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Pseudo First Order Reaction can be defined as a second order or bimolecular reaction that is prepared to act like a first-order reaction. This reaction occurs when a particular reacting material is present in excess or maintained at a consistent pace when compared with other substances. Here, we will learn about the pseudo first order reaction and its examples.
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Key Terms: Pseudo-first-order reactions, lower order kinetics, hydrolysis of the cane sugar, hydrolysis of ester, bimolecular reaction, second order reaction
Pseudo First Order Reaction Definition
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The pseudo-first-order reactions are the reactions that appear to be second order in nature, but taken as a first-order reaction on a close analysis. Thus an example of second-order of the reaction as given by the equation is:
A+B → C+D
This reaction is based on the concentrations of both A and B. However, one component is present in large amounts whose concentration hardly changes as the reaction proceeds.
Hence, if component B is in large amounts and the concentration of B is very high as compared to that of A, the reaction will be considered as a pseudo-first-order reaction with respect to A. Again, if component A is in large amounts and the concentration of A is very high as compared to that of B, the reaction is considered to be pseudo-first order with respect to B.
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| Wittig Reaction | Collision Theory | SN2 Reaction |
| Order of Reaction | Anode and Cathode | Wurtz Reaction |
Pseudo First Order Reaction Example
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Reactions appear to be higher order but they in fact follow lower order kinetics (first order) are known as pseudo first order reactions. Pseudo first order reactions can be understood better with the help of following examples:
Hydration of Alkyl Halide
CH3I + H2O → CH3OH + H+ + I-
Rate of reaction = k [CH3I] [H2O]
The concentration of water is much higher than methyl iodide as methyl iodide is also used in aqueous solution form.
[CH3I] <<< [H2O]
Therefore, concentration of water does not change here and can be considered as no change or constant.
Rate of reaction = k’ [CH3I]
Hence, the reaction is the first order reaction, but it is actually the second order reaction and this is why it is called pseudo first order reaction.
Hydrolysis of Sugarcane
C12H22O11 + H2O → C6H12O6 + C6H12O6
Sucrose, Water, Glucose, Fructose
Rate of reaction = k [ C12H22O11] [ H2O ]
However, H2O >>> C12H22O11
Therefore, here the concentration of water can be estimated as constant as its concentration does not change as such during the reaction.
Now, the rate of reaction can be written as:
r = k’ [C12H22O11]
Where, k’ = k [ H2O ]
Hence, the reaction is the first order reaction, but it is actually the second order reaction and this is why the hydrolysis of the cane sugar is called pseudo first order reaction.
Hydrolysis of sucrose
C12 H22 O11 + H2O ------------ C6 H12 O6 + C6 H12 O6
Rate = k [C12 H22 O11]¹ [H2O]0
This reaction appears as second order but is first order, as water does not have any effect on rate.
Additionally, Hydrolysis of Ester also appears to be second order but water (H2O) does not show any effect on rate so it is a good example for pseudo first order reaction.
Hydrolysis of Ester
CH3COOC2H5 + H2O → CH3COOH + C2H5OH
Ethyl ethanoate, Water, Ethanoic acid, Ethanol
Hydrolysis of Ester
Rate of reaction = k [CH3COOC2H5 ] [H2O ]
But, [H2O ] >>> [CH3COOC2H5 ]
Therefore, we can assume that during the reaction, concentration of water remains almost constant.
So, we can write the rate of reaction as:
r = k’ [CH3COOC2H5 ]
k’ = k [H2O]
Hence, hydrolysis of ester is a pseudo first order reaction.
Things to Remember
- The order of the pseudo first order reaction is made one by altering certain conditions.
- In such a reaction, we basically isolate a reactant by increasing the concentration of the other reactants.
- When the other reactants are in excess, the change in their concentrations does not affect the reaction much.
- The reaction only depends on the concentration of the isolated reactant.
- The concentrations of all the other reactants are considered as constant in the rate law which is why the order of reaction becomes one.
