Which of the following is false for a pseudo first-order reaction?

a) The reaction follows first-order kinetics b) Molecularity of the reaction should be one c) The reactants such as water are present in excess and are assumed to be constant throughout the reaction d) Unit of the rate constant is s^{-1} The order of the reaction can sometimes be altered by taking one of the reactants in large excess compared to the other. Pseudo first order reactions are those reactions in which the molecularity is more than one but follows first order kinetics. therefore,Answer: b

Why are pseudo first-order reactions important in chemical kinetics?

Pseudo first-order reactions are important because they simplify the study of complex reactions. By controlling the concentration of one reactant, scientists can more easily analyze reaction rates and determine rate constants for higher-order reactions.

How does temperature affect the rate of a pseudo first-order reaction?

Temperature affects the rate of a pseudo first-order reaction similarly to other reactions. An increase in temperature generally leads to an increase in the reaction rate and the pseudo first-order rate constant (k'). This relationship is often described by the Arrhenius equation.

What are the limitations of using pseudo first-order conditions in kinetics studies?

Limitations of using pseudo first-order conditions include:

Can pseudo first-order conditions be applied to reactions with more than two reactants?

Yes, pseudo first-order conditions can be applied to reactions with more than two reactants. In such cases, all reactants except one are kept in large excess, making the reaction appear first-order with respect to the limiting reactant.

What is a pseudo first-order reaction?

A pseudo first-order reaction is a second-order (or higher-order) reaction that behaves like a first-order reaction due to one reactant being in large excess. The concentration of the excess reactant remains essentially constant throughout the reaction, simplifying the rate law.

Why is it common to study second-order reactions under pseudo first-order conditions?

Studying second-order reactions under pseudo first-order conditions simplifies the kinetics analysis. It allows researchers to determine rate constants more easily and accurately, as the mathematics involved in pseudo first-order reactions is simpler than that of true second-order reactions.

How can you determine the rate constant (k') from the graphical representation of a pseudo first-order reaction?

To determine the rate constant (k') from the graphical representation, plot ln[A] vs. time (t). The slope of the resulting straight line is equal to -k'. Therefore, k' is the negative of the slope.

What is the relationship between the pseudo first-order rate constant (k') and the true second-order rate constant (k)?

The pseudo first-order rate constant (k') is related to the true second-order rate constant (k) by the equation: k' = k[B], where [B] is the concentration of the reactant in excess. This relationship allows you to calculate the true second-order rate constant from pseudo first-order data.

Can a pseudo first-order reaction become a true second-order reaction? If so, how?

Yes, a pseudo first-order reaction can become a true second-order reaction if the concentration of the excess reactant decreases significantly during the reaction. This transition occurs when the assumption that [B] remains constant is no longer valid, and the reaction rate depends on the concentrations of both reactants.

What is the integrated rate law for a pseudo first-order reaction?

The integrated rate law for a pseudo first-order reaction is: ln[A] = -k't + ln[A]₀, where [A] is the concentration of the limiting reactant at time t, [A]₀ is the initial concentration, and k' is the pseudo first-order rate constant.

How does the rate law for a pseudo first-order reaction differ from a true first-order reaction?

The rate law for a pseudo first-order reaction looks similar to a true first-order reaction, but the rate constant (k') includes the concentration of the excess reactant. For example, if A + B → Products, and [B] is in excess, the rate law becomes: Rate = k'[A], where k' = k[B].

How can you identify a pseudo first-order reaction experimentally?

You can identify a pseudo first-order reaction by observing that the reaction rate depends on the concentration of only one reactant, even though the overall reaction involves two or more reactants. The concentration of the other reactant(s) remains effectively constant throughout the reaction.

How does the half-life of a pseudo first-order reaction compare to a true first-order reaction?

The half-life of a pseudo first-order reaction behaves similarly to that of a true first-order reaction. It is independent of the initial concentration of the limiting reactant and is given by the equation: t₁/₂ = ln(2) / k', where k' is the pseudo first-order rate constant.

What is the graphical representation of a pseudo first-order reaction?

The graphical representation of a pseudo first-order reaction is a straight line when ln[A] is plotted against time (t). The slope of this line is equal to -k', the negative of the pseudo first-order rate constant.

