Question

Write the chemical equation for the overall cell reaction that occurs in a lead storage automobile battery. What species is oxidized in such a battery? What species is reduced? Why can such a battery be "recharged"?

Short answer

The overall cell reaction in a lead storage automobile battery is: Pb(s) $$+ PbO2(s) $$ + 2H2SO4(aq) $$ -> 2PbSO4(s) $$ + 2H2O(l) In this reaction, lead (Pb) is oxidized to form lead sulfate (PbSO4), while lead dioxide (PbO2) is reduced to form lead sulfate (PbSO4) as well. The battery can be recharged because the reaction is reversible, allowing the reactants to be replenished during the recharging process.

Step-by-step solution

Understanding lead-acid batteries

A lead-acid battery consists of a series of lead (Pb) and lead dioxide (PbO2) electrodes, which are submerged in a sulfuric acid (H2SO4) electrolyte solution. Step 2: Write half-reactions for each electrode

Half-reactions for the lead and lead dioxide electrodes

For the lead (Pb) electrode, the half-reaction is the oxidation of Pb to PbSO4: Pb $$ + SO4^2- $$ -> $$ PbSO4(s) $$ + 2e^- For the lead dioxide (PbO2) electrode, the half-reaction is the reduction of PbO2 to PbSO4: PbO2(s) $$+ 4H+(aq) $$+SO4^2-(aq) $$ + 2e^- $$-> $$PbSO4(s) $$+ 2H2O(l) Step 3: Combine the half-reactions

Combining the half-reactions

Sum the two half-reactions, making sure that the number of electrons is balanced on both sides. In this case, the electrons are already balanced (2 electrons are transferred in both half-reactions): Pb $$ + SO4^2- $$ -> $$ PbSO4(s) $$ + 2e^- (Oxidation half-reaction) PbO2(s) $$+ 4H+(aq) $$+SO4^2-(aq) $$ + 2e^- $$-> $$PbSO4(s) $$+ 2H2O(l) (Reduction half-reaction) Add the two half-reactions together: Pb(s) $$+ PbO2(s) $$ + 2H2SO4(aq) $$ -> 2PbSO4(s) $$ + 2H2O(l) Step 4: Identify the oxidized and reduced species

Identifying the species

In this reaction, the lead (Pb) is oxidized to form lead sulfate (PbSO4). The lead dioxide (PbO2) is reduced to form lead sulfate (PbSO4) as well. Step 5: Explain why the battery can be recharged

The battery's recharging ability

This type of battery can be recharged because the reaction is reversible. When charging the battery, an external source of electrical energy is applied, and the chemical reaction proceeds in the opposite direction, converting PbSO4 back to Pb and PbO2. This allows the battery to be used multiple times, as the reactants are replenished during the recharging process.

Key concepts

Electrochemical Cells

Electrochemical cells are the fundamental units of batteries and are responsible for converting chemical energy into electrical energy through chemical reactions. In the case of a lead-acid battery, the cell primarily consists of two types of electrodes: one made of lead and another coated with lead dioxide. These electrodes are immersed in an electrolyte solution of sulfuric acid (H₂SO₄). When the battery is discharging, electrons flow from the lead electrode to the lead dioxide electrode, powering the connected electrical devices.

An electrochemical cell includes different components, such as the anode and cathode, which facilitate the flow of electrons via an external circuit. The sulphuric acid acts as a medium which allows ions to move between electrodes, completing the circuit internally. This system is carefully designed to ensure a safe and efficient energy transfer.

Oxidation-Reduction Reactions

Oxidation-reduction (redox) reactions are processes where electrons are transferred between substances. They are central to the function of electrochemical cells. In the context of a lead-acid battery, oxidation occurs at the lead electrode when it loses electrons, forming lead sulfate as shown in the half-reaction:
Pb(s) + SO₄^2- -> PbSO₄(s) + 2e^-.

Conversely, reduction happens at the lead dioxide electrode where it gains electrons:
PbO₂(s) + 4H⁺ + SO₄^2- + 2e^- -> PbSO₄(s) + 2H₂O(l).

The elegance of a redox reaction in a battery is its reversibility, which is the basis for recharging. By applying an external electrical energy source, these reactions can be reversed, allowing the battery to regain its original chemical constituents and hence its stored energy.

Chemical Equations

Chemical equations are concise representations of chemical reactions, displaying the substances involved before and after the reaction. To write a balanced chemical equation, one must ensure that the numbers of atoms and the charge are the same on both sides of the equation. For the overall reaction in a lead-acid battery, this balance is demonstrated as:
Pb(s) + PbO₂(s) + 2H₂SO₄(aq) -> 2PbSO₄(s) + 2H₂O(l).

This equation reveals the transformation of the electrodes and the electrolyte during the discharge of the battery. Understanding chemical equations is critical for grasping the changes that occur during the battery's operation and also during its recharging process.

Rechargeable Batteries

Rechargeable batteries, such as the lead-acid battery used in automobiles, are designed to undergo multiple cycles of discharging and recharging. The chemical reactions in these batteries are reversible, which is not the case with all batteries. During discharge, the lead-acid battery converts chemical energy into electrical energy, and during charge, it reverts back to its original chemical state by using electrical energy from an external source.

This reversibility is due to the nature of the redox reactions involved, which allows the original reactive substances (lead and lead dioxide) to be recreated from lead sulfate. The longevity and efficiency of rechargeable batteries come from the ability to recover their reactants and thus their energy capacity, making them both practical and cost-effective for use in numerous applications, including vehicles and backup power systems.