Question

Which of the following reactions are spontaneous?
(a) $P{b}^{4+}+S{n}^{2+}\to S{n}^{4+}+P{b}^{2+}$

(b) $S{n}^{4+}+P{b}^{2+}\to P{b}^{4+}+S{n}^{2+}$

($E_{P{b}^{4+}/P{b}^{2+}}^o=1.69V,E_{S{n}^{4+}/S{n}^{2+}}^o=0.15V$)

• A. Only (a)
• B. Only (b)
• C. Both (a) and (b)
• D. Neither (a) nor (b)

Answer 1

Answer: (A)

Only (a)

We know that $E_{cell}^o=E_{cathode}^o-E_{anode}^o$
$E^o$
Also, $S{n}^{2+}\to S{n}^{4+}+2e^{-}$ $-0.15V$
$P{b}^{4+}+2e^{-}\to P{b}^{2+}$ $1.69V$
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$P{b}^{4+}+S{n}^{2+}\to S{n}^{4+}+P{b}^{2+}$ $E_{cell}^o=1.69-0.15=+1.54V$
___________________________

$E^o$

Also, $P{b}^{2+}\to P{b}^{4+}+2e^{-}$ $-1.69V$

$S{n}^{4+}+2e^{-}\to S{n}^{2+}$ $0.15V$

__________________________

$S{n}^{4+}+P{b}^{2+}\to P{b}^{4+}+S{n}^{2+}$ $E_{cell}^o=0.15-1.69=-1.54V$

___________________________

For spontaneous rection $\Delta G<0$

(a) $\Delta G=-nF{E}_{cell}^o=-2\times F\times 1.54\Rightarrow \Delta G<0\Rightarrow spontaneous$
(a) $\Delta G=-nF{E}_{cell}^o=-2\times F\times (-1.54)\Rightarrow \Delta G>0\Rightarrow non-spontaneous$

Option A is the correct answer.