Also Read:
Sample Questions
Ques: Which one of the following is an example of pseudo first order reaction: (1 mark)
a)CH3CHO4(g) → CH4(g) + CO(g)
b)2H2O2(aq) → H2O (1) + O2(g)
c)Isomerisation of cyclopropane to propene
Ans: c.
Is an example of pseudo first order reaction.
Ques: Which one of the following is known as pseudo first order reaction: (1 mark)
a)Decomposition of acetaldehyde
b)Acid hydrolysis of ester
c)Isomerisation of cyclopropane to propene
d)Decomposition of hydrogen peroxide
Ans: The correct answer is b. Acid hydrolysis of an ester.
Ques: Inversion of sucrose in the presence of dilute acid is said to be a pseudo first order reaction. (2 marks)
a)Why is it said to be so?
b)Give another example of pseudo order reaction.
Ans: a. 
The rate of reaction only depends on the concentration of sucrose and it is independent of the concentration of water. Therefore, the reaction is approximated as the second order but actually follows the first order kinetics.
- Hydrolysis of ester in the presence of acid.
Ques: The hydrolysis of ester in acid medium is a first order reaction. (3 marks)
a)What do you mean by first order reaction?
b)State the relation between state constant and Half-Life period of a reaction.
c)Half-life period of a first order reaction is 20 s. How much time will it take to complete 90% of the reaction?
Ans: a. When the sum of the powers of the concentration in terms of the rate expression is one, that reaction is called a first order reaction.
R = k [A]1
- In the case of first order reaction,
Ques: What do you mean by pseudo first order reaction? (2 marks)
Ans: The pseudo-first-order reactions are the reactions that appear to be second order in nature, but taken as a first-order reaction on a close analysis. Thus an example of second-order of the reaction as given by the equation is:
A+B → C+D
This reaction is based on the concentrations of both A and B. However, one component is present in large amounts whose concentration hardly changes as the reaction proceeds.
Ques: How hydrolysis of sugarcane is considered as pseudo first reaction? (2 marks)
Ans: C12H22O11 + H2O → C6H12O6 + C6H12O6
Sucrose, Water, Glucose, Fructose
Rate of reaction = k [ C12H22O11] [ H2O ]
However, H2O >>> C12H22O11
Therefore, here the concentration of water can be estimated as constant as its concentration does not change as such during the reaction.
Now, the rate of reaction can be written as:
r = k’ [C12H22O11]
Where, k’ = k [ H2O ]
Hence, the reaction is the first order reaction, but it is actually the second order reaction and this is why the hydrolysis of the cane sugar is called pseudo first order reaction.
Ques: State another example of pseudo first order reaction. (3 marks)
Ans: Another example of pseudo first order reaction is hydration of alkyl halide:
CH3I + H2O → CH3OH + H+ + I-
Rate of reaction = k [CH3I] [H2O]
The concentration of water is much higher than methyl iodide as methyl iodide is also used in aqueous solution form.
[CH3I] <<< [H2O]
Therefore, concentration of water does not change here and can be considered as no change or constant.
Rate of reaction = k’ [CH3I]
Hence, the reaction is the first order reaction, but it is actually the second order reaction and this is why it is called pseudo first order reaction.
Previous Year Questions
Ques: Define the following terms:
(a) Pseudo first order reaction.
(b) Half life period of reaction (t1/2). (Delhi 2014)
Ans: (a) Those reactions which are not truly of the first order but under certain conditions become first order reactions are known as the pseudo first order reactions.
(b) The time taken for half of the reaction to complete is called half life period.
Ques: i. What is the order of the reaction whose rate constant has the same units as the rate of reaction?
ii)For a reaction A + H2O → B; Rate ∝ [A],
Write the order of this reaction. (Comptt. All India 2017)
Ans:
- The reaction whose rate constant has the same units as the rate of reaction, will have zero order of reaction.
- The reaction A + H2O → B Rate ∝ [A]
The order of this reaction will be a pseudo first order reaction as the rate of reaction depends only on concentration of A only.
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