What are some common examples of pseudo first-order reactions?

Common examples of pseudo first-order reactions include:

What is the difference between a pseudo first-order reaction and a consecutive first-order reaction?

A pseudo first-order reaction is a higher-order reaction that behaves like a first-order reaction due to one reactant being in excess. A consecutive first-order reaction, on the other hand, involves two or more first-order steps occurring in sequence, where the product of one step becomes the reactant for the next step.

What is the significance of the y-intercept in the graphical representation of a pseudo first-order reaction?

In the graphical representation of a pseudo first-order reaction (ln[A] vs. time), the y-intercept represents ln[A]₀, where [A]₀ is the initial concentration of the limiting reactant. This allows for the determination of the initial concentration if it is unknown.

How can you convert a pseudo first-order rate constant to a true second-order rate constant?

To convert a pseudo first-order rate constant (k') to a true second-order rate constant (k), divide k' by the concentration of the excess reactant: k = k' / [B], where [B] is the concentration of the reactant in excess.

How does the order of a reaction change when pseudo first-order conditions are applied?

When pseudo first-order conditions are applied to a second-order (or higher-order) reaction, the observed order of the reaction with respect to the limiting reactant becomes first-order. The overall order of the reaction appears to decrease due to the constant concentration of the excess reactant.

How does the concept of pseudo first-order reactions relate to the method of initial rates?

The concept of pseudo first-order reactions is often used in conjunction with the method of initial rates. By using a large excess of all reactants except one, researchers can determine the order of the reaction with respect to the limiting reactant more easily, as the initial rate depends only on its concentration.

What is the relationship between the half-life and the rate constant in a pseudo first-order reaction?

In a pseudo first-order reaction, the half-life (t₁/₂) is inversely proportional to the pseudo first-order rate constant (k'). The relationship is given by the equation: t₁/₂ = ln(2) / k'. This means that as the rate constant increases, the half-life decreases, and vice versa.

What is the significance of the linearity in the ln[A] vs. time plot for a pseudo first-order reaction?

The linearity in the ln[A] vs. time plot for a pseudo first-order reaction confirms that the reaction follows first-order kinetics with respect to the limiting reactant. Any deviation from linearity suggests that the reaction is not pseudo first-order or that other factors are influencing the reaction rate.

How does the concentration of the excess reactant affect the pseudo first-order rate constant?

The concentration of the excess reactant is directly proportional to the pseudo first-order rate constant (k'). As the concentration of the excess reactant increases, k' increases linearly. This relationship is expressed as k' = k[B], where k is the true second-order rate constant and [B] is the concentration of the excess reactant.

Can pseudo first-order conditions be applied to gas-phase reactions?

Yes, pseudo first-order conditions can be applied to gas-phase reactions. In such cases, one gaseous reactant is kept in large excess, and its partial pressure remains essentially constant throughout the reaction. The rate law and integrated rate law are then expressed in terms of partial pressures instead of concentrations.

How does the presence of a catalyst affect a pseudo first-order reaction?

A catalyst can increase the rate of a pseudo first-order reaction by lowering the activation energy. While the catalyst does not change the order of the reaction, it can increase both the true second-order rate constant (k) and the observed pseudo first-order rate constant (k'). The relationship k' = k[B] still holds in the presence of a catalyst.

What is the difference between a pseudo first-order rate constant and an observed rate constant?

A pseudo first-order rate constant (k') is specific to reactions under pseudo first-order conditions, where one reactant is in large excess. An observed rate constant, on the other hand, is a general term that refers to the experimentally determined rate constant for any reaction, regardless of its order or conditions.

How can you use pseudo first-order conditions to determine the activation energy of a reaction?

To determine the activation energy using pseudo first-order conditions:

What is the significance of the pseudo first-order rate constant in chemical kinetics?

The pseudo first-order rate constant (k') is significant because:

How does the concept of pseudo first-order reactions apply to radioactive decay?

While radioactive decay is a true first-order process, the concept of pseudo first-order reactions can be applied when studying the effect of radioactive decay on chemical reactions. For example, if a radioactive isotope is used as a reactant in large excess, its decay can be treated as a pseudo first-order process in the context of the overall reaction.

