Q31 / 120 · JEE Advanced 2006 · Chemistry
Paragraph:
Tollen's reagent is used for the detection of aldehyde when a solution of $\mathrm{AgNO}_3$ is added to glucose with $\mathrm{NH}_4 \mathrm{OH}$ then gluconic acid is formed
$\begin{array}{rr}\mathrm{Ag}^{+}+e^{-} \rightarrow \mathrm{Ag} & E_{\text {red }}^{\circ}=0.8 \mathrm{~V} \\ \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6+\mathrm{H}_2 \mathrm{O} \rightarrow \text { Gluconic acid }\left(\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_7\right)+2 \mathrm{H}^{+}+2 e^{-} ; & -E_{\mathrm{red}}^{\circ}=-0.05 \mathrm{~V} \\ \mathrm{Ag}\left(\mathrm{NH}_3\right)_2^{+}+e^{-} \rightarrow \mathrm{Ag}(s)+2 \mathrm{NH}_3 ; & E_{\mathrm{red}}^{\circ}=0.337 \mathrm{~V}\end{array}$
[Use $2.303 \times \frac{R T}{F}=0.0592$ and $\frac{F}{R T}=38.92$ at $298 \mathrm{~K}$ ]Question:
When ammonia is added to the solution, $\mathrm{pH}$ is raised to 11 . Which half-cell reaction is affected by $\mathrm{pH}$ and by how much?
- A$E_{\text {oxd }}$ will increase by a factor of $0.65$ from $E_{\text {oxd }}^{\circ}$
- B$E_{\text {oxd }}$ will decrease by a factor of $0.65$ from $E_{\text {oxd }}^{\circ}$
- Correct answerC$E_{\text {red }}$ will increase by a factor of $0.65$ from $E_{\text {red }}^{\circ}$
- D$E_{\text {red }}$ will decrease by a factor of $0.65$ from $E_{\text {red }}^{\circ}$
Solution
On increasing concentration of $\mathrm{NH}_3$, the concentration of $\mathrm{H}^{+}$ion decreases, therefore
$$
\begin{aligned}
E_{\mathrm{red}} & =E_{\mathrm{red}}^{\circ}-\frac{0.0592}{n} \log _{10}\left[\mathrm{H}^{+}\right]^2 \\
& =0-\frac{0.0592}{2} \times 2 \log _{10} 10^{-11} \quad\left(\because \mathrm{pH}=11 ; \therefore\left[\mathrm{H}^{+}\right]=10^{-11} \mathrm{M}\right) \\
& =-0.0592 \times 11=0.6512 \mathrm{~V}=0.65 \mathrm{~V}
\end{aligned}
$$
$E_{\text {red }}$ increased by a factor of $0.65$ from $E_{\text {red }}^{\circ}$.
Hence, the correct answer is option: 'C'.