# Daniel cell

The cells which produce electrical energy from chemical reactions are known as galvanic or voltaic cells. A galvanic cell in which one electrode is Zn plate in $ZnSO_4$ (or $Zn^{++}$ ion) solution and the other is Cu plate in $CuSO_4 (or Cu^{++}$ ion) solution is called the Daniel cell.

A schematic diagram of the cell is shown below:

Daniell cell

The two half reactions,

I.e.

$Zn(s) \to Zn^{2+} (aq) + 2e^-$ At anode

$Cu^{2+}(aq) + 2e^- \to Cu(s)$ at cathode

are made to take place simultaneously with the electron transfer occurring through an extemal conducting wire. The two solutions (i.e., $ZnSO_4 \text{and} CuSO_4$) are linked by an inverted U-shaped tube as salt bridge. The tube is filled with a solution of an electrolyte like KCl, $KNO_3 or NH_4NO_3$ to which gelatin or agar-agar has been added to obtain a semi solid paste. Sometimes, a porous partition is used in place of salt bridge, which separates the two solutions.

At anode, metallic zinc passes into solution as $Zn^{2+}$ ions and liberates electrons. while at cathode, ions gain these electrons and are discharged as metallic copper. Hence net reaction may be represented as:

$Zn(s) + Cu^{2+} (aq) \to Cu(s) + Zn^{2+}(aq)$

It is the chemical energy of this reaction which is converted into electrical energy.

It is evident from the above equation that redox reaction is used as a source of electric energy in a galvanic cell. Some other examples of galvanic cells are given below:

$Zn(s) +2H^+ \to Zn^{2+} (aq.) + H_2 (g) \\[3mm] Cu(s) + 2Ag^+ (aq.) \to Cu^{2+} + 2Ag(s) \\[3mm] Fe(s) + Cu^{2+}(aq.) \to Fe^{2+} (aq.) + Cu(s) \\[3mm] Sn(s) + Cu^{2+} (aq.) \to Sn^{2+} (aq.) + Cu(s)$

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