Understanding Batteries: Exploring Lead Acid Reactions

[OmCheeto]

I used to think I knew how batteries worked, but I've decided that if I did, I now don't.

Being that chemistry is one of my worst subjects, I decided to evade all of the anion, cation, this, that, and other terminology that I find incomprehensible and totally unable to devote to memory.

So here's my plan. I want to build a theoretical lead acid battery, using just single atoms.

I'm pretty sure that if I can understand this, I'll be back up to speed.

So here goes nothing.

Wiki lists the reactions at the negative and positive plates for discharge as follows:

Negative plate reaction: Pb(s) + HSO₄-(aq) → PbSO₄(s) + H+(aq) + 2-e
Positive plate reaction: PbO₂(s) + HSO₄-(aq) + 3H+(aq) + 2-e → PbSO₄(s) + 2H₂O(l)

So my negative plate will consist of a single neutrally charged lead atom and my solution will be a single hydrogen sulphate molecule with a net charge of minus one.

So now some hocus pocus happens and the lead atom and hydrogen sulphate molecule decide that they like each other, mate, and the progeny are lead sulphate, 3 positively charged hydrogen atoms, and 2 electrons.

And now some even more hocus pocus tomfoolery occurs when wiki states:

The total reaction can be written:
Pb(s) + PbO₂(s) + 2H₂SO₄(aq) → 2PbSO₄(s) + 2H₂O(l)

Where did all the electrons and ions run off to? And why do we now have sulphuric acid instead of hydrogen sulphate as our solution molecule? I understand that both sides of the reaction have to match, like an equation, as far as charge and total atoms go.

I guess [STRIKE]that is[/STRIKE] these are my [STRIKE]first[/STRIKE] third and fourth questions:

What is the liquid molecule in a charged battery? Hydrogen sulphate, or sulphuric acid?

Perhaps I should draw a picture...

And where do the Hydrogen gas babies come from?



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ps. I am not adverse to metaphor...

Om likes Evo.
Om is kind of neutral, but his outer shell is somewhat empty.
Evo has been referred to as "negative" by some, but Om and Evo are strangely attracted to each other.
They get married, become one, and have 3 hydrogen and 2 electron babies.

But who the hell is that guy over there? Why is he eating our babies? Why are the electron babies running through that lamp over there? And why is that 3rd guy pooping some weird watery lead sulphate spawn?



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ok to delete, as it appears I've lost my mind...

 

[Borek]

In the total reaction electrons produced by reducing substance were already consumed by the oxidized substance, so they are not listed - but they are transferred during the reaction.

Liquid in the lead acid battery is a concentrated sulfuric acid. Sulfuric acid dissociates in two steps:

H₂SO₄ <-> H⁺ + HSO₄^-

followed by

HSO₄^- <-> H⁺ + SO₄^2-

In concentrated solutions second reaction doesn't go too far - HSO₄^- is a relatively weak acid. H₂SO₄ is very strong, and you can assume even in highly concentrated solutions it is substantially dissociated (so you can expect both H₂SO₄ and HSO₄^- to be present.

It is not uncommon to write reactions without all these details, just in a generalized form - and I see why it can be confusing.

 

[Yanick]

I think a Lead Acid battery is not the best example to use to learn about galvanic cells.

You're probably better off reading the galvanic cell Wikipedia entry. They use Zn and Cu to illustrate the process which can be easier to imagine.

My basic understanding is as follows:

Different metals/elements have different affinities for electrons, some readily give them up while others hold on tight. Therefore if you have an element which is greedy for electrons but is in an oxidized (electron deficient) state, and you mix that ionic compound (ionic because it has lost electrons and has a charge, therefore it will exist with some counter ion to balance the charge) with some kind element in its neutral form, the greedy guy will steal the electrons and become neutral while the other guy will lose his electrons and become charged. You can check out the electrochemical series for more on this.

