Test Index

IIT JEE Advanced 2006 Paper 1

Paragraph for Questions 30–32:

Tollen’s reagent is used for the detection of aldehyde when a solution of

\mathrm{AgNO}_3

is added to glucose with

\mathrm{NH_4OH}

then gluconic acid is formed

\mathrm{Ag}^{+}+e^{-}

→ Ag ;

E_{red}^{+}

= 0.8 V

\mathrm{C_6H_{12}O_6} + \mathrm{H_2O}

→ Gluconic acid

(\mathrm{C_6H_{12}O_7}) + 2\mathrm{H}^{+} + 2e^{-}

;

E_{red}^{\circ}

= - 0.05 V

\mathrm{Ag}(\mathrm{NH}_3)_2^{+} + e^{-}

→ Ag (s) +

2\mathrm{NH}_3

;

E_{red}^{\circ}

= 0.337 V

[Use 2.303 ×

\frac{RT}{F}

= 0.0592 and

\frac{F}{RT}

= 38.92 at 398 K]

Section: Chemistry

Question : 71 of 120

Marks: +1, -0

Solution:

The expression for oxidation reaction is

E_{oxd}

E_{oxd}^{\circ}

\frac{2.303RT}{nF}

log

\frac{[\mathrm{C_6H_{12}O_6}]}{[\mathrm{C_6H_{12}O_7}][\mathrm{H}^+]^2}

The expression for reduction is

E_{red}

E_{red}^{\circ}

\frac{2.303RT}{nF}

log

\frac{1}{[\mathrm{Ag}^+]}

The addition of

\mathrm{NH_3}

results in formation of more

[\mathrm{Ag(NH_3)_2}]^+

complex, thus reducing the concentration of

\mathrm{Ag}^+

ions available for reduction. Thus addition of

\mathrm{NH_3}

causes concentration of

[\mathrm{H}^+]

to decrease and pH to increase. So

E_{red}

increases approximately by a factor of 0.65 V.

E_{red}

E_{red}^{\circ}

\frac{2.303RT}{nF}

log

[\mathrm{H}^+]^2

= 0 -

\frac{0.0592}{2}

× 2 × 11 = 0.6512

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