What is a Pseudo First Order Reaction? (Definition, Rate Law & Examples)
Pseudo First Order Reaction is essential in chemistry and helps students understand various practical and theoretical applications related to this topic. This concept is a fundamental part of chemical kinetics and frequently appears in rate law, order of reaction, and mechanism questions in exams and assignments.
What is Pseudo First Order Reaction in Chemistry?
- A pseudo first order reaction refers to any chemical reaction that is not truly first order, but behaves like a first order reaction under specific conditions.
- This happens when one of the reactants is present in a large excess compared to the other, causing its concentration to remain effectively constant throughout the reaction.
- This concept appears in chapters related to rate law, order of reaction, and hydrolysis reactions, making it a foundational part of your chemistry syllabus.
Molecular Formula and Composition
Pseudo first order reaction does not describe a specific molecule, but a reaction pattern. Typically, the general form of a pseudo first order reaction is:
A + B → Products
If [B] is in large excess, its concentration is nearly constant. The reaction seems to depend only on [A]:
Rate = k’[A], where k’ = k[B]n. This feature is what makes it “pseudo” (false) first order, when in fact it may be second order or higher.
Preparation and Synthesis Methods
Pseudo first order kinetics is not about preparing a compound, but about setting reaction conditions. To observe pseudo first order behavior in the lab, simply use one reactant in large excess over the other.
For example, in hydrolysis reactions, abundant water is used so that its concentration does not change significantly during the reaction.
Frequent Related Errors
- Confusing pseudo first order with a true first order reaction mechanism or order.
- Forgetting to check which reactant is in excess and wrongly applying the pseudo-order formula.
- Writing the wrong rate law (ignoring the reactant in constant concentration).
- Mismatching molecularity and order; pseudo first order affects order, not molecularity.
Uses of Pseudo First Order Reaction in Real Life
Pseudo first order kinetics is widely used in calculating rate constants in industrial, biological, and laboratory processes. Reactions such as hydrolysis of esters, inversion of cane sugar, and decomposition of aspirin often use this approach for easy calculation and experimental simplicity.
It is also helpful in studying enzyme kinetics and catalyzed reactions where one substance, like water or the catalyst, is present in large excess.
Relation with Other Chemistry Concepts
Pseudo first order reaction is closely related to first order reactions, true second order reactions, and the concept of molecularity vs. order. It also helps bridge ideas in types of chemical reactions and is foundational for understanding hydrolysis and substitution reaction mechanisms.
Step-by-Step Reaction Example
- Consider the hydrolysis of ethyl acetate:
CH3COOC2H5 + H2O → CH3COOH + C2H5OH - Write the rate law for the reaction:
Rate = k [CH3COOC2H5][H2O] - Set up experimental conditions with water in large excess.
[H2O] is nearly constant, combine with k: Rate = k’ [CH3COOC2H5], where k’ = k[H2O] - The reaction now follows pseudo first order kinetics.
Lab or Experimental Tips
To identify a pseudo first order reaction, always look for a reactant used in much larger quantity than the other. If the concentration of one reactant remains almost unchanged during the reaction, you can treat the reaction as first order in the variable reactant.
Vedantu educators often recommend plotting ln[A] vs. time to verify a straight line, confirming pseudo first order behavior.
Try This Yourself
- Give two examples of reactions that are usually treated as pseudo first order in the lab.
- Write the effective rate law for the hydrolysis of sucrose.
- Explain why adding excess water helps simplify the determination of the reaction order.
Final Wrap-Up
We explored pseudo first order reaction—its definition, common examples like ester hydrolysis and sugar inversion, and why it matters in chemistry labs. Understanding pseudo first order kinetics helps you solve problems faster and clarifies tricky exam concepts. For more step-by-step guidance and live sessions on chemical kinetics, visit Vedantu.
| Reaction Type | Definition | Order Depends On | Common Examples |
|---|---|---|---|
| First Order | Rate depends only on single reactant | [A]1 | Radioactive decay |
| Pseudo First Order | True order > 1 but reduced due to excess reactant | [A] (with [B] constant) | Ester hydrolysis, cane sugar inversion |
| Second Order | Rate depends on two different reactants | [A][B] | SN2 reactions, many organic substitutions |
FAQs on Pseudo First Order Reaction in Chemistry
1. What is a pseudo first order reaction?
A pseudo first order reaction is a chemical reaction that is truly of higher order but appears to be first order because one reactant is present in large excess and its concentration remains nearly constant during the reaction. This simplification allows the reaction rate to depend only on the concentration of the limiting reactant, following first order kinetics.
2. Give two examples of pseudo first order reactions.
Common pseudo first order reaction examples include:
- Hydrolysis of esters (e.g., ethyl acetate with water in acid medium).
- Inversion of cane sugar (sucrose hydrolysis) in excess water.
3. What is the rate law for a pseudo first order reaction?
The rate law for a pseudo first order reaction is usually written as:
- Rate = k’[A]
- Here, k’ (apparent rate constant) = k[B]n, where [B] is in large excess.
- The reaction appears first order with respect to [A].
4. Is hydrolysis of an ester a pseudo first order reaction?
Yes, the hydrolysis of an ester in the presence of excess water is a classic pseudo first order reaction. Although two reactants are involved, the concentration of water remains almost constant, making the reaction rate effectively dependent on the ester alone.
5. What is the difference between first order and pseudo first order reaction?
First order reaction: Reaction is truly first order, depending only on the concentration of a single reactant.
Pseudo first order reaction: Truly higher order, but behaves like first order due to one reactant being in large excess. The rate appears to depend only on the limiting reactant.
6. Is inversion of cane sugar a pseudo first order reaction?
Yes, inversion of cane sugar in the presence of acid and excess water behaves as a pseudo first order reaction since water's concentration remains constant, and the rate depends mainly on sucrose concentration.
7. What is meant by apparent rate constant (k’) in pseudo first order reactions?
The apparent rate constant (k’) in a pseudo first order reaction is the product of the true rate constant and the constant concentration of the reactant in excess, raised to its order. For example, k’ = k[B]n, where [B] is in large excess.
8. How can you identify a pseudo first order reaction experimentally?
To identify a pseudo first order reaction experimentally:
- Keep one reactant in large excess compared to the other.
- Monitor the rate dependence—if rate appears to depend only on the limiting reactant, it's pseudo first order.
- Plot ln[A] versus time to get a straight line, similar to true first order kinetics.
9. Can SN2 reactions be pseudo first order?
SN2 reactions are normally second order, but if one reactant (usually the nucleophile or substrate) is in great excess, the reaction may exhibit pseudo first order kinetics, with rate appearing to depend on only the limiting reactant's concentration.
10. Why are pseudo first order reactions important in chemical kinetics?
Pseudo first order reactions are important because:
- They simplify complex rate laws by reducing the number of variables.
- Allow easier experimental determination of reaction rates and constants.
- Help in the study of biochemical and environmental reactions where solvents like water are in large excess.
11. What is the graphical representation of a pseudo first order reaction?
For a pseudo first order reaction, plotting ln[A] versus time yields a straight line with a negative slope, similar to a true first order reaction. This indicates the reaction’s rate depends only on the concentration of the limiting reactant.
12. How can you distinguish between a second order and a pseudo first order reaction?
Distinguishing between second order and pseudo first order reactions:
- Second order: Rate depends on both reactant concentrations ([A][B]).
- Pseudo first order: Appears first order as one reactant's concentration remains nearly constant (in excess), so rate ≈ k’[A].
