Question

Give the construction and working of a lead accumulator along with the chemical reactions involved.

Answer 1

Lead Accumulator:

1. The lead accumulator is a secondary electrochemical cell since electrical energy and emf are not developed within the cell but it is previously stored by passing an electric current. Hence it is also called a lead accumulator or lead storage battery.
2. It is reversible since the electrochemical reaction can be reversed by passing an electric current in opposite direction and consumed reactants can be regenerated.
3. Hence battery can be charged after it is discharged.

Construction:

1. In a lead accumulator, the negative terminal (anode) is made up of lead sheets packed with spongy lead, while the positive terminal (cathode) is made up of lead grids packed with $Pb{O}_2$.

Working:

1. In a lead-acid accumulator, the anode and cathode are made of lead dioxide and lead respectively.
2. Electrolyte is dilute sulphuric acid.
3. As power is discharged from the accumulator, both the anode and cathode undergo a chemical reaction that progressively changes them into lead sulphate.
4. When the anode and cathode are connected by a wire, current flows from anode to cathode through the wire.

Chemical Reaction for Discharging:

1. When the battery is discharged, it acts as a galvanic cell and the following chemical reaction occurs:

Negative:

$Pb\left(s\right)+HS{O_4}^{-}+H_{2}O\left(l\right)\to 2e^{-}+PbS{O}_4\left(s\right)+H_3{O}^{+}\left(aq\right)\left(oxidation\right)$

Positive:

$Pb{O}_2\left(s\right)+HS{O_4}^{-}\left(aq\right)+3H_3{O}^{+}\left(aq\right)+2e^{-}\to PbS{O}_4\left(s\right)+5H_{2}O\left(l\right)\left(reduction\right)$

1. Lead sulfate is formed at both the electrodes.
2. Two electrons are also transferred in the complete reaction.
3. The lead-acid battery is packed in a thick rubber or plastic case to prevent leakage of the corrosive sulphuric acid.

Lead Acid Battery Charging:

1. The sulphuric acid existing in the lead discharge battery decomposes and needs to be replaced.
2. Sometimes, the plates change their structure by themselves.
3. Eventually, the battery becomes less efficient and should be charged or changed.

Chemical Reaction for Recharging:

The chemical reaction that takes place when the lead-acid battery is recharging can be found below:

Negative:

$2e^{-}+PbS{O}_4\left(s\right)+H_3{O}^{+}\left(aq\right)\to Pb\left(s\right)+HS{O_4}^{-}+H_{2}O\left(l\right)\left(reduction\right)$

Positive:

$PbS{O}_4\left(s\right)+5H_{2}O\left(l\right)\to Pb{O}_2\left(s\right)+HS{O_4}^{-}\left(aq\right)+3H_3{O}^{+}\left(aq\right)+2e^{-}\left(oxidation\right)$