Advanced Topic 430 of 1102 Kinetics & Equilibrium
Pseudo First Order Reaction
A higher-order reaction behaves as first order when one reactant is in large excess.
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In-depth explanation of Pseudo First Order Reaction
A pseudo first order reaction is a higher order reaction whose rate appears to depend on only one reactant, because another reactant is present in such large excess that its concentration barely changes. The rate law rate = k[A][B] with [B] nearly constant becomes rate = k'[A], where k' = k[B]0 is an observed rate constant. This approximation simplifies kinetic studies, since the excess reagent can be ignored and the data analysed with first order equations. It is widely used for hydrolysis reactions carried out in water, where water is the solvent and is far in excess. The approach is valid only while the excess species remains essentially unchanged.
Going deeper: Pseudo First Order Reaction
Key ideas: The excess reagent should be at least about 10 to 20 times the limiting reagent so its concentration changes by under 5 to 10 percent. Measure k' at several excess concentrations and plot k' against [B]0: the slope is the true second order k. This isolation technique lets you analyse a complex rate law one reagent at a time.
Common mistakes and connections: Students call any reaction in water pseudo first order, even when the solute is not the limiting reagent, or forget that k' contains the constant concentration so its units are per s, not L per mol per s. Another error is quoting the half-life of k' as if it were a true property of the reaction. Examples include acid hydrolysis of esters and inversion of cane sugar.
Visualization of Pseudo First Order Reaction
With one reagent in huge excess (such as water) its concentration is constant, so rate = k[B]0[A] behaves like a first order decay in A. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The hydrolysis of an ester in a large excess of water behaves as a first order reaction in ester, because the water concentration is effectively constant.
Frequently asked questions about Pseudo First Order Reaction
What is Pseudo First Order Reaction?
A higher-order reaction behaves as first order when one reactant is in large excess.
Is Pseudo First Order Reaction a beginner, intermediate or advanced topic?
Pseudo First Order Reaction is a advanced topic in Kinetics & Equilibrium. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Pseudo First Order Reaction?
Study these earlier topics in Kinetics & Equilibrium first: First Order Reaction, Second Order Reaction, Half-life of Reactions.
What comes after Pseudo First Order Reaction?
The next topics in Kinetics & Equilibrium are Method of Initial Rates and Integrated Rate Laws.
Can I practise Pseudo First Order Reaction with quizzes?
Yes. The interactive lesson includes a self-check question on Pseudo First Order Reaction, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Pseudo First Order Reaction fits in Kinetics & Equilibrium
Pseudo First Order Reaction is topic 11 of 91 in Kinetics & Equilibrium.