Question

Tollen's reagent is used for the detection of aldehydes. When a solution of $AgN{O}_3$ is added to glucose with $N{H}_{4}OH$, then gluconic acid is formed.
$A{g}^{+}+e^{-}\to Ag;E_{red}^o=0.80V$
$C_6{H}_{12}{O}_6+H_{2}O\to C_6{H}_{12}{O}_7+2H^{+}+2e^{-};E_{oxi}^o=-0.05V$
$Ag(N{H}_3{)}_2^{+}+e^{-}\to Ag(s)+2N{H}_3;E_{red}^o=0.337V$
$(\text{Take}:2.303\times \frac{RT}{F}=0.06\text{at}298K)$

When ammonia is added to the solution, $pH$ is raised to 11. Which half-cell reaction is affected by $pH$ and by how much?

• A. $E_{oxi}^o$ will increase by a factor of 0.66 from $E_{oxi}^o$
• B. $E_{oxi}^o$ will decrease by a factor of 0.66 from $E_{oxi}^o$
• C. $E_{red}^o$ will increase by a factor of 0.66 from $E_{red}^o$
• D. $E_{red}^o$ will decrease by a factor of 0.66 from $E_{red}^o$

Answer 1

The correct option is C $E_{red}^o$ will increase by a factor of 0.66 from $E_{red}^o$

In second half cell reaction;
$C_6{H}_{12}{O}_6+H_{2}O\to C_6{H}_{12}{O}_7+2H^{+}+2e^{-};E_{oxi}^o=-0.05V$
On increasing concentration of $N{H}_3$, the concentration of $H^{+}$ ion decreases, therefore,
$E_{red}=E_{red}^o-\frac{0.06}{2}\log [H^{+}{]}^2$
$E_{red}=0-\frac{0.06}{2}\times 2\log {10}^{-11}=0.66V$
i.e., $E_{red}$ increases by $0.66V$