Medium
Explain pseudo first order reaction by giving an example.
Pseudo first order reaction: The order of a reaction is sometimes altered by experimental conditions. Consider a chemical reaction between two substances where one reactant is present in large excess,such that its concentration remains almost constant throughout the reaction. Such reactions are called pseudo first order reactions.
Example: During the hydrolysis of $0.01 \ mol$ of ethyl acetate with $10 \ mol$ of water,the amounts of the constituents at the beginning $(t=0)$ and at completion $(t)$ are as follows:
$CH_{3}COOC_{2}H_{5} + H_{2}O \rightarrow CH_{3}COOH + C_{2}H_{5}OH$
At $t=0$: $0.01 \ mol$ ethyl acetate,$10 \ mol$ water,$0 \ mol$ products.
At $t=t$: $0.0 \ mol$ ethyl acetate,$9.99 \ mol$ water,$0.01 \ mol$ products.
The concentration of water does not change significantly during the course of the reaction. Therefore,in the rate equation,
Rate $= k^{\prime}[CH_{3}COOC_{2}H_{5}][H_{2}O]$
the term $[H_{2}O]$ can be treated as a constant. The equation thus becomes:
Rate $= k[CH_{3}COOC_{2}H_{5}]$
where $k = k^{\prime}[H_{2}O]$.
Since the rate depends only on the concentration of one reactant,the reaction behaves as a first order reaction. Such reactions are called pseudo first order reactions.
Example: During the hydrolysis of $0.01 \ mol$ of ethyl acetate with $10 \ mol$ of water,the amounts of the constituents at the beginning $(t=0)$ and at completion $(t)$ are as follows:
$CH_{3}COOC_{2}H_{5} + H_{2}O \rightarrow CH_{3}COOH + C_{2}H_{5}OH$
At $t=0$: $0.01 \ mol$ ethyl acetate,$10 \ mol$ water,$0 \ mol$ products.
At $t=t$: $0.0 \ mol$ ethyl acetate,$9.99 \ mol$ water,$0.01 \ mol$ products.
The concentration of water does not change significantly during the course of the reaction. Therefore,in the rate equation,
Rate $= k^{\prime}[CH_{3}COOC_{2}H_{5}][H_{2}O]$
the term $[H_{2}O]$ can be treated as a constant. The equation thus becomes:
Rate $= k[CH_{3}COOC_{2}H_{5}]$
where $k = k^{\prime}[H_{2}O]$.
Since the rate depends only on the concentration of one reactant,the reaction behaves as a first order reaction. Such reactions are called pseudo first order reactions.
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