# Where do the electrons in lead–acid battery come from? [closed]

\begin{align} \ce{PbO2 + H2SO4 &-> PbSO4 + H2O + O} &\quad &\text{(anode)}\\ \ce{Pb + H2SO4 &-> PbSO4 + H2} &\quad &\text{(cathode)} \end{align}

The cathode’s $$\ce{H2}$$ joins the anode’s $$\ce{O}$$ making $$\ce{H2O}$$ in solution. The book says the cathode gets four electrons, but from where? Two liberated hydrogens have one each, that’s two. Where’d the other two come from?

• That's lead storage battery. – Zenix May 15 '20 at 15:07
• I adjusted the formatting of the reactions, but the first one looks wrong. It would be nice if you could add the source you've taken them from and cite the textbook. – andselisk May 15 '20 at 15:16
• These aren't equations for battery, but just for reactions with conc. H2SO4. @andselisk Question could use your closehammer, I think ;) – Mithoron May 15 '20 at 15:47
• @Mithoron I'm not sure about single-handedly closing this one. I don't think this is necessarily a bad or a homework question; it's just somewhat hard to trace the OP's reasoning in the absence of the source they took the reactions from. – andselisk May 15 '20 at 15:56
• @andselisk Well, that would be like "trihandedly" now ;) and it seems real unclear. – Mithoron May 15 '20 at 15:59

The half-equations are not correctly written. There is no Oxygen atom released, and no H atom emitted, as the author proposes. And the cathode does not get 4 electrons, as he or she states. The correct half-equations should be, first at the anode : $$\ce{Pb + SO_4^{2-} -> PbSO4 + 2 e^-}$$ And at the cathode it is : $$\ce{PbO_2 + 4H+ + SO_4^{2-} + 2 e^- -> PbSO_4 + 2 H2O}$$
• Can we consider $\ce{PbO2}$ as a positive electrode – Adnan AL-Amleh May 15 '20 at 20:45
• Yes ! $\ce{PbO_2}$ is a positive electrode. – Maurice May 16 '20 at 10:26