Question

What are the anode and cathode reactions in a lead-acid storage battery? What happens when the battery is recharged?

Short answer

During discharge, the anode reaction is \( Pb + SO4^{2-} \rightarrow PbSO4 + 2e^- \) and the cathode reaction is \( PbO2 + 4H^+ + SO4^{2-} + 2e^- \rightarrow PbSO4 + 2H2O \). During recharge, the reactions are reversed.

Step-by-step solution

Understand the Lead-acid Battery Composition

The lead-acid battery consists of a negative plate of lead (Pb) and a positive plate of lead dioxide (PbO2) in an electrolyte solution of sulfuric acid (H2SO4).

Identify the Anode and Cathode Reactions During Discharge

During the discharge process, the anode (negative plate) undergoes oxidation by losing electrons and forming lead sulfate (PbSO4). The half-reaction at the anode is: \( Pb(s) + SO4^{2-}(aq) \rightarrow PbSO4(s) + 2e^- \). The cathode (positive plate) undergoes reduction by gaining electrons to also form lead sulfate. The half-reaction at the cathode is: \( PbO2(s) + 4H^+(aq) + SO4^{2-}(aq) + 2e^- \rightarrow PbSO4(s) + 2H2O(l) \).

Describe the Reactions During Recharge

When the battery is recharged, the direction of the chemical reactions is reversed. The anode reaction converts lead sulfate back into lead, and the cathode reaction converts lead sulfate and water back into lead dioxide and sulfuric acid. The reactions are: Anode: \( PbSO4(s) + 2e^- \rightarrow Pb(s) + SO4^{2-}(aq) \) and Cathode: \( PbSO4(s) + 2H2O(l) \rightarrow PbO2(s) + 4H^+(aq) + SO4^{2-}(aq) + 2e^- \).

Key concepts

Electrochemical Cells

An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or facilitating chemical reactions through the introduction of electrical energy. A lead-acid battery, like the one described in our textbook exercise, is a classic example of a rechargeable electrochemical cell.
In such cells, electricity is produced through a series of redox (reduction-oxidation) reactions occurring in separate compartments called half-cells, connected by an electrolyte solution that allows ions to move while keeping the reactants apart to avoid direct reaction. The two half-cells consist of an anode and a cathode, each hosting one half of the redox reactions during the cell's discharge process.
To optimize understanding of these concepts, it's vital to visualize the flow of electrons from the anode to the cathode during the discharge cycle, representing the flow of electric current. Understanding the role of the sulfuric acid electrolyte, which facilitates the ion transfer necessary for the cell's operation, also strengthens the grasp of how these units function as a whole.

Oxidation and Reduction

Oxidation and reduction are two halves of a whole in electrochemical reactions, commonly referred to as redox reactions. In the context of a lead-acid battery, the anode reaction represents oxidation, where lead (Pb) loses electrons and transforms into lead sulfate (PbSO4). This loss of electrons is key to understanding oxidation.

Oxidation:

The reaction at the anode can be represented as: \[ Pb(s) + SO4^{2-}(aq) \rightarrow PbSO4(s) + 2e^- \].Here, 's' denotes a solid state, 'aq' an aqueous solution, and 'e^-' the electrons that are lost during oxidation.

Reduction:

The cathode reaction demonstrates reduction, where the lead dioxide gains electrons to become lead sulfate as well: \[ PbO2(s) + 4H^+(aq) + SO4^{2-}(aq) + 2e^- \rightarrow PbSO4(s) + 2H2O(l) \].'Reduction' here refers to the gain of electrons. For ease of understanding, always remember: Oxidation is loss of electrons (OIL - Oxidation Is Loss), and Reduction is gain of electrons (RIG - Reduction Is Gain).
It is beneficial for the student to allocate the equation to each half-cell reaction physically, so they can see where each process takes place within the battery structure, thereby solidifying their understanding of the spatial dynamics.

Battery Recharge Process

Recharging a battery essentially means reversing the discharge reactions. In a lead-acid battery, applying an external electrical current reverses the electrochemical reactions that occur during discharge. This ensures that the lead and lead dioxide plates are regenerated from lead sulfate, and the sulfuric acid electrolyte is replenished.
During the recharge phase:

• The anode reaction is: \[ PbSO4(s) + 2e^- \rightarrow Pb(s) + SO4^{2-}(aq) \].
• The cathode reaction is: \[ PbSO4(s) + 2H2O(l) \rightarrow PbO2(s) + 4H^+(aq) + SO4^{2-}(aq) + 2e^- \].

To enhance comprehension, it's crucial to recognize that the external power source supplies the electrons that facilitate these reverse reactions. As this can be a complex concept, picturing the flow of electrons against the natural discharge direction can help. Moreover, knowing that the recharge process is not 100% efficient due to energy losses mainly as heat provides students with a more comprehensive understanding of real-world battery operations.