Question
Using the standard electrode potential, find out the pair between which redox reaction is not feasible. $${E^ \circ }$$ values : $$\frac{{F{e^{3 + }}}}{{F{e^{2 + }}}} = + 0.77;\frac{{{I_2}}}{{{I^ - }}} = + 0.54;$$ $$\frac{{C{u^{2 + }}}}{{Cu}} = + 0.34;\frac{{A{g^ + }}}{{Ag}} = + 0.80\,V$$
A.
$$F{e^{3 + }}\,{\text{and}}\,\,{I^ - }$$
B.
$$A{g^ + }\,{\text{and}}\,\,Cu$$
C.
$$F{e^{3 + }}\,{\text{and}}\,\,Cu$$
D.
$$Ag\,{\text{and}}\,\,F{e^{3 + }}$$
Answer :
$$Ag\,{\text{and}}\,\,F{e^{3 + }}$$
Solution :
For the reaction, $$2F{e^{3 + }} + 2{I^ - } \to 2F{e^{2 + }} + {I_2}$$
$$\eqalign{
& E_{{\text{cell}}}^ \circ = E_{\frac{{F{e^{3 + }}}}{{F{e^{2 + }}}}}^ \circ - E_{\frac{{{I_2}}}{{{I^ - }}}}^ \circ \cr
& \,\,\,\,\,\,\,\,\,\,\,\, = 0.77 - \left( {0.54} \right) \cr
& \,\,\,\,\,\,\,\,\,\,\,\, = + 0.23\,V \cr} $$
Here, $$E_{{\text{cell}}}^ \circ $$ is $$ + ve$$ so, reaction is feasible.
For the reaction, $$Cu + 2A{g^ + } \to C{u^{2 + }} + 2Ag$$
$$\eqalign{
& E_{{\text{cell}}}^ \circ = E_{\frac{{A{g^ + }}}{{Ag}}}^ \circ - E_{\frac{{C{u^{2 + }}}}{{Cu}}}^ \circ \cr
& \,\,\,\,\,\,\,\,\,\,\,\, = 0.80 - \left( {0.34} \right) \cr
& \,\,\,\,\,\,\,\,\,\,\,\, = + 0.46\,V \cr} $$
Here, $$E_{{\text{cell}}}^ \circ $$ is $$ + ve$$ so, the reaction is feasible.
For the reaction, $$2F{e^{3 + }} + Cu \to 2F{e^{2 + }} + C{u^{2 + }}$$
$$\eqalign{
& E_{{\text{cell}}}^ \circ = E_{\frac{{F{e^{3 + }}}}{{F{e^{2 + }}}}}^ \circ - E_{\frac{{C{u^{2 + }}}}{{Cu}}}^ \circ \cr
& \,\,\,\,\,\,\,\,\,\,\,\, = 0.77 - \left( {0.34} \right) \cr
& \,\,\,\,\,\,\,\,\,\,\,\, = + 0.43\,V \cr} $$
Here, $$E_{{\text{cell}}}^ \circ $$ is $$ + ve$$ so, the reaction is feasible.
For the reaction, $$Ag + F{e^{3 + }} \to A{g^ + } + F{e^{2 + }}$$
$$\eqalign{
& E_{{\text{cell}}}^ \circ = E_{\frac{{F{e^{3 + }}}}{{F{e^{2 + }}}}}^ \circ - E_{\frac{{A{g^ + }}}{{Ag}}}^ \circ \cr
& \,\,\,\,\,\,\,\,\,\,\,\, = 0.77 - \left( {0.80} \right) \cr
& \,\,\,\,\,\,\,\,\,\,\,\, = - 0.03\,V \cr} $$
Here, $$E_{{\text{cell}}}^ \circ $$ is negative so, the reaction is not feasible.