Can a pseudo first-order reaction have a non-zero reaction order with respect to the excess reactant?

In a strict sense, a pseudo first-order reaction has a zero-order dependence on the excess reactant. However, in some cases, there may be a small, non-zero order with respect to the excess reactant if its concentration changes slightly during the reaction. This situation is sometimes referred to as a "mixed-order" reaction.

How does the initial concentration of the limiting reactant affect the rate of a pseudo first-order reaction?

In a pseudo first-order reaction, the initial concentration of the limiting reactant does not affect the rate constant (k') or the half-life. However, it does affect the initial reaction rate. A higher initial concentration of the limiting reactant will result in a faster initial rate, following the rate law: Rate = k'[A].

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

Chemistry
Chemical Kinetics
pseudo first order reaction

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

Q: How does the concept of pseudo first-order reactions apply to enzyme kinetics?

In enzyme kinetics, pseudo first-order conditions are often used to study the initial rates of enzyme-catalyzed reactions. When the substrate concentration is much higher than the enzyme concentration, the reaction follows pseudo first-order kinetics with respect to the enzyme concentration.

Team Careers360Updated on 02 Jul 2025, 04:48 PM IST

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.

This Story also Contains

Pseudo First-Order
Variants and Examples Explored
Applications to Real Life and Academics
Some Solved Examples
Summary

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.

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}
$

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.

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 $\rightarrow$ 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 Chemistry Notes :

Frequently Asked Questions (FAQs)

Q: How does the concept of pseudo first-order reactions apply to enzyme inhibition studies?

In enzyme inhibition studies, pseudo first-order conditions can be used to:

Q: What is the relationship between pseudo first-order kinetics and steady-state approximation?

While pseudo first-order kinetics and steady-state approximation are distinct concepts, they are both used to simplify complex reaction kinetics. Pseudo first-order conditions simplify the rate law by keeping one reactant in excess, while steady-state approximation assumes that the concentration of an intermediate remains constant. In some cases, both approximations can be applied to the same reaction system to facilitate kinetic analysis.

Q: How can you use pseudo first-order conditions to study the kinetics of complex reaction mechanisms?

To study complex reaction mechanisms using pseudo first-order conditions:

Q: What is the difference between a pseudo first-order reaction and a pseudo zero-order reaction?

A pseudo first-order reaction appears first-order in the limiting reactant due to the excess of other reactants. A pseudo zero-order reaction, on the other hand, appears to have a constant rate independent of reactant concentrations. This can occur when a reactant is in such excess that changes in its concentration do not affect the rate, or when a limiting factor (e.g., enzyme saturation) controls the rate.

Q: How does the ionic strength of a solution affect the rate of a pseudo first-order reaction?

The ionic strength of a solution can affect the rate of a pseudo first-order reaction by:

Q: Can you apply the concept of pseudo first-order reactions to photochemical processes?

Yes, the concept of pseudo first-order reactions can be applied to photochemical processes. For example, in photodegradation studies, if the light intensity is kept constant and in excess, the reaction can be treated as pseudo first-order with respect to the degrading compound. This simplifies the kinetic analysis of complex photochemical systems.

Q: How does the solvent affect the rate of a pseudo first-order reaction?

The solvent can affect the rate of a pseudo first-order reaction by:

Q: What is the significance of the pseudo first-order approximation in atmospheric chemistry?

In atmospheric chemistry, the pseudo first-order approximation is often used because many reactive species (e.g., OH radicals) are present in very low concentrations compared to their reaction partners (e.g., pollutants). This approximation simplifies the analysis of complex atmospheric reactions and helps in modeling air quality and climate change.

Q: How can you use pseudo first-order conditions to study the kinetics of acid-base reactions?

To study acid-base reactions using pseudo first-order conditions:

Q: What is the relationship between the pseudo first-order rate constant and the reaction's rate-determining step?

The pseudo first-order rate constant (k') is related to the rate-determining step of the reaction. If the rate-determining step involves only the limiting reactant, k' directly reflects its rate constant. If the rate-determining step involves both reactants, k' incorporates the concentration of the excess reactant and the true rate constant of that step.

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