Order of a reaction

The order of a reaction is defined as the relationship present between the rate of a chemical reaction and the concentration of the reactants taking part in that chemical reaction. We can also define the order of a reaction as the sum of powers or orders on the concentration in terms of rate law expression.

If we can find the order of a reaction, then the entire composition of the mixture in the reaction can be understood.

The order of a reaction can be determined by adding individual orders or powers of the concentration of the reactants. For example, if reactant A is of the first order (power of 1) and reactant B is of the first order (power of 1), then the overall reaction would be of second order.

The order of reaction does not depend on the coefficients that correspond to each species present in a reaction. The order of the reaction is found in the form of an integer or fraction. It can also have a value of zero (0).

How to find the order of a reaction 

Several methods are used to determine the order of a reaction.

• We use differential rate law or the integrated rate law to express the order of a reaction for the data used in an experiment. Mainly one from these three (1,2 or 0) orders is present in a reaction. Often, the exponents or powers in the rate law are positive.

• We can also use a natural logarithm to express the order of a reaction.

For example ln r = ln k + a.ln[X] + b.ln[Y] + ….

• If the partial order of B is being determined, then the order of the given rate equation is r = x.ln[A] + C, where C is a constant.

• We can also use a graph for determining the order of a reaction. For example, if a graph can be plotted by taking in r as the function of [B], then the slope will be given by x.

• There also exist other different ways to define the order of a reaction. For example: flooding. In this, the value or concentration of a single reactant is measured, and all other components of the reaction are present in large amounts.

Values of the order of reaction 

1. Zero-order reaction 

• A zero-order reaction is a reaction that is independent of the concentration of the reactants present in a reaction. The exponent or power of a reactant in this rate reaction is zero.

Rate= -∆[A]/∆(t)

=k[reactant]0=k(1)

• In a zero-order reaction, the rate constant has the same units as the reaction rate, generally moles per litre per second.

• In this rate reaction, a change in the concentration of the reactants does not affect the speed of the reaction.

• In a zero-order reaction, the product of the change of concentration of a reactant and time always produces a straight line when we plot it on a graph.

• Integrated rate law of zero-order reaction is given in the form of 

[A]=[A]0-kt

• An example of a zero-order reaction is the enzyme-catalysed oxidation of CH3CH2OH (ethanol) to CH3CHO (acetaldehyde).

2. First-order reaction 

• In a first-order reaction, the rate of reaction is directly proportional to the concentration of one of the reactants. If we double the concentration of reactant A, then the reaction rate doubles.

Rate=  – ∆[A] /∆(t) =k[A]

• The unit of first-order reaction rate constant is reciprocal of seconds(s-1).

• In a first-order reaction, only one reactant is first order, while the rest of the reactants would be zero order.

• Integrated rate law of first-order reaction is given as 

[A]=[A]0 e-kt

• Expression of the logarithm of the relationship between the concentration of A and t:

ln[A] = -kt + ln[A]

• Example of a first-order reaction

2H2O2→ 2H2O + O2

3.  Second order reaction 

• In a second-order reaction, the rate of reaction is proportional to the square of the concentration of one reactant.

• The second kind of second-order reaction is present, which has a reaction rate that is proportional to the product of the concentration of the two reactants.

Rate =-∆[A]/∆t=k[A]2

• The unit of second-order reaction rate constant is inverse of a mole per litre per second.

• Integrated rate law of second-order reaction 

1/[A]t -1/[A]0 =kt

• Example of a second-order reaction

NO2+ CO → NO + CO2

Uses of the order of reaction 

• It is used in the baking industry.

• It is used in the Haber process, which produces ammonia from hydrogen and nitrogen gas.

• Used to test, identify and analyse a wide range of reactions.

Conclusion 

In chemical kinetics, the order of reaction for a certain reactant is defined as the power to which its concentration term in the rate equation is raised.

Order of reaction is used to determine the rate of the reaction with the help of the rate constant.

There are different values of the order of reaction like zero, first and second. Order is an important part of a reaction. For example, in the baking industry, with the help of order, we can easily define the rate at which cookies will be baked. Order of reaction states a positive relationship between the rate of any chemical reaction and the concentration of reactants taken in that chemical reaction. There are different types of methods for obtaining the order of a reaction.