KOH Solubility: Negative Heat of Solution Explained

[JonnyG]

This is a quick question. My textbook lists the heat of solution of KOH as a negative number, and thus the solubility of KOH decreases as temperature is raised. Word for word, my book says "Negative heat of solution: The solute solubility decreases with rising solution temperature". But this wikipedia page: https://en.wikipedia.org/wiki/Solubility_table

shows that KOH solubility increases with rising temperature. Can someone please explain this contradiction?

 

[Gary Howard]

Where I worked we would make solutions of KOH in water to 34% KOH. The crystals dissolved and heated the water considerably. Don't know if this helps.

 

[Kevin McHugh]

It is due to the heat of solvation. Dissolution of KOH is exothermic. Heat is a by product of the reaction. You slow the solvolysis down by adding heat. You will speed solvolysis up by taking heat out of the system.

 

[Merlin3189]

I do not profess to have more than a very vague idea about this topic, but enough to understand your question and wonder what the real answer is. After some research I am inclined to believe that you are right - the book has an incomplete (and hence inaccurate) explanation which *may* give a partial general picture, but will allow of many exceptions.and not give accurate quantitive predictions. Entropy seems to be the missing factor, which can negate the effect of heat of solution, particularly as temperature rises.

Can I suggest you look at this link Chemguide: Solubility of Group 2 Compounds. Then at the role of entropy in solubility.

 

[JonnyG]

It is due to the heat of solvation. Dissolution of KOH is exothermic. Heat is a by product of the reaction. You slow the solvolysis down by adding heat. You will speed solvolysis up by taking heat out of the system.

Suppose to put a solution of KOH on the stove and heated it up. I am adding heat into the system and thus I decrease the solubility, right? But the table on the WIKI website says that the solubility actually increases with increasing temperature of the substance. I am assuming that increasing temperature of the substance means adding more heat into the system.

 

[BvU]

My textbook lists the heat of solution of KOH as a negative number, and thus the solubility of KOH decreases as temperature is raised

Is that really what it says ? I believe the negative number: dissolving the stuff is exothermic. But whence the "and thus.." ?

 

[JonnyG]

Is that really what it says ? I believe the negative number: dissolving the stuff is exothermic. But whence the "and thus.." ?

No that is my interpretation of what I read - that if solvation is exothermic then an increase in temperature decreases solubility. What my textbook actually says is "Negative heat of solution: The solute solubility decreases with rising solution temperature" and "Solutes with positive heats of solution ordinarily become more soluble as the temperature of their solutions is raised...the effect of temperature on saturated solutions of solutes with negative heats of solution is the reverse of the process just described".

Perhaps KOH is an exception to the rule?

 

[Borek]

if solvation is exothermic then an increase in temperature decreases solubility

If solvation is exothermic then an increase in temperature decreases solvation.

 

[JonnyG]

If solvation is exothermic then an increase in temperature decreases solvation.

Solvation is the process of the solvent molecules surrounding the solute molecules, so if solvation decreases then wouldn't solubility also decrease?

 

[Borek]

Apparently it doesn't

 

[JonnyG]

But what about my book listing the heat of solution of KOH as a negative number and then saying "Negative heat of solution: The solute solubility decreases with rising solution temperature"

 

[Borek]

It doesn't separate solubility from solvation - it lists total. Not that it is trivial to measure each separately, so what we list as a "heat of solution" is actually a sum of heats of processes taking part at the same time.

At least that's my understanding of the situation. You are perfectly right the statement and the data listed are contradicting each other.

 

[Kevin McHugh]

Solvation is the process of the solvent molecules surrounding the solute molecules, so if solvation decreases then wouldn't solubility also decrease?

No, just the rate of solvolysis. Now granted, at extreme temps, you can affect total solubility.

 

[symbolipoint]

But what about my book listing the heat of solution of KOH as a negative number and then saying "Negative heat of solution: The solute solubility decreases with rising solution temperature"

Which is the description, "exothermic".

 

[Bystander]

You will notice the "state" of aggregation of potassium hydroxide? Pellets? 'Tain't "fish, flesh, or fowl," but a semi-solid solution that must be standardized. You cannot prepare a known strength solution gravimetrically. Arguably 100% "miscible," whatever that might mean/indicate in such a case.

 

[NascentOxygen]

Word for word, my book says "Negative heat of solution: The solute solubility decreases with rising solution temperature".

This is an intriguing question, so I went looking for an answer...

If I read this discussion correctly, enthalpy is only half the story. You need to also consider entropy.

Also, an interesting point arises from relying on tables of heat of solution giving ∆H for dissolving in water: but when dissolving extremely soluble substances, as you add more and more you are no longer dissolving into water, you are dissolving into a solution already containing much of that substance...and this may no longer be highly exothermic---∆H may be much lower---and near saturation it can even have become endothermic.