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Question Video: Identifying the Half-Equation Which Shows the Reaction Occurring at the Negative Plate during the Discharge Phase of a Battery Chemistry

Which of the following half-equations shows the reaction occurring at the negative plate during the discharge phase of a battery? [A] Pb(s) + H₂SO₄(aq) ⟶ PbSO₄(s) + 2H⁺(aq) + 2e⁻ [B] PbO₂(s) + H₂SO₄(aq) + 2H⁺(aq) + 2e⁻ ⟶ PbSO₄(s) + 2H₂O(l) [C] PbSO₄(s) + 2H⁺(aq) + 2e⁻ ⟶ Pb(s) + H₂SO₄(aq) [D] PbSO₄(s) 2H₂O(l) ⟶ PbO₂(s) + H₂SO₄(aq) + 2H⁺(aq) + 2e⁻

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Video Transcript

Which of the following half equations shows the reaction occurring at the negative plate during the discharge phase of a battery? (A) Pb solid plus H2SO4 aqueous forms PbSO4 solid plus two H+ aqueous plus two e−. (B) PbO2 solid plus H2SO4 aqueous plus two H+ aqueous plus two e− forms PbSO4 solid plus two H2O liquid. (C) PbSO4 solid plus two H+ aqueous plus two e− forms Pb solid plus H2SO4 aqueous. Or (D) PbSO4 solid plus two H2O liquid forms PbO2 solid plus H2SO4 aqueous plus two H+ aqueous plus two e−.

The first observation we should make when answering this question is to identify the type of battery to which the question refers. Given our knowledge of secondary galvanic cells, we should realize that lead and sulfuric acid are common components of the lead–acid battery. This type of battery is a secondary galvanic cell so can be run as a galvanic cell in the discharge phase or as an electrolytic cell in the charge phase.

The question tells us that the battery is in the discharge phase, so it is operating as a galvanic cell. The question asks for the reaction occurring at the negative plate. In a galvanic cell, the anode is the negative plate. Lead-metal plates are commonly used for the anode and cathode. These plates have a grid structure that allows either spongy lead to fill the grids in the case of the anode or lead(IV) dioxide in the case of the cathode. At the negative plate, solid lead and sulfuric acid react, and oxidation occurs.

Thus, the half equation that shows the reaction occurring at the negative plate during the discharge phase of a battery is (A). Pb solid plus H2SO4 aqueous forms PbSO4 solid plus two H+ aqueous plus two e−.

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