determination of order of reaction
A series of standards is used to make a Beer's Law plot, the best-fit equation of which is used to determine the concentration of the isolated reactant. To test for the order of reaction with regard to that reactant, three plots are made. The first is concentration of the isolated reactant versus time.
The order of a reaction for each reactant can be estimatedfrom the variation in initial rate with the concentration of that reactant, using the natural logarithm of the typical rate equation
ln r=\ln k+x\ln[A]+y\ln[B]+...
For example, the initial rate can be measured in a series of experiments at different initial concentrations of reactant A with all other concentrations [B], [C], ... kept constant, so that
ln r=x\ln[A]+ constant
The slope of a graph of {\displaystyle \ln r} as a function of {\displaystyle \ln[A]}then corresponds to the order x with respect to reactant A.
However this method is not always reliable because
1.)measurement of the initial rate requires accurate determination of small changes in concentration in short times (compared to the reaction half-life) and is sensitive to errors, and
2.)the rate equation will not be completely determined if the rate also depends on substances not present at the beginning of the reaction, such as intermediates or products.
i) Integration method: This method consists in performing the reaction between required amounts of the reactants. The products formed are analysed from time to time and the concentration of reactant is determined. These values are substituted in equations for various order reactions.
(ii)Graphical Method: We know that the reaction velocity in a first order reactions varies as one concentration term; while in second order reactions, the rate is determined by variation of two concentration terms and so on.
iii)Isolation method: In this method all the reactants are taken in very large excess except one and the order of the reaction is determined by the first method. The concentrations of the substances taken in excess are considered to be constant. The sum of the orders, obtained by the process of isolation of each reactant is the order of the reaction. The method may be illustrated by the reaction between sodium formate and silver acetate.
iv)Half period reaction Method: In this method, the time at which one-half of the substance has disappeared is determined and the experiment is repeated with different initial concentrations of the reactants.