Pseudo First Order Reaction - Rate Law, Graphical Representation, Examples, FAQs

In chemical kinetics, most reactions do not necessarily proceed according to some simple first, second, or higher-order kinetics. An interesting example is that of the pseudo-first-order reaction. The concept, outside of a laboratory setup, has far-reaching implications on everyday life and provides a greater understanding of the dynamics of reactions and rate laws. Suppose some medication degrades inside a body. It is of interest to find out the time of its decay so that one can identify dosage intervals. Many decays such as these nearly follow pseudo-first-order kinetics, depending on temperature and concentration. With this in mind, scientists worked on these kinetics to maximize the efficiency of drugs.

The pseudo-first-order, although apparently a first-order reaction, is basically derived from mechanisms more complex than one reactant or step. We will look at the details in this article, defining what it is and its main characteristic features. This will be illustrated by showing how the application comes in chemical reactions, biological processes, and environmental studies.

Pseudo First-Order

Concept Definition and the Basics

A pseudo-first-order reaction is a complex process involving more than one reactant or including complicated steps; however, it apparently obeys the first-order kinetics. Under certain conditions of the experiments, it simplifies to a first-order rate law, which usually happens when the concentration of one reactant is in large excess or if reaction intermediates are involved.

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Variants and Examples Explored

Types of Pseudo-First-Order Reactions

Describe a number of situations to which pseudo-first-order kinetics apply, including enzymatic reactions, radioactive decay, and chemical degradation processes. For each of these examples, describe how factors like catalysts or limiting reactants impact the rate of reaction.

If one reactant is present in large excess a 2^nd order reaction is converted to 1^st order reaction. This is called a pseudo-first-order reaction.

acid-catalyzed hydrolysis of ester

$
\mathrm{C}_3 \mathrm{H}_6 \mathrm{O}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{CH}_3 \mathrm{COOH}+\mathrm{CH}_3 \mathrm{OH}
$$

This reaction is first order with respect to ester. The 2nd order reaction with respect to that reactant which has been taken in more amount. For example:
$
\begin{aligned}
& r=k\left[\mathrm{CH}_3 \mathrm{CO}_2 \mathrm{CH}_3\right]\left[\mathrm{H}_2 \mathrm{O}\right]=k^{\prime}\left[\mathrm{CH}_3 \mathrm{CO}_2 \mathrm{CH}_3\right] \\
& k\left[\mathrm{H}_2 \mathrm{O}\right]=k^{\prime}
\end{aligned}
$

Read more :

• NCERT solutions for Class 12 Chemistry Chapter 4 Chemical Kinetics
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Applications to Real Life and Academics

Real-Life Applications

Look at applications in relation to pharmaceuticals, whereby problems of stability and shelf life of drugs are directly associated with issues of pseudo-first-order kinetics. The other topics in this respect are pollutant degradation and industrial processes linked with kinetic parameters of this nature.

Chemical Engineering Relevance

Reactor design and optimization in chemical engineering rely on perfect kinetics. Discuss some of the new developments or challenges in modeling and consequently, predicting these reactions.

Recommended topic video on (Pseudo First Order Reaction )

Some Solved Examples

Example 1

Question:

Reactions which are biomolecular and yet follow first order kinetics are called

1)Complex order reaction

2)Simple reactions

3)Pseudo unimolecular reactions

4)Elementary reactions

Solution:

Pseudo-first order reaction - present in large excess a 2^nd order reaction is converted to 1^st order reaction. This is called a pseudo-first-order reaction or Pseudo unimolecular reaction.

Hence, the answer is the option (3).

Example 2

Question:
Reactions which are bimolecular and yet follow first-order kinetics are called:

1) Complex order reactions

2) Simple reactions

3) Pseudo unimolecular reactions

4) Elementary reactions

Solution:
Reactions that are bimolecular but follow first-order kinetics occur when one of the reactants is present in a large excess. This makes the reaction behave as if it is first-order with respect to the other reactant. These are called pseudo-unimolecular reactions.

Hence, the answer is option (3).

Example 3

The rate of certain hypothetical reaction A + B + C products, is given by
$\mathrm{r}=\frac{\mathrm{dA}}{\mathrm{dt}}=\mathrm{k}[\mathrm{A}]^{1 / 2}[\mathrm{~B}]^{1 / 3}[\mathrm{C}]^{1 / 3}$
The order of a reaction is?

1)1

2)0

3)15/12

4) (correct)7/6

Solution

The order of the reaction is the sum of the power of reactant concentration.

Order of reaction $=\frac{1}{2}+\frac{1}{3}+\frac{1}{3}$ Order of reaction $=\frac{3+2+2}{6}=\frac{7}{6}$

Hence, the answer is the option (4).

Example 4

Consider the reaction of ethyl acetate as given below

$\mathrm{CH}_3 \mathrm{COOEt}+\mathrm{H}_2 \mathrm{O} \longrightarrow \mathrm{CH}_3 \mathrm{COOH}+\mathrm{EtOH}$

The apparent order of the hydrolysis reaction of ethyl acetate is

1) (correct)1

2)2

3)3

4)4

Solution

Hydrolysis of Esters is a pseudo-first order reaction.

Because water is in excess. So 2^nd order reaction is converted to 1^st order reaction.

So, the order of reaction will be 1.

Hence, the answer is the option(1).

Summary

The pseudo-first-order reaction therefore exposes a unique view of reaction kinetics which relates theoretical models applied to practical fields. One central point that underlines its importance in an academic setting and an industrial application is its simplification of complex reactions without necessarily losing its accuracy.

Also check-

• NCERT Exemplar Class 11th Chemistry Solutions
• NCERT Exemplar Class 12th Chemistry Solutions
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NCERT Chemistry Notes :

• NCERT notes Class 11 Chemistry
• NCERT notes Class 12 Chemistry
• NCERT Notes For All Subjects