How does temperature affect solutions?

[DaveE]

TL;DR

Why can I dissolve more salt in hot water, than cold? Why does more air dissolve in cold water than hot?

- Why can I dissolve more salt in hot water, than cold?
- Why does more air dissolve in cold water than hot?
- What about non-polar solvents? Is it the water or the solute that does this?
- Are there combinations that do the opposite of the normal solid/gas dichotomy?

If you want me to understand you, I'll need a freshman undergraduate level explanation. Everything I learned about chemistry or thermodynamics has left long ago. Those were the classes I struggled to get through without wasting enough effort to really learn much. One thing I learned as an undergraduate was triage; those were the subjects that were mostly left to die. Plus, I have a whole diatribe about how chemistry was taught at my schools that you don't need to hear.

 

[anuttarasammyak]

I am not sure I can make you understand by my explanation but the key word is Gibb's free energy in thermodynamics and statistical mechanics.

We add salt, NaCl, to lower the temperature of chilly water. It means that we need energy to solve NaCl and this process does not go spontaneously. On the other hand we observe the process of the ions leaving from crystal into water increase entropy of the system. It means the same process goes spontaneously. These plus-minus effects balance at a equilibrium which temperature of the system decides. In lower temperature energy wins more and equilibrium point goes less solvable. In increasing temperature entropy wins more and equilibrium goes more solvable.

 

[Borek]

Lithium carbonate has a solubility of 1.5g/100mL at 0°C and 0.7g/100mL at 100°C.

 

[DaveE]

I am not sure I can make you understand by my explanation but the key word is Gibb's free energy in thermodynamics and statistical mechanics.

We add salt, NaCl, to lower the temperature of chilly water. It means that we need energy to solve NaCl and this process does not go spontaneously. On the other hand we observe the process of the ions leaving from crystal into water increase entropy of the system. It means the same process goes spontaneously. These plus-minus effects balance at a equilibrium which temperature of the system decides. In lower temperature energy wins more and equilibrium point goes less solvable. In increasing temperature entropy wins more and equilibrium goes more solvable.

Thanks, this is helpful.

So would you say the same for N₂ and water. The gas in water is the low entropy side, with weak binding to the water molecules, favored at low temps?

 

[anuttarasammyak]

Ref to my post #2 I find much better explanation in web text https://chem.libretexts.org/Bookshelves/General_Chemistry/Book:_CLUE_(Cooper_and_Klymkowsky)/6:_Solutions/6.4:_Gibbs_Energy_and_Solubility
I hope it will help you.

As for gas we see cola pours out when we open the lid. We observe higher the pressure, more solvable CO2 gas. Higher the temperature, less solvable CO2.
For increasing pressure under constant temperature, we would fill more gas into the system of fixed volume vessel or shrink the vessel removing heat. So gas in water would increase.
For increasing temperature under constant pressure, we would remove gas from the vessel or inflate the vessel adding heat. So gas in water would also decrease.
I am not succeeding to connect above thought with Gibb's Free Energy. I appreciate advice of colleagues.

 

[DaveE]

Ref to my post #2 I find much better explanation in web text https://chem.libretexts.org/Bookshelves/General_Chemistry/Book:_CLUE_(Cooper_and_Klymkowsky)/6:_Solutions/6.4:_Gibbs_Energy_and_Solubility
I hope it will help you.

As for gas we see cola pours out when we open the lid. We observe higher the pressure, more solvable CO2 gas. Higher the temperature, less solvable CO2.
For increasing pressure under constant temperature, we would fill more gas into the system of fixed volume vessel or shrink the vessel removing heat. So gas in water would increase.
For increasing temperature under constant pressure, we would remove gas from the vessel or inflate the vessel adding heat. So gas in water would also decrease.
I am not succeeding to connect above thought with Gibb's Free Energy. I appreciate advice of colleagues.

Thanks again. That's a really helpful explanation. I've always had trouble with how you equate ΔS and energy, I've always understood that it takes energy to create order, but I never understood how much, how to use it. I think this is a result of all of the analogies of colored marbles mixing and such. It sort of teaches you about the concept of entropy, but no one ever talks about how you actually measure it, or the underlying processes (energy) require to sort those marbles.

Unfortunately, it also reaffirms why I didn't like or really understand chemistry. It always seemed to me that it was too complex to really understand. It always seemed like there were general rules that applied most of the time, but as soon as you think you have a method in hand, you run across and exception, then a different sort of exception. I always felt like the only way to understand it was to either learn about the quantum mechanics of every sort of chemical bond, or to give up and look up the answer in tables from someone else's experiment. If you really do understand it, it must be very difficult to teach to people like me that aren't happy with memorization in place of understanding.

In the example link, I completely followed their explanation. But, I have no idea why dissolving NaCl is endothermic and CaCl₂ is exothermic. Something about the strength of the ionic bonds in these salts compared to the entropy change, I guess. Maybe because the Ca²⁺ ions have more charge than Na⁺ so it takes more energy to dissociate? Is the ΔH term essentially the energy of breaking the ionic bonds?

 

[Borek]

I have no idea why dissolving NaCl is endothermic and CaCl2 is exothermic.

There are two processes - one (endothermic) is breaking the ionic bonds in the crystal structure of the salt, the other (exothermic) is solvation of ions by water molecules. Overall effect is a sum of both, so in general it depends on the type of the crystal structure, sizes of ions involved and their charges.