Pseudo First Order Reaction

Pseudo means fake or false. A pseudo-first-order reaction is defined as a reaction that appears like a higher order reaction but is actually of a lower order. A pseudo-first-order reaction is a bimolecular reaction that appears like a first order (1st order) reaction due to more concentration of reactants than the other reactants. But, it is different from the first-order reaction. In this article, we are going to learn about the pseudo-first-order reaction. We can determine the order of the reaction by the dependency of the reaction rate on reactants concentration. We can say an order is a first-order reaction if the rate of reaction is directly proportional to the first power of concentration of the reactants. The order of the pseudo-first-order reaction is defined as the sum of the powers of concentration of reactants in the rate law expression, which is also known as the order of that particular chemical reaction. We can classify the order of the reactions as a first-order reaction, second-order reaction, zero-order reaction, or pseudo-first-order reaction depending on the reactants’ concentration.

Rate Of Reaction

The change in the concentration of any one of either reactants or products of the chemical reaction is commonly known as the rate of that particular reaction. Units of the rate of a reaction are mol/Ls or M/s or atm/time units.

Rate Constant

We can define the Rate constant as the rate of reaction at a specific time when the molecular concentration of the reactants is taken as unity. For example, for a chemical reaction, A + B ——> AB,

Rate = K[A][B]

where, 

• [A] is the concentration of A,
• [B] is the concentration of B.

Units of the rate constant are K = (1/Time) * (1/[Conc.]^n-1).

Graph of a Pseudo-Reaction

In the case of pseudo, a first-order reaction chemical reaction takes place between two substances and one reactant is present in access. Let’s discuss the pseudo-first-order reaction in detail. 

Let’s take a reaction like a A + b B —–> c C + d D. Its rate of reaction is given by,

Rate = K (A^x B^y)

where,

• x and y are considered as the concentrations of reactants A and B.

Order the whole reaction will be x + y. As x is the order of reaction concerning A and y is concerning B.

Graphical representation of Pseudo-reaction

Examples Of Pseudo first-order Reaction

Hydrolysis of Ester (or ethyl Acetate)

CH₃COOC₂H₅ + H₂O ——–> CH₃COOH + C₂H₅OH

Rate = K [CH₃COOC₂H₅] [H₂O]

In the above reaction, concentration of water is constant during the reaction.

So, we can write the rate of the following reaction as below,

Rate = K [CH₃COOC₂H₅]

In the above reaction, hydrolysis of ethyl acetate results in the formation of Ethanoic acid and Ethanol.

Therefore, hydrolysis of the ester is a perfect example of a pseudo first order reaction.

Hydrolysis of Sugar (or sucrose)

C₁₂H₂₂O₁₁ + H₂O ———> C₆H₁₂O₆ + C₆H₁₂O₆

Rate = K [C₁₂H₂₂O₁₁] [H₂O]

In the above reaction, concentration of water is constant during the reaction.

So, we can write the rate of the following reaction as below.

Rate = K [C₁₂H₂₂O₁₁]

In the above reaction sucrose hydrolysis results in glucose and fructose formations.

Therefore, we can say hydrolysis of cane sugar (sucrose) is the perfect example pseudo first order reaction.

FAQs on Pseudo First Order Reaction

Question 1. What do you mean by pseudo-first-order reaction?

Answer:

Pseudo means fake or false. The reaction which appears like a higher order reaction but is actually of a lower order is called a pseudo first order reaction. Pseudo first order reaction is a bimolecular reaction that appears like first order (1st order) reaction due to more concentration of reactants than the other reactants. But, pseudo first order reaction is different from first order reaction.

Question 2. Give two examples of pseudo-first-order reactions.

Answer:

Example-1: Hydrolysis of ethyl ethanoate
Reaction : CH₃COOC₂H₅ +H₂O —-> CH₃COOH + C₂H₅OH

Example-2: Hydrolysis of sugar
Reaction : C₁₂H₂₂O₁₁ + H₂O —–> C₆H₁₂O₆ + C₆H₁₂O₆

Question 3. How can we say that the given reaction is a pseudo-first-order reaction?

Answer:

The reactive is a bimolecular reaction and the order of the reaction is one first order reaction. This happens only when one of the reactants is excess in amount. In this way, we can figure out whether the given reaction is a pseudo first order reaction or not.

Question 4. What is the rate constant? Explain with an example.

Answer:

We can define the Rate constant as the rate of reaction at a specific time when the molecular concentrations of the reactants are taken as unity. For a chemical reaction, A + B ——> AB, the formula is given by, R = K[A][B] where, [A] is the concentration of A and [B] is the concentration of B.

Question 5. How can we say a reaction is whether a reaction is first order or second order or zero order reactions?

Answer:

We can classify the order of the reactions as first order reaction second-ordered reaction or zero order reaction or pseudo-first order reaction depending on the concentration of the reactants.