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Old Nov 16th 2018, 09:19 PM   #1
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Acid Stratification

This is something said to occur in lead acid batteries ...

It's suggested that if undisturbed sulfuric acid would "stratify" that the acid would "sink to the bottom " separate out from the water because it's denser ...

This seems insane to me , and I suggest any observed stratification occurs because the topping up water is added from above and not well mixed ...

Conciser a beaker of water . Concentrated sulfuric acid is added with stirring to make 10% concentration ...A great deal of heat is emitted...

To suggest if left, this solution will separate out with a layer of concentrated acid at the bottom is insane ! You could stir it up and get more heat out! It would violate laws of thermodynamics and entropy!!
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Old Nov 19th 2018, 04:03 AM   #2
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I was not aware of this phenomena,
but I do not see any fundamental problem with it.

The battery does not represent an isolated system.
As the stratification occurs the battery will be exchanging energy with its surroundings
as the acid stratifies it will get colder,
but this is very gradual so it will absorb heat from its surroundings.
The energy you get back on stirring is this heat that has been gradually absorbed.
It is the result of the gravitational potential energy of the heavier acid molecules.
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Old Nov 19th 2018, 05:14 AM   #3
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Originally Posted by Woody View Post
I was not aware of this phenomena,
but I do not see any fundamental problem with it.

The battery does not represent an isolated system.
As the stratification occurs the battery will be exchanging energy with its surroundings
as the acid stratifies it will get colder,
but this is very gradual so it will absorb heat from its surroundings.
The energy you get back on stirring is this heat that has been gradually absorbed.
It is the result of the gravitational potential energy of the heavier acid molecules.
Thanks for the reply Woody ...

Lets take this in stages .Forget the battery for now ..

We have 20% sulfuric acid in a bottle . You seem to believe , that over time this will stratify , absorbing heat from the environment...

I find that hard to accept .. where did the heat of dilution come from in the first place ?? It must be a chemical or ionic reaction , what could cause this to reverse???

I've also been informed that if you leave two stroke petrol (petrol with 4% oil dissolved) then the oil will , over time sink to the bottom , I find this hard to believe too . Surely this would mean entropy has moved in the wrong direction ??
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Old Nov 19th 2018, 06:29 AM   #4
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All chemical reactions are reversible,
and in a mixture of the reactants and products there will always be some of all of them.
For some reactions the balance is 99.999% in one direction (e.g. Hydrogen, Oxygen, and Water) but it is never completely 100%.

The same can be said for dissolving.
As each species of molecule comes (even briefly) out of solution
it will be subject to buoyancy forces which will move it up or down relative to the bulk solution.
Eventually this will lead to stratification.
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Old Nov 19th 2018, 08:08 AM   #5
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Originally Posted by Woody View Post
All chemical reactions are reversible,
and in a mixture of the reactants and products there will always be some of all of them.
For some reactions the balance is 99.999% in one direction (e.g. Hydrogen, Oxygen, and Water) but it is never completely 100%.

The same can be said for dissolving.
As each species of molecule comes (even briefly) out of solution
it will be subject to buoyancy forces which will move it up or down relative to the bulk solution.
Eventually this will lead to stratification.
I'm sorry Woody all of that is wrong ...

Your first sentence ..."All chemical reactions are reversible" !!!

Just put in search Reversible vs. Irreversible Reactions and you'll get a 100 links
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Old Nov 19th 2018, 09:18 AM   #6
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In my youth I studied chemistry and I stick by what I said.
All reactions reach a balance,
"reversible" reactions are where the balance is sensibly close to the 50/50 position.
"irreversible" reactions are where the balance is hugely over one way or another.

However even with "irreversible" reactions there is a tiny tendency for molecules to occasionally disassociate.

Take water for example;
It is thought that part of the reason for the dryness of Mars is that over millennia the water would disassociate, a molecule at a time,
then the hydrogen would be lost at the top of the atmosphere while the oxygen reacts with the minerals in the ground.
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Old Nov 20th 2018, 03:04 AM   #7
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Originally Posted by Woody View Post
In my youth I studied chemistry and I stick by what I said.
All reactions reach a balance,
"reversible" reactions are where the balance is sensibly close to the 50/50 position.
"irreversible" reactions are where the balance is hugely over one way or another.

However even with "irreversible" reactions there is a tiny tendency for molecules to occasionally disassociate.

Take water for example;
It is thought that part of the reason for the dryness of Mars is that over millennia the water would disassociate, a molecule at a time,
then the hydrogen would be lost at the top of the atmosphere while the oxygen reacts with the minerals in the ground.
Looking more into this , there is something in what you say ... "Irreversible" reactions , to all intents and purposes under normal conditions go only one way ... I'm sure sulfuric acid dissolving in water would be classified as irreversible , being very exothermic , I am confident it would not stratify .. A practicing chemist should know ... he would need to regularly shake up his bottles of dilute acid ...

But what about oil in petrol ???
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Old Nov 20th 2018, 06:26 AM   #8
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When is it a mix and when is it a solution?

A solution can be viewed very much like a chemical reaction
just with much weaker bonds.

I think that we can count oil and petrol as a solution,
So all of the above arguments should apply.

If a mix/solution of oil and petrol is left to stand,
it should stratify.
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