Methods Of Finding The Order Of Reaction

Methods of Finding the Order of Reaction Ostwald Isolation Method This method is introduced by Ostwald and is used to determine the order of a reaction with respect to one reactant at a time. The total order of the reaction can be calculated by adding all the orders of the reaction for individual reactant. The principle behind this method is that if the concentration of all but one reactant is taken in excess, then during the course of reaction, the concentration of those reactants taken in excess will remain constant and the variation in the rate will correspond to the concentration of that reactant whose concentration is small. The process is repeated for other reactants present in the chemical reaction one by one and order with respect to each reactant can be determined. By adding all the orders of the individual reactant we will get overall order of the reaction .For example,

Suppose, we isolate A by taking B and in large excess and get order of reaction with respect to A (say p) Similarly, q and r are the order of reactions for B and C by taking C and A in excess for q order of B and A, B in excess for r order of C.

Then the overall order of the reaction will be,

Half Life Period Method Half-life of a reaction is defined as the time during which the concentration of a reactant is reduced to half of its initial concentration. It can also be explained as the time in which half of the reactant is completed. Half-life is represented as t1/2. With the help of half-life we can determine the order of the reaction. FOR FIRST ORDER REACTION Consider a reaction,

Half –life period means the time in which initial concentration [A]0 is reduced to half. i.e.

Then the half life period t1/2 becomes

Thus, half-life period of first order reaction is independent of the concentration of the initial concentration of the reactant. From the equation, it also clear that half-life period for the first order reaction is inversely proportional to the rate constant.

Similarly, we can calculate the relation for the time required to reduce the concentration of the reactant to any fraction of the initial concentration. Take example, i)

ii) Time required to complete 3/4 of the reaction will be given as:

FOR ZERO ORDER REACTION

Graphs between t1/2 and concentration i.e.t1/2 vs. [A] 0, for zero order, t1/2 vs.[A]0 for first order , t1/2 vs. 1/ [A]0 for second order.

From the above graphs it is clear that only the half-life of first order reaction s are independent of the concentrations.

PSEUDO CHEMICAL REACTION There are some reactions which appear to be second order but in fact these are first order reaction with respect to the different reactants i.e.

Now, if one of the reactant is present in high concentration (solvent) then there is very little change in its concentration. In other words the concentration of that reactant remains practically constant during the reaction. For example, If [A] = 0.01 M and that of solvent water [B] = 55.5 M, the concentration of B changes only from 55.5 to 55.49 M even after the completion of the reaction. Under such conditions, the rate of the given reaction will be

The reaction therefore, behaves as a first order reaction not a second order reaction. Pseudo reaction can be explained by taking an example of hydrolysis of ethyl acetate:

The molecularity of the reaction is two because it involves two reacting species, ethyl acetate and water. But the concentration of ethyl acetate changes during the reaction while the concentration of water remains constant since it is present in such large excess. Therefore, the rate of reaction depends on the concentration of ethyl acetate and hence the order of the reaction is one.

[H2O] can be taken as constant so that,

Thus, the reaction appears to be second order but follows the first order kinetics. Such reactions which appear to be of higher order but actually are of lower order are called Pseudo chemical reactions. But the above reaction is of first order reaction. Similarly, Hydrolysis of cane sugar also shows pseudo order reaction.

In the above reaction, molecularity is two and order of reaction is one. *****************************************************