The effect of pH on the rate constant for the | vedclass

Name: Exam Paper Generator | JEE NEET Paper Generator | Vedclass
Brand: Vedclass
Rating: 5 (35 reviews)

Question

(B) The hydrolysis of an ester is a reaction catalyzed by both $H^+$ ions (acid catalysis) and $OH^-$ ions (base catalysis). The rate law for the hydr

Vedclass · Accepted Answer

Vedclass · Answer

(B) The hydrolysis of an ester is a reaction catalyzed by both $H^+$ ions (acid catalysis) and $OH^-$ ions (base catalysis). The rate law for the hydrolysis of an ester can be expressed as: $Rate = k_0[Ester] + k_H[H^+][Ester] + k_{OH}[OH^-][Ester]$.Thus,the observed rate constant $k$ is $k = k_0 + k_H[H^+] + k_{OH}[OH^-]$.At low $pH$ (acidic medium),$[H^+]$ is high,so $k \approx k_H[H^+]$. Taking logarithms,$\log k = \log k_H + \log[H^+] = \log k_H - pH$. This shows a linear decrease of $\log k$ with increasing $pH$.At high $pH$ (basic medium),$[OH^-]$ is high,so $k \approx k_{OH}[OH^-]$. Since $pH + pOH = 14$,$[OH^-] = 10^{-(14-pH)}$. Thus,$\log k = \log k_{OH} + \log[OH^-] = \log k_{OH} - 14 + pH$. This shows a linear increase of $\log k$ with increasing $pH$.At neutral $pH$ $(pH \approx 7)$,the rate is minimum due to the combined effect of both acid and base catalysis. The graph of $\log k$ versus $pH$ shows a $V$-shaped curve with a minimum at $pH \approx 7$.

The effect of $pH$ on the rate constant for the hydrolysis of an ester is given by:

Similar Questions

For the following parallel chain reaction,what will be the value of the overall half-life of $A$ in minutes?
Given that $\frac{[B]_t}{[C]_t} = \frac{16}{9}$
$A \xrightarrow{k_1 = 2 \times 10^{-3} \ s^{-1}} 4B$
$A \xrightarrow{k_2} C$

The catalyst for the hydrolysis of cane sugar is ........

The temperature coefficient for a reaction in which food deteriorates is $2$. Then food deteriorates ...... times as rapidly at $25\,^oC$ as it does at $5\,^oC$.

Which of the following statements is correct?

Vedclass Test Series

Exam Paper Generator

Online Exam Module