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Electrochemistry — Chemistry STD 12 Science — Question

Maharashtra BoardEnglish MediumSTD 12 ScienceChemistryElectrochemistry4 Marks
Question
Explain the construction and working of a dry cell (or Leclanche’s cell).###Write a note on dry cell.
✓

Answer

(A) Principle :
  • Leclanche’s cell is a primary voltaic cell.
  • It doesn’t contain mobile liquid electrolyte but contains moist viscose aqeuous paste of the electrolytes.
  • It is an irreversible voltaic cell which can’t be recharged.
(B) Construction :
(i) It consists of a small zinc vessel which serves as an anode (negative electrode).
Image
(ii) The zinc vessel contains a porous paper bag containing an inert graphite (C) electrode which serves as cathode, immersed in a paste of $MnO _2$ and carbon black. This paper bag divides the dry cell into two compartments, namely anode and cathode compartments.
(iii) The rest of the cell is filled with a moist paste of $NH _4 Cl$ and $ZnCl _2$ which acts as an electrolyte for zinc anode.
(iv) The graphite rod is fitted with a metal cap and the cell is sealed to prevent the drying of moist paste by evaporation.

(C) The dry cell can be represented as,
$- Zn \left| ZnCl _{2( aq )}, NH _2 Cl _{( aq )}, MnO _{2( s )}\right| C ^{+} \text {. }$

(D) Reactions in the dry cell :
(i) Oxidation at zinc anode :
$Zn _{( s )} \rightarrow Zn _{( aq )}^{2+}+2 e ^{-} \text {(oxidation half reaction) }$
(ii) Reduction at graphite (C) cathode :
The electrons released in the oxidation reaction at anode, flow to cathode through external circuit. Hydrogen in $NH _4$ ion is reduced to molecular hydrogen which reduces $MnO _2$ to $Mn _2 O _3$.
$\begin{aligned}
& 2 NH _{4( aq )}^{+}+2 e ^{-} \longrightarrow 2 NH _{3( aq )}+ H _{2( g )} \\
& 2 MNO _{2( s )}+ H _{2( g )} \longrightarrow Mn _2 O _{3( s )}+ H _2 O _{( l )}
\end{aligned}$
The overall cell reaction is
$\begin{array}{r} 
Zn _{( s )}+2 NH _{4( aq )}^{+}+2 MnO _{2( s )} \longrightarrow Zn _{( aq )}^{2+}+2 NH _{3( aq )} \\
+ Mn _2 O _{3( s )}+ H _2 O _{( l )}
\end{array}$
(iii) $Zn ^{2+}$ react with $NH _3$ and form a complex.
$Zn _{( aq )}^{2+}+4 NH _{3( aq )} \longrightarrow\left[ Zn \left( NH _3\right)_4\right]^{2+}( aq )$
Since $Zn ^{2+}$ ions are removed, the overall cell reaction can't be reversed.

(E) Uses of dry cell :
  • Dry cell is used as a source of electric power in radios, flashlights, torches, clocks, etc.
  • Since they are available in small size and portable, they can be used conveniently.

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