Corrosion is a process of deterioration and consequent loss of solid metallic materials through an unwanted chemical or electrochemical attack by its environment starting at the surface. The chemical corrosion occurs due to the direct chemical action of the environment (e.g., inorganic liquid) or atmospheric gases as O2, H2S , SO2, halogens and ammonia. The extent of chemical corrosion depends on the chemical affinity of solid metal with the corrosive environment and ability of the reaction product to form protective film on the metal surface. The electrochemical corrosion occurs when a metal is in contact with the conducting liquid or when two dissimilar metals or alloys are dipped partially or completely in a solution. The electrochemical corrosion occurs due to the existence of separate anodic or cathodic areas between which there occurs a flow of current through the conducting solution. The corrosion always occurs at the anodic areas. The rusting of iron occurs due to corrosion. The electrode reactions in the rusting process are as follows:
Anode: \(Fe \rightarrow Fe^{2+} + 2e^{-};\) \(E^0 = - 0.44V\)
Cathode: \(O_2 + 2H_2O + 4e^- \rightarrow 4OH^-;\) \(E^0 = 1.23 V\)
The overall reaction is
\(Fe + O_2 + H_2O \rightarrow Fe^{2+} + 2OH^- ;\) \(E^0 = 1.67 V\)
The Fe2+ and OH− ions combine to form Fe(OH)2 which is oxidized to Fe(OH)3 in excess of oxygen. The product formed corresponds to Fe2O3.xH2O. If the supply of oxygen is limited, the corrosion product is black magnetite (Fe3O4).
When a lead storage battery is discharged then
Answer & explanation
Correct answer: option 4
The correct answer is option 4. When a lead storage battery is discharged, sulfuric acid is consumed.
A lead storage battery (or lead-acid battery) operates by converting chemical energy into electrical energy through electrochemical reactions. During discharge, the battery produces electrical energy, and the following reactions occur:
Components:
Anode : Made of lead \((Pb)\).
Cathode : Made of lead dioxide \((PbO_2)\).
Electrolyte: Sulfuric acid \((H_2SO_4)\).
Discharge Reactions:
1. At the Anode (Negative Electrode):
Lead \((Pb)\) reacts with the bisulfate ion \((HSO_4^-)\) from sulfuric acid, forming lead sulfate \((PbSO_4)\) and releasing two electrons \((2e^-)\).
\(\text{Pb} + \text{HSO}_4^- \rightarrow \text{PbSO}_4 + \text{H}^+ + 2e^-\)
2. At the Cathode (Positive Electrode):
Lead dioxide \((PbO_2)\) reacts with the bisulfate ion \((HSO_4^-)\), protons \((H^+)\), and electrons \((2e^-)\) to form lead sulfate (PbSO₄) and water \((H_2O)\).
\(\text{PbO}_2 + \text{HSO}_4^- + 3\text{H}^+ + 2e^- \rightarrow \text{PbSO}_4 + 2\text{H}_2\text{O}\)
Overall Discharge Reaction:
Combining the reactions at the anode and cathode, the overall reaction during discharge is:
\(\text{Pb} + \text{PbO}_2 + 2\text{H}_2\text{SO}_4 \rightarrow 2\text{PbSO}_4 + 2\text{H}_2\text{O}\)
Key Points:
1. Consumption of Sulfuric Acid \((H_2SO_4)\):
During discharge, sulfuric acid \((H_2SO_4)\) is consumed in the reactions at both the anode and the cathode. This reduces the concentration of sulfuric acid in the electrolyte.
2. Formation of Lead Sulfate \((PbSO_4)\):
Lead \((Pb)\) at the anode and lead dioxide \((PbO_2)\) at the cathode are converted to lead sulfate \((PbSO_4)\). Lead sulfate forms on both electrodes as a product of the discharge reactions.
3. Water \((H_2O)\) Production:
Water \((H_2O)\) is produced at the cathode as a byproduct of the reaction.
Incorrect Options Clarified:
\(SO_2\) is evolved: This is incorrect. Sulfur dioxide \((SO_2)\) is not involved in the discharge reactions of a lead-acid battery.
Lead is formed: This is incorrect. Lead \((Pb)\) is actually consumed at the anode.
Lead sulfate is consumed: This is incorrect. Lead sulfate \((PbSO_4)\) is formed, not consumed, during discharge.
During the discharge of a lead storage battery, sulfuric acid \((H_2SO_4)\) is consumed, lead sulfate \((PbSO_4)\) is formed on both electrodes, and water \((H_2O)\) is produced. The correct answer is: sulphuric acid is consumed