. Explain four different methods of determining order of reaction


Best Answer

1. Initial rate method

Initial rate method:  A number of experiments are carried out by varying concentration of a reactant w.r.t. which the order is to be determined keeping the concentrations of all other reactants constant.  The initial rate of the reaction at each concentration is determined by concentration-time curves.  The order of the reactant is then calculated from the rates at various concentrations.  The experiments are repeated with other reactants in a similar way.  Likewise, the orders w.r.t. all reactants are determined.  The overall order is the sum of the orders of all the reactants.

In this method we use the integrated rate law equations. Substituting the various experimental values of a, x, t etc. As you can see, each order of reaction has a unique input and output variable that produces a straight line. 

2. Integrated rate law method

For example: if we graph the following rate data for the decomposition of H2O2 assuming that it could be zero, first, or second order, we find that the graph for a 2nd order reaction (1/[A] versus t) gives a straight line. Therefore, the reaction has the rate law rate = k [H2O2]2.

We calculate the rate constant.  If we get a constant value of K1(rate constant for first order), it is a first-order reaction; and similarly, for constant values of k2 and k3, the reaction are of second and third order respectively.

3. Graphical Method 

A graph is plotted between time t and the concentration of reactant (a-x) or product (x), slope of which gives the rate of reaction (dx/dt) for the selected time instant. (figure 1 & 2)The various values of the rate (dx/dt) are now plotted against the corresponding concentration (a-x) or (a-x)2 or (a-x)3 from which we draw the following graphical conclusions:

graph

 

 

 

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