Reversible and Irreversible Cells
A Galvanic cell always has two electrodes in which there is difference in the tendency to give up electrons. The electrodes are properly arranged so that current can flow. Galvanic cells may be either reversible or irreversible in the thermodynamic sense. A reversible cell is one which does not give any current when it is connected in an opposite sense to an outside source of e.m.f., exactly equal to that of the cell. If the e.m.f. of the external source is infinitesimally higher than that of the cell then the current will go in the opposite direction, i.e., the cell reaction will be reversed; on the other hand, if the value is slightly lower current will flow from the cell. Daniel cell is an example of a reversible cell.
The chemical reaction of the cell is reversed and the current flows in opposite direction when the external emf is slightly higher than that of the cell. Any other cell, which does not obey the above two conditions, is termed as irreversible. Daniel cell is reversible but Zn, H2SO4, Ag etc., cell is irreversible in nature
When in a cell reaction a gas is evoked or a precipitate is formed the cell is irreversible because the cell cannot be brought back to its original condition by reversing the current. As an example of an irreversible cell consider zinc and copper electrodes which dip into a solution of sulphuric acid. When the cell is producing current the cell reaction will be
Zn (s) + 2 H+ (aq) ↔ Zn2+ (aq) + H2 (g)
The zinc electrode dissolves to form zinc ions, and hydrogen is liberated at the copper electrode. When the current is reversed the above reaction is not reversed. Instead the following reaction takes place-
Cu (s) + 2 H+ (aq) ↔ Cu2+ (aq) + H2 (g)
Copper goes into solution and hydrogen gas is liberated.
Irreversible cells are not amenable to exact theoretical treatment. For reversible cells thermodynamic principles may be applied. The work done in a reversible cell is the maximum and the maximum work can be related to other thermodynamic quantities. In reversible cells both the electrodes are reversible with respect to one or other of the ions in solution.