Reversible and Irreversible cells

REVERSIBLE AND IRREVERSIBLE CELLS

Daniell cell has the emf value 1.09 volt. If an opposing emf exactly equal to 1.09 volt is applied to the cell, the cell reaction,

                           Zn + Cu2+ --> Cu + Zn2+

stops but if it is increased infinitesimally beyond 1.09 volt, the cell reaction is reversed.

                           Cu + Zn2+ --> Zn + Cu2+

Such a cell is termed a reversible cell. Thus, the following are the two main conditions of reversibility:

(i) The chemical reaction of the cell stops when an exactly equal opposing emf is applied.

(ii) The chemical reaction of the cell is reversed and the current flows in opposite direction when the opposing emf is slightly greater than that of the cell.

Any other cell which does not obey the above two conditions is termed as irreversible. A cell consisting of zinc and copper electrodes dipped into the solution of sulphuric acid is irre­versible. Similarly, the cell

                Zn|H2S04(aq)|Ag

is also irreversible because when the external emf is greater than the emf of the cell, the cell reaction,

                         Zn + 2H+ --> Zn2+ + H2

is not reversed but the cell reaction becomes

                        2Ag + 2H+ --> 2Ag+ + H2

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