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The reaction that appears to be an nth order reaction, whereas it belongs to some different order is known as the pseudo order reaction. These are the reactions that are generally not first-order but appear to be first order due to increased concentrations of one or more reactants than the other reactants. For example the pseudo first-order reactions include Acid-catalysed hydrolysis of ethyl acetate, and inversion of sugarcane. As we all know that the order of the reaction can be determined by the rate of reaction’s dependence on the concentration of reactants. The order of the reaction in these cases is zero when the rate is independent of the concentrations of reactants. Similarly, the order of the reaction is the one in which the rate of reaction is proportional to the first power of the reactant concentration.
Any reaction which is not first-order reaction naturally but it is made first order via increasing or decreasing the concentration of one or the other reactant is called Pseudo first order reaction. Pseudo refers to ‘fake’. Thus, we get clear information from the name itself that a reaction is not a first-order reaction by nature. The order of reaction is made one by altering certain conditions.
Examples of Pseudo First Order Reaction
Hydrolysis of an ester: an example of this type of reaction is seen in, ethyl acetate which upon hydrolysis in aqueous solution using a mineral acid as catalyst results in the formation of acetic acid and ethyl alcohol.
ethyl acetate (is formed in excess) acetic acid ethyl alcohol. Here a huge amount of water is used and the rate law can now be written as
The reaction is mainly second-order but in practice it is observed to be first-order. Hence, it represents a pseudo-1st order reaction.
Hydrolysis of sucrose: Sucrose upon hydrolysis in the presence of a dilute mineral acid results in the formation of glucose and fructose.
Pseudo First Order Reaction vs First Order Reaction
The major difference is usually that, in a first order reaction, the order of reaction is one by nature. A pseudo first-order reaction is a second order reaction by nature but it has been altered to make it a first order reaction. The second point of difference is that in a first order reaction, the rate of reaction is based on all the reactants whereas, in a pseudo-first-order reaction, the rate of reaction based only on the isolated reactant as a difference in the concentration of the reactant in excess usually will not affect the reaction.
Half-Life in a Pseudo-1st Order reaction
Half-life is defined as the time needed to decrease the concentration of a reactant by half, so we must solve it for t. Here, [B] represents the reactant in excess, and its concentration will remain constant. [A]o represents the initial concentration of A; so the half-life concentration of A is calculated to be 0.5[A]o.
The pseudo-1st-order reaction equation is written as:
[A] = [A]oe−[B]kt or [A]/[A]o = e−k′t
By taking natural logs on both sides of the pseudo-1st-order equation, we get: ln([A]/[A]o)=k’t
As the concentration of A for a half-life t1/2 is 1/2[A]o :
ln(1/2[A]o / [A]o)= ln(1/2) = −k’t1/2
So again recalling that k’ = k[B], [B]≈[B]o and that −ln(1/2)=ln2: ln(2) = k[B]ot1/2
t1/2 = ln2k[B]o
Conclusion
In this article, we have learned about the pseudo-first-order reaction. With the help of two different examples of pseudo-first-order reactions, we have helped you to know different reactions that are not of first-order reaction but appear to be of first order because of the higher concentration of the reactants. We hope that you were able to grasp a clear concept of pseudo- first order reaction.