The situation described above is not a battery because both atoms/ions are mixed in one vessel and there will be no net flow of charge and no current, though there will be a redox reaction occurring. This is not useful for someone trying to make a battery.

In order to make a battery you need to separate the two elements in space, but allow the flow of electrons between them via a wire. As the reaction proceeds via the wire allowing electrons to move from one electrode to the other, charge buildup will start causing electrons and ions to want to migrate towards the oppositely charged electrode. A salt bridge allows for ions to diffuse in such a way as to maintain overall electrical neutrality of the solution.

So in the case of a Zn/Cu cell. The Cu2+ will steal the electrons of the Zn metal (neutral), but the opposite will not happen (Zn2+ will not steal the electrons of neutral Cu metal) because the Cu2+ is the greedy guy. Thus we can already see that there will be a certain directionality to the flow of electrons and ions.

The two neutral metal electrodes are placed in separate containers with the Zn neutral, metal electrode immersed in a solution containing Zn2+ with some counter ion. The same for the Cu neutral metal electrode (Cu2+ with counter ion). So each vessel contains a solid neutral metal electrode, water, the oxidized/charged metal in solution and some counter ion to balance the charge.

So the Cu2+ really wants its 2 electrons so it can become a solid neutral Cu. these electrons are stolen from the Zn metal electrode via the wire because Zn just doesn't care as much (Zn is a slacker).

As the Zn loses its electrons it becomes charged and goes into solution (falls off of the solid electrode), the electrons travel down the wire towards the Cu electrode (this is where we get our current).

When the electrons get to the Cu electrode, the Cu2+ ions which are close to the Cu electrode, but still in the solution, will take these electrons and become solid Cu (neutral). The solid Cu then precipitates out of solution but doesn't fall to the bottom, it just deposits itself onto the existing solid metal Cu electrode (this phenomenon is used to electroplate things).

The last thing to take care of is the electrical neutrality issue.

The Zn solution will constantly be getting more and more Zn2+ making it overall more positively charged and the Cu will constantly be losing the Cu2+ which will give that solution an overall negative charge. Left unabated this situation would lead to termination of the reaction and loss of current (note it may just slow the reaction, I'm not sure as I'm no electrochemistry expert).

In order to avoid this we provide another connection between the vessels in the form of a salt bridge. This salt bridge allows the positively charged ions, from the Zn solution to migrate toward the increasingly negatively charged Cu solution and vice versa for negatively charged ions moving toward the increasingly positively charged Zn solution. This keeps everything net neutral and everybody happy.

The electrons will keep producing current as long as there is enough of the solid Zn to keep giving up it's electrons and there is enough dissolved Cu2+ to keep accepting them in the other vessel. As these species are diminished the current will get smaller and then just stop.

This is the general idea behind a battery. Learning the terminology will help you analyze any galvanic cell. They all have minimum requirement of electron source (slacker guy), electron sink (greedy guy), charged, dissolved salts of each metal with counter ion, salt bridge etc.

Rechargeable batteries use an outside EMF to drive everything backwards and restore the battery.

There are many other things which can happen in galvanic cells depending on the system and conditions used, such as water electrolysis. These are typically side reactions and make the whole system more complicated, possibly less efficient or even dangerous. I'd get a good handle on the basic mechanism of a simple galvanic cell (like the Zn/Cu cell where everything is different enough so you can imagine what's going where) before I moved onto the more complicated stuff.

Lead acid confused me as we'll because I wasn't used to seeing the same metal at each electrode. You can see though, that the species are not the same as one is just solid neutral Pb metal while the other is lead oxide PbO2, plus they both become lead sulfate at the end etc. Its not the easiest system to learn if you don't know much Chemistry and are trying to get a handle on galvanic cells.

 

[DrDu]

So my negative plate will consist of a single neutrally charged lead atom and my solution will be a single hydrogen sulphate molecule with a net charge of minus one.

