Electrolyte used in a lead-acid cell is

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Examoogle · Accepted Answer

dilute $H_2SO_4$

Explanation: The electrolyte used in a lead-acid battery is dilute sulfuric acid ($H_2SO_4$). It acts as the medium for ion transfer between the lead (anode) and lead dioxide (cathode) plates during the charge and discharge processes.

Working Principle: During discharge, the $H_2SO_4$ dissociates into hydrogen and sulfate ions. Sulfate ions react with the active material on the plates (Pb and $PbO_2$) to form lead sulfate ($PbSO_4$), while the hydrogen ions contribute to the formation of water. This process causes a reduction in the specific gravity of the electrolyte, which is a key indicator of the battery's state of charge.

Key Formulas: $Pb + PbO_2 + 2H_2SO_4 \rightarrow 2PbSO_4 + 2H_2O$ — Overall chemical reaction during discharge

Key Points: The concentration of $H_2SO_4$ is typically around 30-40% by weight when fully charged.; Specific gravity of the electrolyte is measured using a hydrometer to determine charge status.; Using pure water or concentrated acid can lead to battery damage or ineffective chemical reactions.; The electrolyte is consumed during discharge, producing water as a byproduct.

Advantages: High surge current capability; Low internal resistance for high-load applications

Disadvantages: Heavy weight due to lead content; Requires maintenance of electrolyte levels (topping up with distilled water)

Applications: Automotive SLI (Starting, Lighting, Ignition) systems; Uninterruptible Power Supply (UPS) systems

Additional Info: Concentrated $H_2SO_4$ is not used because it would corrode the plates and cause rapid self-discharge.. Option B is incorrect because 'only' implies pure acid, which is highly reactive and not suitable as an electrolyte.. Option C is incorrect because pure water is a poor conductor of electricity.

Answer: D is correct — The electrolyte in a lead-acid cell is a mixture of water and sulfuric acid, known as dilute $H_2SO_4$.

Exam Tip: Always remember that in lead-acid batteries, the electrolyte becomes denser during charging and lighter (more water content) during discharging.

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