Question

Give reasons : (I) Why does a mercury cell deliver a constant voltage for its entire life ? (II) Why is it necessary to use a salt bridge in a galvanic cell ?

Hint:

Mercury cell: concentrations fixed; salt bridge: completes circuit & balances charge.

Answer 1

(I) answer: A mercury cell gives a steady voltage right through its life because nothing in it changes concentration. Inside the cell the reactants and products are all solids (zinc and mercuric oxide turning into zinc oxide and mercury), and the electrolyte is a thick paste, not a normal solution. Since no dissolved ion gets more or less concentrated as the cell works, the cell potential, which would otherwise depend on ion concentrations, has nothing pulling it up or down. So the voltage stays constant until the cell is used up.

(II) answer: A salt bridge is needed for two reasons. First, it completes the circuit by connecting the two half-cells so that the current can flow as a full loop. Second, as the reaction runs, one half-cell tends to build up positive charge and the other negative charge. The salt bridge lets its ions move into each side to cancel this build-up and keep both solutions electrically neutral. It also stops the two solutions from mixing directly. Without it the charge build-up would quickly stop the cell from working.