Pseudo First Order
Mediumchemistry
Hydrolysis of ester in large excess water shows pseudo first-order because:
Select the correct option:
Solution
Incorrect! Answer:
[Water] is effectively constant
- Theoretical Rate Law: Rate=k[Ester][H2O]. This is second-order overall.
- Concentration Effect: If the concentration of one reactant (H2O) is present in huge excess (e.g., 55.5 M), its concentration change during the reaction is negligible.
- Simplification: [H2O]≈constant. We can define a new rate constant k′=k[H2O].
- Apparent Law: Rate=k′[Ester].
- Definition: Such reactions, which are not truly first-order but behave as such under specific conditions, are called pseudo first-order reactions.
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About This Question
- Subject
- chemistry
- Chapter
- chemical kinetics
- Topic
- pseudo first order
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
[Water] is effectively constant
- Theoretical Rate Law: Rate=k[Ester][H2O]. This is second-order overall.
- Concentration Effect: If the concentration of one reactant (H2O) is present in huge excess (e.g., 55.5 M), its concentration change during the reaction is negligible.
- Simplification: [H2O]≈constant. We can define a new rate constant k′=k[H2O].
- Apparent Law: Rate=k′[Ester].
- Definition: Such reactions, which are not truly first-order but behave as such under specific conditions, are called pseudo first-order reactions.
This medium difficulty chemistry question is from the chapter chemical kinetics, covering the topic of pseudo first order. It appeared in the 2025 exam.
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