:  Gives an insight into the mechanisms of changes involved, and  Allows a prediction of the degree of the change that will occur after a given time has elapsed. ORDER OF REACTION This is the number of concentration terms that determine the rate. Consider the reaction: A + B C + D The rate of the reaction is proportional to the concentration of A to the power of x, [A] x and also the rate may be proportional to the concentration of B to the power of y, [B] y. The overall equation is, Rate = k [A] x [B] y The overall order of reaction is x+y RATE CONSTANT A rate constant is a proportionality constant that appears in a rate law. For example, k is the rate constant in the rate law d [A]/dt = k [A]. Rate constants are independent of concentration but depend on other factors, most notably temperature. ZERO ORDER REACTION When the reaction rate is independent of concentration of the reacting substance, it depends on the zero power of the reactant and therefore is zero order reaction. In this type of reaction, the limiting factor is something other than concentration, for example, solubility or absorption of light in certain photochemical reactions. The rate of decomposition can be described mathematically as: Rate of concentration decrease =-dCx = K ………………… (1) dt Integrating the equation yields X= Kt + constant ……………… (2) A plot of X v/s Time results in straight line with slope equal to K. The value of K indicate the amount of drug that is degraded per unit time, and intercept of line at time zero is equal to constant in equation (2). The unit of K is conc time-1 , with typical units of mole L-1 s-1. Half-life is given by equation t 1/2 = Co/2k. Examples:-Vitamin A acetate to anhydrous vitamin A. Photolysis of cefotaxime. Loss in color of multi sulfa product.