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When a Lead Storage Battery is Discharged, is Sulphuric Acid Consumed?

Published in Battery Chemistry 3 mins read

Yes, sulphuric acid is indeed consumed when a lead storage battery is discharged. This consumption is a fundamental part of the electrochemical reactions that generate electricity.

The Discharge Process Explained

During the discharge cycle of a lead-acid battery, chemical energy is converted into electrical energy. This process involves the lead (Pb) electrode and the lead dioxide (PbO₂) electrode reacting with the sulphuric acid (H₂SO₄) electrolyte.

Here's a breakdown of what happens:

  • At the Negative Electrode (Anode - Lead Plate):
    Lead (Pb) reacts with sulfate ions (SO₄²⁻) from the sulphuric acid to form lead sulfate (PbSO₄) and release electrons.
    • Pb + SO₄²⁻ → PbSO₄ + 2e⁻
  • At the Positive Electrode (Cathode - Lead Dioxide Plate):
    Lead dioxide (PbO₂) reacts with hydrogen ions (H⁺) and sulfate ions (SO₄²⁻) from the sulphuric acid, accepting electrons from the external circuit to form lead sulfate (PbSO₄) and water (H₂O).
    • PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O

Overall Reaction During Discharge:
When these two half-reactions are combined, the net result shows the consumption of sulphuric acid:

Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O

As clearly demonstrated by the overall reaction, sulphuric acid (H₂SO₄) is a reactant and is thus consumed during discharge. Simultaneously, water (H₂O) is produced.

Impact on the Electrolyte

The consumption of sulphuric acid and the production of water have a direct impact on the electrolyte's composition:

  • Depletion of Sulphuric Acid: As the battery discharges, the concentration of sulphuric acid in the electrolyte decreases.
  • Increase in Water Content: Water is generated during the reaction, which further dilutes the electrolyte.
  • Lower Specific Gravity: The specific gravity of the electrolyte (a measure of its density and acid concentration) drops significantly as the battery discharges, becoming more watery. This change in specific gravity is often used to determine the battery's state of charge.

This depletion of sulphuric acid means that a fully discharged battery's electrolyte will consist largely of water, with a very low acid concentration.

Recharging the Battery

Fortunately, the process is reversible in a lead-acid battery. When the battery is recharged, an external electrical current forces the reverse chemical reactions to occur:

  • Lead sulfate (PbSO₄) on both electrodes is converted back into lead (Pb) at the negative plate and lead dioxide (PbO₂) at the positive plate.
  • Crucially, water (H₂O) is consumed, and sulphuric acid (H₂SO₄) is regenerated. This replenishes the acid in the electrolyte, returning the sulfate to the acid and increasing its specific gravity.

Summary of Battery States

The following table summarizes the key changes in the electrolyte during discharge and charge cycles:

Aspect During Discharge During Charge
Sulphuric Acid Consumed Produced (regenerated)
Water Produced Consumed
Electrolyte Density Decreases (becomes more watery) Increases (becomes more concentrated)
Specific Gravity Decreases (indicator of low state of charge) Increases (indicator of high state of charge)
Electrodes Lead sulfate (PbSO₄) forms on both plates Lead and lead dioxide regenerate from lead sulfate

Understanding these chemical changes is vital for maintaining the performance and lifespan of lead-acid batteries. For more detailed information on lead-acid battery chemistry, you can refer to resources like Wikipedia's Lead-Acid Battery page.