The key difference between first order and pseudo first order reaction is that first order reactions proceed at a rate that depends linearly only on one reactant concentration whereas pseudo first order reactions are second order reactions that are made to behave as first order reactions.
The term “pseudo” means “not genuine”. Therefore, a pseudo reaction has the properties of a particular type of chemical reactions, but the reaction actually is a completely different chemical reaction.
CONTENTS
1. Overview and Key Difference
2. What are First Order Reactions
3. What are Pseudo First Order Reactions
4. Side by Side Comparison – First Order vs Pseudo First Order Reaction in Tabular Form
5. Summary
What are First Order Reactions?
First order reactions are chemical reactions where the rate of reaction depends on the molar concentration of one of the reactants that is involved in the reaction. According to the definition of “the order of reaction”, the sum of the powers to which the reactant concentrations are raised in the rate law equation will always be 1 for a first order reaction. There can be either a single reactant that takes part in these reactions or more than one reactant. At the first situation, the concentration of that single reactant determines the rate of the reaction. At the second situation, one of the reactants that take part in the reaction will determine the rate of the reaction.
In order to understand this concept, let us consider the decomposition reaction of N2O5, which forms NO2 and O2 gases as products. Since it has only one reactant, we can write the reaction and the rate equation as follows.
2N2O5(g) → 4NO2(g) + O2(g)
Rate = k[N2O5(g)]m
In this rate equation, k is the rate constant for this reaction and m is the order of the reaction. Then the experimental determinations should give the value of m as 1. Therefore, we can identify that this is a first order reaction.
What are Pseudo First Order Reactions?
Pseudo first order reactions are second order chemical reactions that are made to behave as first order reactions. Therefore, these reactions can be called bimolecular reactions as well. This type of reactions occurs when one reacting material occurs a great excess concentration in the reaction mixture, and thus, it appears as a constant concentration compared to the concentrations of other substances.
A sample second order reaction can be given as follows:
In this reaction, the rate of the reaction depends on both “A” concentration and “B” concentration. But the concentration of “A” is very high compared to the concentration of “B”, and the change in concentration of “A” during the progression of the reaction appears to be negligible. Then we can predict the order of this reaction as 1 because the change in concertation appears for only “B”. However, the reaction is actually a second order reaction because the rate of the reaction depends on both reactants. Therefore, we can categorize this type of reactions as pseudo first order reactions.
What is the Difference Between First Order and Pseudo First Order Reaction?
First order reactions and pseudo first order reactions are different from each other. The key difference between first order and pseudo first order reaction is that first order reactions proceed at a rate that depends linearly only on one reactant concentration whereas pseudo first order reactions are second order reactions that are made to behave as first order reactions.
Below is a summary tabulation of the difference between first order and pseudo first order reaction.
Summary – First Order vs Pseudo First Order Reaction
First order reactions and pseudo first order reactions are different from each other. The key difference between first order and pseudo first order reaction is that first order reactions proceed at a rate that depends linearly only on one reactant concentration whereas pseudo first order reactions are second order reactions that are made to behave as first order reactions.
Reference:
1. “Pseudo First Order Reaction – Rate Law, Order of Reaction & Examples.” BYJUS, 12 Aug. 2020, Available here.