Explaining the Immiscibility of Oil and Water: A Thermodynamic Perspective

[Gokul43201]

Where does entropy/enthalpy come into play with an oil/water mixture? Again, my understanding here is thin, so I am in need of confirmation/correction, but it was my understanding that with a mixture of something like oil and water, there is no change in enthalpy/entropy because nothing chemical happens.

The question in the OP is: (in the above situation) why does nothing chemical happen? Or, why does oil only form a suspension (or dispersion) in water, and not a solution?

The answer follows by assuming that if it did, then there would be an increase in free energy (as explained earlier) which is unphysical. Hence, by a method of reductio ad absurdum, you prove that oil and water are immiscible.

The same argument arrives at a reduction in free energy if ethanol and water form a solution, making this the favorable final state for that system.

 

[ShawnD]

I've always found that adding 10mL of water to 1L of say, 10M HCl makes about as much heat as adding 10mL of 10M HCl to 1L of water.

That's because HCl is a gas, so "HCl solution" already has water in it.
H2SO4, the fuming kind in particular, contains absolutely no water. If you add 50mL of water to 50mL of H2SO4, the solution becomes so hot you can't even hold it with your bare hands.
This works well with sodium hydroxide too. Add water to 10M NaOH and nothing will happen. Add water to solid 99% NaOH and it will get very hot.edit:
I just realized that I didn't exactly answer your argument. You said water into acid is the same heat as acid into water, and my response was that HCl is a weak ***** acid.

To answer your question: it's different not because of the heat, but because of a difference in boiling points. Water boils at 100C, nitric acid is like 120C, and sulfuric acid is way up there past 200C (don't have an exact number). If you add sulfuric acid to water, the solution will only boil if 1). the small amount of H2S04 boils (at 200C) or 2). the entire amount of water boils (100C, but larger quantity). It takes a lot of heat to get H2SO4 up to 200C, and a lot of heat to get the majority of the water up to 100C. Adding water to sulfuric acid has the opposite rules of 1) the small amount of water boils (100C) or 2) the large amount of H2SO4 boils (200C).
Assuming both water into acid and acid into water create the same amount of heat, is it easier to reach a temperature of 100C or 200C? 100C. Adding water is bad

 

[Gokul43201]

To answer your question: it's different not because of the heat, but because of a difference in boiling points. Water boils at 100C, nitric acid is like 120C, and sulfuric acid is way up there past 200C (don't have an exact number). If you add sulfuric acid to water, the solution will only boil if 1). the small amount of H2S04 boils (at 200C) or 2). the entire amount of water boils (100C, but larger quantity).

What? A solution is a single phase system. Its constituent components do not retain their pure state boiling points. When sulfuric acid dissolves in water (and unless it dissolves, there isn't any heat released), the solution acquires a new boiling point (more correctly, a tiny range of boiling points within a narrow two-phase region flanked below by the single-phase solution and above by the single-phase vapor). When you add a little sulfuric acid to a lot of water, the new BP is going to be pretty close to 100C. When you add a little water to a lot of acid, the new BP will be closer to 200C.

Either you're wrong, or I've completely misunderstood you.