Order of Reaction - Definition, Explanation of Reaction Order, FAQs

Chemical reactions lie at the core of many natural and industrial processes that shape the world around us. The rate at which these reactions proceed is crucial for various scientific and engineering applications, from the rusting of iron to the digestion of food. This rate of reaction is deeply influenced by the "order of reaction." The order of reaction quantifies how the concentration of reactants affects the speed of a chemical reaction. In simpler terms, it is the sum of the powers to which the concentration terms are raised in the rate law expression.

We will also discuss methods used to determine the order of reaction and why understanding this concept is important both academically and practically. Whether you are a chemistry student or a curious learner, this guide aims to provide comprehensive information on this fundamental concept.

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Definitions and Explanations

Order of Reaction

The order of reaction is defined as the sum of the exponents of the concentration terms in the rate law expression for a chemical reaction. For a general reaction ${aA} + {bB} \rightarrow{cC} + {dD}$ , the rate law is given by:

$Rate ={k}[{A}]^{x}[{B}]^{y}$

Here, ( x ) and ( y ) are the orders of the reaction with respect to reactants ( A ) and ( B ), respectively, and ( k ) is the rate constant. The overall order of the reaction is ( x + y ).

Experimental Determination

The reaction constant and the order of the reacting species must be determined experimentally and cannot be directly predicted from the balanced chemical equation. The order of such reacting species and other experimental values may be zero, negative, or fractional and all vary with conditions like temperature (T), pressure (P), and reactant concentrations.

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Types and Examples

Elementary Reactions

In a simple or elementary single-step reaction, the rate law can be directly written from the balanced equation. For example, consider the reaction:

$A + B \rightarrow C$

The rate law is:

$Rate ={k}[{A}]^{x}[{B}]^{y}$

where x and y are determined experimentally.

Initial Rate Method

Since this is the rate of reaction at the very beginning, the concentration of reactants is known. By comparing the initial rates at different concentrations, one can determine the correct rate law and order of reaction.

Relevance and Applications

Academic Importance

Understanding the order of reaction is crucial for chemistry students as it forms the basis for more advanced topics in kinetics and reaction mechanisms. It helps predict the behavior of reactions under various conditions.

Applications in Real-Life Situations

Knowing the order of the reaction is important industrially because it can help optimize reaction conditions for maximum yield and efficiency. For example, in the pharmaceutical industry, closely monitoring reaction rates is crucial for proper drug production.

Related Topics Link,

• Endothermic Reaction
• Exothermic Reaction
• Etard Reaction Mechanism
• Neutralization Reaction

Some Solved Examples

Example 1
Question: For the reaction $A + 2B \rightarrow C$ , if the rate is given by ${R}=[{A}][{B}]^2$ , what is the order of the reaction?

1) (correct)3

2)6

3)5

4)7

Solution

Order of a Reaction -

The order of a reaction is determined as the sum of the powers of the concentration terms that appear in the experimental rate equation. It is an experimental quantity.

Formula: $\begin{aligned} & a A+b B \rightarrow c C+d D \\ & r=K[A]^a[B]^b \\ & \text { order }=(a+b)\end{aligned}$

The order has a relation with stoichiometry & is determined experimentally. It can be zero/fraction/negative/positive

So,

For reaction $A+2 B \rightarrow C$

Rate Law $=\mathrm{k}[\mathrm{A}][\mathrm{B}]^2$

Order =1 + 2 = 3

Hence, the answer is the option (1).

Example 2
Question: For a reaction $A + B \rightarrow C + D$ , if the concentration of A is doubled and the rate of reaction doubles, and when the concentration of B is increased by 9 times, the rate gets tripled. What is the order of the reaction?

1)2

2)1

3) (correct)3/2

4)4/3

Solution

Order of a Reaction -
The order of a reaction is determined as the sum of the powers of the concentration terms that appear in the experimental rate equation. It is an experimental quantity.

Rate $=k[A][B]^{1 / 2}$

Order $=1+\frac{1}{2}=\frac{3}{2}$

It can be zero/fraction/negative/positive fractional.

Order = 3/2 =1.5
Hence, the answer is the option (3).

Example 3
Question: What are the possible orders of a reaction?

1)Zero

2)Positive

3)Fractional

4) (correct)All of the above

Solution

The order of the reaction can be zero/fraction/negative/positive/fractional.

The order of a reaction has no relation with stoichiometry & is determined experimentally. It can be zero/fraction/positive.
Hence, the answer is the option (4).

Example 4
Question: For the reaction $A + B \rightarrow C$ , what is the order of the reaction?

1)2

2)1

3)Can not be more than 2

4) (correct)Can not be determined

Solution

Order of a Reaction -
The order of a reaction is determined as the sum of the powers of the concentration terms that appear in the experimental rate equation. It is an experimental quantity.

Formula: $\begin{aligned} & a A+b B \rightarrow c C+d D \\ & r=K[A]^a[B]^b \\ & \text { order }=(a+b)\end{aligned}$

The order has a relation with stoichiometry & is determined experimentally. It can be zero/fraction/negative/positive

So, for the given reaction order can not be determined because experimentally determined stoichiometry of powers of the concentration is not given.

Hence, the answer is the option (4).

Example 5
Question: For the reaction $2{NO} + {Cl}_2 \rightarrow 2 {NOCl}$ , if doubling the concentration of both reactants increases the rate by a factor of 8, but doubling ${Cl}_2$ alone only doubles the rate, what is the order of the reaction with respect to ${NO}$ and ${Cl}_2$ ?

1)1,1

2)1,2

3) (correct)2,1

4)2,2

Solution

Rate $=\mathrm{k}[\mathrm{NO}]^{\mathrm{m}}\left[\mathrm{Cl}_2\right]^{\mathrm{n}}$

Given,

$\begin{aligned} & \mathrm{R}_1=\mathrm{k}[\mathrm{NO}]^{\mathrm{m}}\left[\mathrm{Cl}_2\right]^{\mathrm{n}} \\ & \mathrm{R}_2=\mathrm{k} 2^{\mathrm{m}}[\mathrm{NO}]^{\mathrm{m}} 2^{\mathrm{n}}\left[\mathrm{Cl}_2\right]^{\mathrm{n}}=2^{\mathrm{m}+\mathrm{n}} \mathrm{R}_1 \\ & \mathrm{R}_3=\mathrm{k}[\mathrm{NO}]^{\mathrm{m}} 2^{\mathrm{n}}\left[\mathrm{Cl}_2\right]^{\mathrm{n}}=2^{\mathrm{n}} \mathrm{R}_1\end{aligned}$

According to the experimental data given,

$2^{\mathrm{m}+\mathrm{n}}=8$ and $2^{\mathrm{n}}=2$

Upon solving,

$\mathrm{m}=2, \mathrm{n}=1$

Hence, the answer is the option(3).

NCERT Chemistry Notes:

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Summary

The order of reaction is a fundamental concept in chemical kinetics that defines how the concentration of reactants influences the rate of a reaction. It is experimentally determined and can vary widely depending on the conditions. This concept is essential for both academic studies and practical applications in various industries. By exploring definitions, types, and methods to determine the order of reaction, this article provides a comprehensive overview that benefits students and professionals alike.

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