Order Of A Chemical Reaction

"Gulberg" & "Waage" in 1867 studied the effect of concentration on the rate of reaction. In a chemical reaction, the concentrations of reactant and product molecules change over time.

The above graph shows the variation in the concentrations of the reactants and products over time. As the reaction proceeds, the concentration of the molecules of the reactants decreases while that of the molecules of the products increases. 

Affect on the Rate of reaction????

The rate of reaction also decreases with time. This is because the rate of a reaction is directly proportional to the concentration of the reactants. Therefore, as the concentration of the reactants decreases, the rate of the reaction also decreases with time. 

Generally, the order of a reaction expresses how much the concentration of reactant can affect the rate of reaction. Now, we understand about the order of a reaction or how one can found out the order of a reaction????

Order Of A Reaction

Order of a reaction is an experimental term. It can not be directly determined by the observed rate law. To determine the reaction order, the rate expression must be known.

Let us understand about the order of a reaction....

The order of a reaction is defined as;

...."The order of a reaction is the sum of the exponents of the molar concentration terms of all the reactants present in the rate law obtained from the rate determination step".....

The order of the reaction represents the number of species whose concentration directly affect the rate of reaction. The order of the reaction can be in the form of an Integer or a Fractional value. Its value can also be Zero.

Let's have a look an example of general reaction to clearly understand about the reaction order-

                aA + bB + cC ...... ➝ products 

The rate law for the above reactions can be expressed as;

             ➩ r = k [A]ˣ [B]ʸ [C]ᶻ........ 

   Here,

      x, y& z are the exponents of concentration terms 

        r = rate of reaction

        k = reaction rate constant 

     a,b and c are stoichiometric coefficients

     ( x,y and z may not be equal to a,b and c )   

In the rate law x, y& z are the exponents of the concentration of reactants. They bear no relation with the stoichiometric coefficients "a, b & c". According to the above definition, the order of reaction is given by,   

➩ [ Order of reaction, n = x+y+z+....... ]

If x=a and y=b and z=c; then the order of a reaction,   

➩ [ Order of reaction, n = a+b+c+....... ]

Here, n represent the order of reaction. The value of x,y and z may be equal to the stoichiometric coefficients a,b and c if and only if the reaction is an elementary reaction. Thus, the order of the reaction corresponds to the stoichiometric coefficients only for elementary reactions. While in case of complex reactions, the order of reaction corresponds to the stoichiometric coefficients of the rate determining step (slowest step).

We can also define the order of reaction with respect to each reactant....

For example in the above reaction, 

The order of a reaction is equal to 'x' with respect to reactant 'A' and the order of a reaction is equal to 'y' with respect to 'B' and similarly the order of a reaction is equal to 'z' with respect to 'C' and so on..........

!!!!Hence, x,y And z Are The Partial Orders Of Reaction And The Summation Of All The Partial Orders Yields The Overall Reaction Order!!!! 

......"The powers to which the concentration of reactants are raised in the rate law, is called as order of the reaction".......

For Example

                 2NO + O₂ ➝ 2NO₂

      Rate of reaction, r= [NO]²[O₂]

The above reaction is a second order reaction with respect to [NO], first order reaction with respect to [O₂] and third order reaction overall because "x+y=3" here.

Determination Of Reaction Order

There are several methods which is used to determine the reaction orders and these are;

1. Integral method (one can find out the reaction order by integrating the rate law)
2. Differential method (directly deal with the rate law)
3. Half life method (finding out the half-life of reaction as a function of the initial concentration and reaction rate constant when integrated rate equations are known)
4. Method of excess (used to determine individual orders by taking one of the reactant species in higher concentration)
5. Method of fractional life (this method is used when half life method is not applicable or unable to find reaction orders)
6. Method of initial rate (used for reversible reaction and it is useful where the differential method fails)
7. Method of least square

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