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April 8, 2025Pseudo first-order reactions are reactions that are not first-order but appear to be first order due to larger concentrations of one or more reactants than the other reactants. Some of the examples of pseudo first-order reactions involve Acid-catalysed hydrolysis of ethyl acetate, and inversion of sugarcane. The unit of the rate of the reaction
In this article, we will provide detailed information on Pseudo First-order reactions. This article will give one example of pseudo-first-order reaction and thus explain pseudo-first-order reaction. Scroll down to learn more about pseudo-first-order reactions and pseudo first-order reaction examples.
We know that the order of the reaction is determined by the rate of reaction’s dependency on the concentration of reactants. The order of the reaction is zero if the rate is independent of the concentrations of reactants. Similarly, the order of the reaction is one if the rate of reaction is proportional to the first power of the reactant concentration.
However, changing the concentration of the reactants, that is, increasing or decreasing the concentration of one or the other reactant, can sometimes change the order of a reaction.
Pseudo first-order reaction is a reaction that is not inherently first-order but is made by increasing or decreasing the concentration of one or the other components. The word ‘pseudo’ denotes ‘fake.’
As a result, it is evident from the name that it is not a first-order reaction. Certain conditions are changed to change the reaction sequence. Let’s look at some examples to learn more about pseudo first-order reactions.
Consider the general reaction,
Rate
However, if one of the reactants is present in a high concentration (solvent), then there is very little change in its concentration during the reaction.
In other words, the concentration of that reactant remains practically constant during the reaction. For example, if
Rate
The reaction, therefore, behaves as a first-order reaction in
Such reactions which are not truly of the first order but under certain conditions become reactions of the first order are called pseudo-first-order reactions.
Acid-catalysed hydrolysis of ethyl acetate:
Acid-catalysed inversion of cane-sugar:
Both the above reactions are bimolecular but are found to be of the first order; we can observe these reactions as
For the first reaction, rate of reaction
and for the second reaction, rate of reaction
The reason for such a behaviour is obvious from the fact that water is present in such a large excess that its concentration remains almost constant during the reaction.
The kinetics of the above reactions have been studied as follows:
In this reaction, acetic acid is one of the products, the amount of which can be found by titration against standard NaOH solution. But being an acid-catalyzed reaction, the acid present originally as a catalyst also reacts with
Combining results
Combining results
The maximum amount of
But the maximum amount of
Hence,
From equations
Substituting the values of a and
The hydrolysis of sucrose in the presence of a mineral acid takes place according to the equation:
An important characteristic of the reaction is that sucrose is dextro-rotatory, whereas the products glucose and fructose are dextro-rotatory and laevorotatory, respectively. Further, the laevo-rotation of fructose is more (being –
The kinetics of the above reaction is studied by noting the angle of rotation at different intervals of time with the help of a polarimeter. Suppose
It is evident that the reading at zero time will be positive and would decrease with the passage of time, pass through zero and ultimately become negative.
A little careful consideration shows that
Angle of rotation at any instant of time,
i.e.,
i.e.,
Angle of rotation at infinite time,
i.e.,
i.e.,
From equation
i.e.,
Substituting the values of
The applicability of this equation for the inversion of sucrose was first shown by Wilhelm
We are isolating a reactant in pseudo-first-order reactions by increasing the concentration of the other reactants. Changes in the concentrations of the other reactants do not affect the reaction when they are in excess; therefore, the reaction now just depends on the concentration of the isolated reactant. In the rate law, the concentrations of all other reactants are assumed to be constant. As a result, the order of reaction becomes one.
The unit of the rate of the reaction
For a pseudo first-order reaction
So,
Pseudo-first-order reactions are described as reactions that are in the higher order but under particular conditions behave as first-order reactions. Pseudo-first order kinetics is an important example. When a reaction is a
Q.1: What is the difference between first-order and pseudo-first-order reactions?
Ans: A first-order reaction is when the concentration of only one reactant changes or in which the reaction rate is proportional to the first power of the concentration of the reactant.
The reactions which are not truly of the first order but under certain conditions become reactions of the first order are called pseudo-first-order reactions.
Q.2: How do you solve a pseudo-first-order reaction?
Ans: In any reaction, if one of the components is in excess quantity, then the concentration change will be constant. The reaction is belonging to a pseudo-first-order reaction.
Q.3: What is the pseudo-first-order reaction?
Ans: The reactions apparently seem to be of higher order but under certain circumstances behave as first-order reactions referred to as pseudo-first-order reactions.
Q.4: What is the pseudo rate constant?
Ans: The unit of the rate of the reaction
where
For a pseudo first-order reaction
So,
Q.5: Does temperature affect the pseudo rate constant?
Ans: The reaction rate was found to increase with increasing temperature, indicating that a highly solvated transition state was formed during the reaction.
We hope this article on Pseudo First order reaction has helped you. If you have any queries, drop a comment below, and we will get back to you.
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