So now some hocus pocus happens and the lead atom and hydrogen sulphate molecule decide that they like each other, mate, and the progeny are lead sulphate, 3 positively charged hydrogen atoms, and 2 electrons.

For single atoms this would really be hocus pocus. The reason why substances react is that entropy increases during reaction. Entropy is a concept which makes most sense for macroscopic quantities of a substance.

 

[OmCheeto]

In the total reaction electrons produced by reducing substance were already consumed by the oxidized substance, so they are not listed - but they are transferred during the reaction.

Liquid in the lead acid battery is a concentrated sulfuric acid. Sulfuric acid dissociates in two steps:

H₂SO₄ <-> H⁺ + HSO₄^-

followed by

HSO₄^- <-> H⁺ + SO₄^2-

In concentrated solutions second reaction doesn't go too far - HSO₄^- is a relatively weak acid. H₂SO₄ is very strong, and you can assume even in highly concentrated solutions it is substantially dissociated (so you can expect both H₂SO₄ and HSO₄^- to be present.

It is not uncommon to write reactions without all these details, just in a generalized form - and I see why it can be confusing.

So can we say that sulphuric acid spontaneously dissociates into H+ and HSO₄^-? Thereby leading to the assumption that there are 3 distinct molecules present in the solution? This would make things easier to resolve.

ps. I'm trying to stay away from terms which confuse the hell out of dyslexics. Reading about "Red-Ox" makes me crazy:

"Reduction is the gain..." --> less is more?
"oxidation originally implied reaction with oxygen, but now it doesn't" --> I gave up.

 

[Borek]

So can we say that sulphuric acid spontaneously dissociates into H+ and HSO₄^-? Thereby leading to the assumption that there are 3 distinct molecules present in the solution?

Yes & yes (although I would prefer to call two of the three "ions").

 

[OmCheeto]

Yes & yes (although I would prefer to call two of the three "ions").

I can handle ions. They are not ambiguous.

 

[OmCheeto]

I think a Lead Acid battery is not the best example to use to learn about galvanic cells.

You're probably better off reading the galvanic cell Wikipedia entry. They use Zn and Cu to illustrate the process which can be easier to imagine.

I would not come here asking this question if I had not read the wiki article 3 times.

I believe it was Astronuc who once said; "Try and figure it out yourself first, and then, if you still don't get it, ask your question". But that was 5 years ago, so I'm sure I've garbled his words.

This is not useful for someone trying to make a battery.

I'm not trying to make a battery. I just want 2 electrons to run through the lamp.

I'm assuming that there are two electrons that run through the lamp, as Evo and I had 2 electron babies, which were eaten by that "other" guy.

And this looks like redneck math.

We had only one Hydrogen baby, but "Mr. Positive" just ate three of our Hydrogen babies.

Perhaps I was not meant to be a chemist.

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ps. I chose lead acid, as I once received a letter of commendation from the commander of the submarine pacific fleet, for my trend analysis of a flaw in two cells of my submarines lead acid battery, way back in 1982. I may be senile, but I wasn't always stupid... :grumpy:

 

[OmCheeto]

So do diprotic acids dissociate into sub-ions, because of entropy?

I imagine that there is some type of chart that tells me that H₂SO₄ and H₂O are more energetic than HSO₄^-, H⁺, and H₂O.

 

[DrDu]

The degree of dissociation increases the more you dilute an acid but the energetics always remains the same. What determines the degree of dissociation (via the equilibrium constant K) is the change in free enthalpy
$R\ln K=-\Delta G /T=-\Delta H/T +\Delta S$. The first term on the right is the entropy change of the surrounding due to the heat generated in the reaction (this is so to say the energetic part), the second term is the entropy change of the reaction system itself.
So everything boils down to entropy change

 

[OmCheeto]

...the change in free enthalpy...

I used to know what enthalpy was also. Thank you DrDu, I will study this weekend, and attempt to respond with a semi-educated next question.