What is the Difference Between First Order and Pseudo First Order Reaction?

The main difference between a first-order reaction and a pseudo first-order reaction is that a first-order reaction has an inherent order of one, while a pseudo first-order reaction is a second-order reaction that has been altered to behave like a first-order reaction under certain conditions.

First-Order Reactions:

The rate law for a first-order reaction is given by: $$rate = -k[A]$$
The half-life (t1/2) of a first-order reaction is related to the rate constant (k) as follows: $$t1/2 = \frac{\ln{2}}{k}$$
First-order reactions are characterized by a decrease in the concentration of a reactant by a constant percentage per unit time.

Pseudo First-Order Reactions:

A pseudo first-order reaction is a second-order reaction that has been altered to appear as a first-order reaction under certain conditions.
The rate law for a pseudo first-order reaction is given by: $$rate = -k[A]$$
The half-life (t1/2) of a pseudo first-order reaction is related to the pseudo rate constant (k') as follows: $$t1/2 = \frac{\ln{2}}{k'}$$
Pseudo first-order reactions simplify the quantification of reaction dynamics, making it easier to analyze second-order reactions.

In summary, the key difference between first-order and pseudo first-order reactions is that first-order reactions are genuine reactions with an inherent order of one, while pseudo first-order reactions are second-order reactions that have been altered to behave like first-order reactions under certain conditions.

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What is the Difference Between First Order and Pseudo First Order Reaction?

Comparative Table: First Order vs Pseudo First Order Reaction

First-order reactions and pseudo first-order reactions are different types of chemical reactions. Here is a comparison between the two:

First-order reactions	Pseudo first-order reactions
The reaction's order is one by nature.	The reaction is a second-order reaction made to behave like a first-order reaction.
Occur when the rate of the reaction depends on the concentration of only one reactant.	Occur when the rate of the reaction depends on the concentration of two reactants, but one of them is in great excess or constant concentration.
The rate law is given by: $$\frac{d[A]}{dt} = -k[A]$$.	The rate law is given by: $$\frac{d[A]}{dt} = -k[A]t$$ or $$\frac{d[A]}{dt} = -k^{'}[A]$$.
Used to simplify the analysis and quantification of reaction dynamics, especially when two reactants must be measured simultaneously.	Used to simplify the analysis and quantification of reaction dynamics, especially when two reactants must be measured simultaneously.

In summary, first-order reactions have an order of one by nature, while pseudo first-order reactions are second-order reactions that are made to behave like first-order reactions. Both types of reactions can simplify the analysis and quantification of reaction dynamics, but the underlying mechanisms and rate laws differ.

Guilherme Mazui

Guilherme Mazui is graduated in journalism from the Federal University of Minas Gerais (UFMG) and a master's degree in Communication from the University of São Paulo (USP). In addition, he has experience in advertising writing and has worked as a content editor in several companies.