Lead-Acid Battery Discharge: PbO2 + Pb + 2H2SO4  2PbSO4 + 2H2O

[ally1h]

Homework Statement

The overall cell reaction during discharge of the lead-acid battery is:
PbO2 (s) + Pb (s) +2H2SO4 (aq)  2PbSO4 (s) + 2H2O (l)

Which statement is incorrect?
a) the lead serves as the anode
b) the PbO2 electrode would be positive during discharge
c) upon recharging, the above reaction occurs in reverse
d) upon recharging, the density of the electrolyte will increase
e) upon recharging, the lead electrode is negative

Homework Equations

The Attempt at a Solution

Lead DOES serve as the anode and PbO2 serves as the cathode, so option A is out. From what little I know of lead-acid batteries, upon discharge electrons are put through the PbO2 cathode making the charge negative. I think. So B is a possible answer. C, however, I think is possible as well because according to my book the E˚cell given for the above reaction is -2.046 V making the recharging reaction non-spontaneous... an outside voltage = +2.046 is needed to recharge.


But as for the rest... I'm not certain. There's very little mentioned in my book and I'm having trouble thinking it all through. Will someone please help explain this to me?

 

[chemisttree]

The PbO2 electrode is the positive electrode. C doesn't ask if the recharging reaction occurs spontaneously. It assumes that it does occur. If it occurs because you are pumping electrons in reverse, is the reaction going in reverse or is it going in the forward direction?

Any help?

 

[Blueshift5]

The correct answer is d) upon recharging, the density of the electrolyte will increase. When a lead-acid battery is discharged, the electrolyte (sulfuric acid) is consumed in the chemical reaction and the density decreases. During the charging process, the chemical reaction is reversed and the electrolyte is replenished, causing the density to increase. The lead electrode does become negative during recharging, but this is not the incorrect statement.