Electrochemistry MCQ Questions & Answers in Physical Chemistry | Chemistry

Primary cells are those cells, in which the reaction occurs only once and after use over a period of time, it becomes dead and cannot be reused again. e.g., Leclanche cell and mercury cell.

$$C{u^{2 + }} + {e^ - } \to C{u^ + };E_1^ \circ = 0.15\,V,$$       $$\Delta G_1^ \circ ,{n_1} = 1$$
$$C{u^{2 + }} + 2{e^ - } \to Cu;E_2^ \circ = 0.34\,V,$$       $$\Delta G_2^ \circ ,{n_2} = 2$$
$$C{u^ + } + {e^ - } \to Cu;E_3^ \circ = ?,\Delta G_3^ \circ ,$$       $${n_3} = 1$$
$$\Delta G_3^ \circ = \Delta G_2^ \circ - \Delta G_1^ \circ \Rightarrow - {n_3}FE_3^ \circ $$       $$ = - {n_2}FE_2^ \circ + {n_1}FE_1^ \circ $$
$$ - E_3^ \circ = - 2 \times 0.34 + 1 \times 0.15$$
$$E_3^ \circ = 0.68 - 0.15 = + 0.53\,V$$

Nickel-Cadmium battery
Anode - $$Cd;$$  Cathode - $$Ni{O_2};$$  Electrolyte - $$KOH$$
$$\eqalign{ & {\text{At anode :}}\,C{d_{\left( s \right)}} + 2OH_{\left( {aq} \right)}^ - \to Cd{\left( {OH} \right)_{2\left( s \right)}} + 2{e^ - } \cr & \underline {{\text{At cathode}}:\,Ni{O_{2\left( s \right)}} + 2{H_2}{O_{\left( l \right)}} + 2{e^ - } \to Ni{{\left( {OH} \right)}_{2\left( s \right)}} + 2OH_{\left( {aq} \right)}^ - } \cr & \underline {C{d_{\left( s \right)}} + Ni{O_{2\left( s \right)}} + 2{H_2}{O_{\left( l \right)}} \to Cd{{\left( {OH} \right)}_{2\left( s \right)}} + Ni{{\left( {OH} \right)}_{2\left( s \right)}}\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \cr} $$

When the value of $$\Delta {G^ \circ }$$  is negative, the cell reaction is spontaneous.
$$\Delta {G^ \circ } = - nF{E^ \circ }$$
where, $$n =$$  number of electrons take part
           $$F = $$  Faraday constant
         $${E^ \circ } = $$  $$EMF$$  of the cell
Thus, for a spontaneous reaction, the $$EMF$$  of the cell must be positive.

Electrolyte $$X$$ is strong electrolyte as on dilution the number of ions remain same, only interionic attraction decreases and hence not much increase in $${ \wedge _m}$$  is seen. While $${ \wedge _m}$$  for a weak electrolyte increases significantly.

$$\eqalign{ & Cu + 2A{g^ + }\left( {aq} \right) \to C{u^{2 + }}\left( {aq} \right) + 2Ag\left( s \right) \cr & {\text{Here,}}\,n = 2,\,E_{cell}^ \circ = + 0.46\,V \cr & \Delta {G^ \circ } = - n{E^ \circ }F \cr & = \frac{{ - 2 \times 0.46 \times 96500}}{{1000}}kJ \simeq - 89\,kJ \cr} $$

TIPS/FORMULAE :
(i) In a galvanic cell oxidation occurs at anode and reduction occurs at cathode.
(ii) Oxidation occurs at electrode having higher oxidation potential and it behaves as anode and other electrode acts as cathode.
(iii) $${E_{Cell}} = {E_C} - {E_A}$$
(substitute reduction potential at both places).
$$\eqalign{ & F{e^{2 + }} + Zn \to Z{n^{2 + }} + Fe \cr & \because \,Zn \to Z{n^{ + + }} + 2{e^ - }\,{\text{and}}\,F{e^{2 + }} + 2{e^ - } \to Fe \cr} $$
∴ $$Zn$$ is anode and $$Fe$$ is cathode.
$${E_{Cell}} = {E_C} - {E_A} = - 0.41 - \left( { - 0.76} \right) = 0.35V.$$

$$\eqalign{ & {\text{Mass of }}Ag{\text{ in coated layer}} = V \times d \cr & = 1 \times {10^{ - 3}} \times 100 \times 1.05 \cr & = 0.105\,g \cr & W = \frac{{I \times t \times {\text{Eq}}.\,wt.}}{{96500}} \cr & t = \frac{{W \times 96500}}{{I \times {\text{Eq}}.\,wt.}} \cr & \,\,\,\, = \frac{{0.105 \times 96500}}{{5 \times 108}} \cr & \,\,\,\, = 18.7\,s \cr} $$

$$\eqalign{ & {\lambda _\infty } = {\lambda _a} + {\lambda _c} \cr & \,\,\,\,\,\,\,\,\, = 315 + 35 \cr & \,\,\,\,\,\,\,\,\, = 350\,mho\,c{m^2}\,e{q^{ - 1}} \cr} $$

$$\eqalign{ & Pt + C{l_2} \to P{t^{2 + }} + 2C{l^ - };E_{{\text{cell}}}^ \circ = 0.15\,V \cr & \underline {P{t^{2 + }} + 2{e^ - } \to Pt;{E^ \circ } = 1.20\,V\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \cr & \underline {C{l_2} + 2{e^ - } \to 2C{l^ - };{E^ \circ } = 1.35\,V\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} \cr} $$