Question

What is the sulphuric acid concentration of a fully charged battery electrolyte?

Answer 1

Hint: An electrolyte is a substance which has a tendency to produce an electrically conducting solution on dissolving it in polar solvent like water. On dissolving, the electrolytes dissociate into its respective cations and anions which disperse uniformly in the solvent and hence show electrical conduction.

Complete answer:
A lead-acid battery is composed of two lead plates which are separated by a liquid containing sulphuric acid in water. These types of batteries are rechargeable and consist of charging and discharging chemical reactions. On discharging of battery i.e., when a battery is being used, then electrons move from the negatively charged lead plate towards the positively charged plate. The reaction on negative plate and positive plate can separately be represented as follows:
Negative plate reaction:
 $ Pb(s) + HSO_4^ - (aq) \to PbS{O_4}(s) + {H^ + }(aq) + 2e $
Positive plate reaction:
 $ Pb{O_2}(s) + HSO_4^ - + 3{H^ + }(aq) + 2e \to PbS{O_4}(s) + 2{H_2}O(l) $
On combining both the reactions, the overall reaction for the lead-acid battery is as follows:
 $ Pb(s) + Pb{O_2}(s) + 2{H_2}S{O_4}(aq) \to 2PbS{O_4}(s) + 2{H_2}O(l) $
When the battery is fully charged, lead is considered as the negative plate, lead oxide is the positive plate and about 35 % of concentrated sulphuric acid and 65 % of water together behave as the electrolyte in the chemical process. If the battery is overcharged, the water gets electrolysed and formation of hydrogen gas and oxygen gas takes place. Therefore, to cover up this loss some lead-acid batteries allow water to add in it.
Hence, the sulphuric acid concentration of a fully charged battery electrolyte is $ 35 % $ .

Note:
Remember that when a battery is discharged, the reverse reaction takes place due to which lead sulphate is formed on both positively and negatively charged plates. If in case, a battery is fully discharged, then both the plates become identical and at that point, the battery will be considered as completely dead and cannot be recovered again.