Understanding Solute Activities: Debunking the Myth of Incoherence in Chemistry

[Qube]

My prof says there is no coherent theory regarding activities of solutes, and he's been performing independent research to update his class notes/textbook. However, I've flipped through the chapter on activities in Harris' Quantitative Chem. Analysis and it seems to have a fair bit of discussion on activities. Granted, I haven't found time to really read it yet.

Is there really no coherent theory regarding activities?

 

[Borek]

Debye Huckel theory is derived from the first principles, the only thing that has to be determined experimentally is the ion radius. Trick is, DH theory doesn't work for solutions with the ionic strength higher than 0.1.

We have some semi-empirical theories describing more concentrated solutions, but we don't know why they work, plus, they require experimental determination of interaction coefficients (whatever they are called in English) for each ion pair (unless it was for each pair of substances, I don't remember details ATM). That means ammonium phosphate solution requires at least 30 experimental parameters. And not too many have been measured and published.

 

[Qube]

Debye Huckel theory is derived from the first principles, the only thing that has to be determined experimentally is the ion radius. Trick is, DH theory doesn't work for solutions with the ionic strength higher than 0.1.

We have some semi-empirical theories describing more concentrated solutions, but we don't know why they work, plus, they require experimental determination of interaction coefficients (whatever they are called in English) for each ion pair (unless it was for each pair of substances, I don't remember details ATM).

What are these first principles? I'm not familiar.

That means ammonium phosphate solution requires at least 30 experimental parameters. And not too many have been measured and published.

That solution exists? I thought the two underwent a large extent reaction, making the solution more of an ammonia/hydrogen phosphate anion solution. Also what might these parameters be in general?

Also just in general, in considering equilibrium systems, how important is it to consider aqueous/vapor equilibria? I'm interested because one, I notice that there is extensive vapor released each time I uncork a bottle of 18 M nitric acid.

Also, Harris' book explicitly tells us in one example problem to ignore the H2S(aq) <-> H2S (g) equilibrium, and offers no further insight into how to incorporate this equilibrium. I have a feeling that if I had to, I could figure it out. Just wondering why the book chose to omit this - does the omission symbolize the trivial significance of this equilbria or is it signifying that this is too "complex" or beyond the scope of current scientific understanding (as with activities)?

Finally, what are your thoughts on introducing all this material in a 2000 level "general chemistry" course? I personally don't mind and am thoroughly fascinated by the rigorous analysis we have done in this class (often beyond that of any 4000 level course).

It's also quite amusing to ask students of other "general chemistry" instructors about the pH of a 1.0 * 10^-7 M HCl solution and have them report me that the solution has a pH of 7.0, knowing that they have only done a superficial analysis of equilibrium systems.

 

[Chestermiller]

My prof says there is no coherent theory regarding activities of solutes, and he's been performing independent research to update his class notes/textbook. However, I've flipped through the chapter on activities in Harris' Quantitative Chem. Analysis and it seems to have a fair bit of discussion on activities. Granted, I haven't found time to really read it yet.

Is there really no coherent theory regarding activities?

Try the Handbook of Aqueous Electrolyte Thermodynamics: Theory and Application, by Zemaitis et al.

Chet

 

[Borek]

What are these first principles? I'm not familiar.

Also what might these parameters be in general?

Check the book Chet mentioned. Or any other book where these things are explained/derived.

That solution exists? I thought the two underwent a large extent reaction, making the solution more of an ammonia/hydrogen phosphate anion solution.

How do you call the solution prepared by dissolving ammonium phosphate in water?

 

[Qube]

How do you call the solution prepared by dissolving ammonium phosphate in water?

An "ammonium phosphate" solution, complete with scare quotes to indicate that the name is insufficient to describe what is actually in solution in high concentration (which is not ammonium nor phosphate).

On the other hand if we had a solution made from dissolving ammonium and sulfate ions, I would just call it an ammonium sulfate solution because the protonation of sulfate to make hydrogen sulfate ion is very limited.

 

[Borek]

An "ammonium phosphate" solution, complete with scare quotes

You have just made it.