Video Transcript
Depending on how the dissociation
of sulfuric acid is considered, the half equations of a lead–acid battery can vary
slightly in different sources. Which of the following pairs shows
two half equations that correctly combine to generate the overall reaction equation
for the entire cell? Pair one, Pb solid plus HSO4−
aqueous forms PbSO4 solid plus H+ aqueous plus two e− and PbO2 solid plus SO42−
aqueous plus four H+ aqueous plus two e− forms PbSO4 solid plus two H2O liquid. Pair two, Pb solid plus SO42−
aqueous forms PbSO4 solid plus two e− and PbO2 solid plus HSO4− aqueous plus three
H+ aqueous plus two e− forms PbSO4 solid plus two H2O liquid. Pair three, Pb solid plus H2SO4
aqueous forms PbSO4 solid plus two H+ aqueous plus two e− and PbO2 solid plus H2SO4
aqueous plus two H+ aqueous plus two e− forms PbSO4 solid plus two H2O liquid. (A) Pair one, (B) pair three, or
(C) pair two.
Our question asks us to combine
pairs of reactions to determine if they will represent an overall reaction for a
lead–acid battery. Regardless of any slight
differences in half reactions, the overall reaction should always be the same. This means that if we combine the
pairs given to us in the question, the correct answer should be the following
equation. Pb solid plus PbO2 solid plus two
H2SO4 aqueous forms two PbSO4 solid plus two H2O liquid.
Now all we need to do to solve our
problem is combine our pairs of equations. Firstly, we will combine pair
one. We can see that we have ions which
are not in our correct solution. Therefore, pair one is not
correct.
Let’s now combine pair two. Again, we are left with ions which
are not present in the correct equation already stated. Therefore, pair two is not the
answer.
Finally, we combine the pair three
equations. As we can see, only pair three
results in an overall equation that is indicative of an overall equation for a
lead–acid battery. Therefore, only answer choice (B)
is correct, pair three.
