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−.
