Electrochemical Cells (or) Galvanic Cells

ELECTROCHEMICAL CELLS OR GALVANIC CELLS

Galvanic cells are electrochemical cells in which the electrons, transferred due to redox reaction, are converted into electrical energy.

Cell device (Construction)

Galvanic cell consists of a zinc electrode dipped in 1 M ZnSO₄ solution and a copper electrode dipped in 1 M CuSO₄ solution. Each electrode is known as a half cell. The two solutions are inter connected by a salt bridge and the two electrodes are connected by a wire through a voltmeter.

Reactions occurring in the cell

At anode: Oxidation takes place in the zinc electrode by the liberation of electrons, so this electrode is called negative electrode or anode.

At cathode: Reduction takes place in the copper electrode by the acceptance of electrons, so this electrode is called the positive electrode or cathode.

The electrons liberated by the oxidation reaction flow through the external wire and are consumed by the copper ions at the cathode.

Salt bridge

It consists of a U‒tube containing saturated solution of KCl or NH₄NO₃ in agar‒agar gel. It connects the two half cells of the galvanic cells.

Functions of salt bridge

(i) It eliminates liquid junction potential.

(ii) It provides the electrical continuity between the two half cells.

Conditions for a cell to act as standard cell

The conditions for an electrochemical cell to act as a standard cell are

(i) The e.m.f of the cell is reproductive.

(ii) The temperature‒coefficient of e.m.f (change in e.m.f with temperature) should be very low.

1. Representation of a galvanic cell (or) Cell diagram

(i) A galvanic cell consists of two electrodes anode and cathode.

(ii) The anode is written on the left hand side while the cathode is written on the right hand side.

(iii) The anode must be written by writing electrode metal first and then electrolyte. These two are separated by a vertical line or a semicolon. The electrolyte may be written by the formula of the compound (or) by ionic species.

Examples

(a) Zn/Zn²⁺ (or) Zn/ZnSO₄ (or) Zn; Zn²⁺

(b) Standard hydrogen electrode

 Pt, H₂ (1 atm); H⁺ (1 M)

(iv) The cathode must be written by writing electrolyte first and then the electrode metal. These two are separated by a vertical line or a semicolon.

Example: Cu²⁺/Cu (or) CuSO₄/Cu (or) Cu²⁺; Cu

(v) The two half cells are separated by a salt bridge, to which is indicated by two vertical lines.

Using the above representation, the galvanic cell is represented as follows.

 Zn / ZnSO₄ (1 M) // CuSO₄ (1 M) / Cu

 (or) Zn/Zn²⁺ (1 M) // Cu²⁺ (1 M)/Cu

 (or) Zn; Zn²⁺ (1 M) // Cu²⁺ (1 M); Cu