Question

Why do we use Sulfuric acid in batteries?

Answer 1

Hint: Galvanic cells are used to transform chemical energy produced in a redox reaction into electrical energy. Batteries are a combination of these types of cells. Sulfuric acid is utilised in Lead storage batteries.

Complete Step by Step Solution:
In the lead storage battery, each cell consists of a lead anode and a grid of lead packed with lead dioxide \[(Pb{O_2})\] as a cathode. A solution of Sulfuric acid is utilised as an electrolyte.

Sulfuric acid provides the sulphate ions for the facilitation of the reaction.

Reactions happening at different electrodes are as follows:

Anode:\[Pb\left( s \right){\rm{ }} + {\rm{ }}SO_4^{2 - }{\rm{ }}\left( {aq} \right){\rm{ }} \to PbS{O_4}\left( s \right){\rm{ }} + {\rm{ }}2{e^ - }\]

Cathode: \[Pb{O_2}\left( s \right) + SO_4^{2 - }\left( {aq} \right){\rm{ }} + {\rm{ }}4{H^ + }{\rm{ }}\left( {aq} \right){\rm{ }} + {\rm{ }}2{e^ - }{\rm{ }} \to 2PbS{O_4}\left( s \right) + 2{H_2}O\left( l \right)\]

Overall reaction:
\[\begin{array}{*{20}{l}}{Pb\left( s \right){\rm{ }} + {\rm{ }}Pb{O_2}\left( s \right) + {\rm{ }}2{H_2}S{O_4}\left( {aq} \right){\rm{ }} \to 2PbS{O_4}\left( s \right){\rm{ }} + {\rm{ }}2{H_2}O{\rm{ }}\left( l \right)}\\\;\end{array}\]

On charging the battery, the reverse reaction occurs.
\[PbS{O_4}\left( s \right)\]is converted to Lead oxide and Lead.

Dilute Sulfuric acid gets bulkier than water while recharging, sinking to the foot of the battery. This speeds up the dissociation of sulfuric acid into Hydrogen ions and sulfuric ions. These sulfuric ions combine with lead and facilitate the oxidation process. Hence, facilitating the lead plate to be recharged instantly.
This acid has a huge affinity for water, so it combines again with water after charging.

If any acid other than Sulfuric acids like Hydrochloric acid or Nitric acid is used. It leads to the formation of Lead nitrates and Lead chlorates.

These compounds do not work well as lead sulphates work with Lead and Lead oxide.
Lead sulphate formed during discharge is solid and is glued to the electrodes. It is in a situation to either gain or lose electrons during electrolysis.

Thus, Hydrochloric and Nitric acids cannot be used for these batteries.
Moreover, Sulfuric acid is inexpensive and readily accessible.

So, Sulfuric acid is used in batteries.

Note: Anode is the electrode where oxidation occurs i.e., loss of electrons. At the cathode, reduction occurs i.e., a gain of electrons. The electrode which works as an anode (Lead) during discharge works as a cathode during charging. The electrode that works as a cathode during discharge (Lead dioxide) works as an anode during charging.