Rate equation is given by law of mass action for a chemical reaction, The sum of the exponents by which the concentration terms are raised in rate law is known as Order of Reaction.
It is an experimental quantity and can be predicted by balanced chemical equations.
Order of reaction can be O, positive, negative or can be fractional values.
Generally, Zero order, First order, Second order and Third order Reaction are of great importance.
METHOD OF DETERMINING ORDER OF REACTION
Graphical Method:
Rate Law expressions for various types of reactions,
dx/dt = k zero order
dx/dt = k[a-x] first order
dx/dt = k[a-x]2 second order
dx/dt = k[a-x]n nth order
The plot between dx/dt vs concentration is plotted and if it is a straight line passing through origin than corresponding concentration power is the order of reaction.
Initial Rate Method:
The data’s are given in term of different rates at different concentrations, thus we calculate the rate expression and then find the order of reaction.
Example:
Reaction A + B → Product
S.No. |
Concentration of A |
Concentration of B |
Rate |
1. 2. 3. |
x_{1} x_{2} x_{3} |
y_{1} y_{2} y_{3} |
r_{1} r_{2} r_{3} |
According to rate law
Here we have three unknown quantities k, m, n and we have three equations thus the solution is possible and k, m, n have unique value,
Thus order of reaction = m + n
By integration Methods:
According to this method the concentration of reactants at different points of time are measured and put into the rate equation of different order, the rate equation which gives constant value of k is the proposed equation of that reaction.
By Fractional Change Method:
Time required for any fractional change is related with initial concentrations of reactants as –
Vant Hoff’s Differential Method:
According to vant hoff the rate of n^{th} order reaction is proportional to n^{th} power of concentration.
Isolation Method:
In this method one reactant is taken in large excess and order is determined with respect to other the process is repeated and the actual order is the sum of the order of each isolated reaction.
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