Order of Reaction

Changing the concentration of chemical substances in a reaction usually causes the rate of the reaction to alter. A rate equation mathematically depicts this impact. The rate equation includes response orders. Let us go through the concept of order of reaction in further detail.

What is Order of Reaction?

The order of reaction can be described as the power dependence of rate on the concentration of all reactants in a chemical reaction on the rate law expression.

In other words, the order of a chemical reaction is defined as the summation of the powers of the concentration of the reactants in that chemical reaction’s rate equation. The order of a reaction is an experimental value. It is an empirically determined parameter. It may also have a fractional value.

Let us consider an example,

If the rate of a reaction,

aA + bB + cC –> Products

is given by the rate law as:

k[A]^p[B]^q[C]^r

Rate = – dx/dt = k[A]^p[B]^q[C]^r

Then, the order of reaction (n) is,

n = p + q + r

where p, q, and r are the orders of the individual reactants and the overall order of the reaction is the total of these exponents, i.e., p + q + r

Characteristics of the Order of Reaction

The number of reactants whose concentration directly impacts the rate of reaction is represented by the order of reaction. Some of the characteristics of order of reaction are as follows:

• The number of species whose concentration directly impacts the rate of reaction is represented by reaction order.
• It can be calculated by summing the exponents of all the concentration terms in the rate statement.
• The order of the reaction is determined by the concentration of the reactants.
• The concentration of the products is not the determining element in the reaction order.
• A reaction’s order can be zero, integer, or fraction.
• A positive order means that the concentration of that species has a direct influence on the rate of a reaction.
• A negative order means that the concentration of a species affects the rate of a reaction inversely.

Relation to Rate Law

The power-law form of the rate equation is commonly used to calculate the reaction order.

For the reaction:

aA + bB –> P

The rate law is as follows:

rate = k[A]^x[B]^y

where

• [A] is the concentration of species A,
• xis the order with respect to species A.
• [B] is the concentration of species B,
• y is the order with respect to species B
• k is the rate constant.
• n is the order reaction for the entire chemical reaction. n = x + y

Methods to Determine the Order of Reaction

There are numerous approaches that can be taken in order to identify the order of reaction. Some of these methods are described below.

Differential Method

This is the simplest way for determining the order of the reactions. The rate of a reaction is first expressed as R= k [A]^x [B]^y. The reaction’s final order is given by x+y.

Initial Rates Method

• First, we extract the power law’s natural log form as ln r = ln k + x.ln[A] + y.ln[B] + ….
• The partial order is calculated for each reactant. This is accomplished by varying the concentrations of the reactants in question while maintaining the concentrations of the other reactants constant.
• If the partial order of A is found, the rate equation’s power-law form now becomes ln r = x.ln[A] + C, where C is a constant
• By plotting the graph of ‘ln r’ as a function of ln[A], the corresponding slope is the partial order, denoted by x.

Integral Method

This method is commonly used to validate the order of reaction derived from the initial rates method. The rate law for the first-order reaction is verified by determining if the value of ln[A] is a linear function of time.

A first-order reaction’s integrated rate equation is as follows: In[A] = -kt + In [A]⁰

Different Values of Order of Reaction

Order of Reaction might take the form of integers, zeros, or fractions. Chemical reactions are classified into the following kinds based on the rate of reaction’s dependency on concentration.

1. Zero Order Reactions

A zero-order reaction is one whose rate is independent of reactant concentration. The concentration of the reactants does not change over time, and the rate of concentration remains constant. The enzyme-catalyzed oxidation of CH₃CH₂OH (ethanol) to CH₃CHO (acetaldehyde) is one example of a reaction.

2. First-Order Reactions

The rate of a first-order reaction is proportional to the concentration of a single component. In these reactions, the rate of reaction is determined solely by the concentration of one component. The reaction can involve many reactants, but only the concentration of one reactant impacts the pace of the reaction. Example,

2H₂O₂ –> 2H₂O + O₂

Rate = k[H₂O₂]

3. Pseudo-First Order Reactions –

The concentration of one component remains constant in a pseudo-first-order reaction. The reactant with a constant concentration is either present in excess relative to the other reactant or acts as a catalyst. Example,

CH₃I + H₂O –> CH₃OH + H⁺ + I^–

Rate of reaction = k [CH₃I] [H₂O]

Because methyl iodide is also utilized in an aqueous solution, the concentration of water is significantly greater than that of methyl iodide.

[CH₃I] <<< [H₂O]

As a result, the concentration of water does not change considerably and can be approximated as constant or no change.

Now, the Rate of reaction = k [CH₃I]

4. Second-Order Reactions

When the order of a reaction is 2, it is referred to as a second-order reaction. Rates like r = k[A]² and R = k[A][B] can be identified. Example,

2NO₂ → 2NO + O₂

Rate = k [NO₂]²

Rate Constant

The rate constant is the proportionality constant between the reactant concentration and the rate of a chemical reaction. It is represented by the letter ‘k.’ The reaction rate constant is also known as the reaction rate coefficient.

Order of the Reaction	Unit of the Rate Constant
Zero order reaction	mol L-1 s-1
First order reaction	s-1
Second order reaction	L mol-1 s-1
Third order reaction	L2 mol-2 s-1

Difference between Molecularity and Order of Reaction

Molecularity	Order of Reaction
It refers to the number of molecules involved in the rate-determining phase.	It depicts the relationship between reactant concentration and reaction rate.
It is not affected by pressure or temperature.	It depends on temperature, pressure and concentration.
It is always a whole number.	It can be a zero, integer or a fraction.
It can be calculated using the balanced chemical equation.	It must be validated experimentally.
Molecularity cannot be negative.	The order of reaction can be a negative number.

Frequently Asked Questions on Order of Reaction

Q1. Define order of reaction.

Answer. The order of reaction can be described as the power dependence of rate on the concentration of all reactants in a chemical reaction on the rate law expression.

Q2. If the rate constant of a reaction is k = 5 x 10^-3 s^-1, determine the reaction order.

Answer. For k = 5 × 10^-3 s^-1. The SI unit is s^-1, which is the rate constant of the first-order reaction. As a result, it is a first-order reaction.