# Solid-State Acid-Base Reaction?

1. Jan 23, 2014

### Qube

1. The problem statement, all variables and given/known data

Which of the following CAN be made as a pure substance?

Choices are a list of molecular salts.

2. Relevant equations

According to my teacher the salt cannot be made as a pure substance if the components undergo a large extent acid-base reaction.

The correct choice was $H_{4}NF$ (this can exist as a pure substance).

3. The attempt at a solution

Wait, what?

Can acid-base reactions occur in the solid-state or in the gaseous state? Is this what the problem is implying? I don't see any issue sticking solid ammonium to solid fluoride as long as there is no water present as a solvent.

I thought acid-base reactions occur only in solution.

The correct answer according to my teacher is $H_{4}NF$ and the rationale is that the below reaction is small extent (note that the product acid is stronger than the reactant acid). Thus $H_{4}NF$ can exist as a pure substance.

$H_{4}N^{+} + F^{-} \leftrightharpoons H_{3}N + HF; Extent: small$

2. Jan 23, 2014

### SteamKing

Staff Emeritus
Fluorine is a gas at STP. IDK where you get solid fluorine from. Hydrogen fluoride HF is also a gas, as is ammonia NH3.

3. Jan 23, 2014

### Qube

A/B reactions can occur in the gaseous state?

I think I put solid down because some of the salts were solids at STP (I believe). But you're right, fluorine and the other things I listed are gaseous at STP.

4. Jan 24, 2014

### Staff: Mentor

No idea what the question is about, no idea what your teacher means, but the most important problem is: no idea what definition of "pure substance" is at use here.

Do you remember substances listed and exact wording of the problem?

5. Jan 24, 2014

### Qube

That is one question. Do you see what he's saying?

6. Jan 24, 2014

### Qube

Also Hen is short for...

7. Jan 24, 2014

### Staff: Mentor

My bet is that is a poor question.

I can understand the logic behind ammonium carbonate - it decomposes relatively easy, so it will be always contaminated by ammonia, water and carbon dioxide. However, I am not convinced other molecules listed don't suffer from other problems - whenever you see hydrated cations (like Fe(H2O)63+ or Al(H2O)63+) in the presence of other ligands (like SO42- or F-) you can expect some water molecules to be replaced by these other ligands, and the solution to contain a mixture of complexes. I am not convinced solid would be free of them.

8. Jan 24, 2014

### Qube

We haven't gotten to ligands yet. We're only concerned about acid/base reactions. Can such reactions occur in the solid-state? Or gaseous state?

9. Jan 24, 2014

### Staff: Mentor

Ammonia reacting with hydrogen chloride is definitely an acid-base reaction* and they react in the gaseous phase. Actually it is not that difficult, just take open bottles of concentrated ammonia and concentrated hydrochloric acid and put them close to each other - as both gases are volatile they will mix in the air and you will see fog appearing from nowhere.

In the solid phase... I suspect that mixing some solid acids (like benzoic) and some solid bases (say, NaOH) will soon yield a wet salt. Won't be initially a fast process, and the water produced will soon make it a reaction in solution.

*especially if we take into account all possible acid and base definitions.

10. Jan 25, 2014

### Qube

Very interesting. I should have used my common sense as well and realized that yes indeed acid-base reactions can occur in the solid state. I remember a former chemistry teacher who managed to destroy a brand-new classroom installation of tile flooring by storing a potent acid in a metal tub over the weekend.

Also, my teacher mentioned smelling salts as an example of a solid-state acid/base reaction, since smelling salts are just ammonium carbonate. The two undergo a large extent A/B reaction and create a potent smell.

$H_{4}N^{+} + CO_{3}^{2-} \leftrightharpoons H_{3}N + HOCO_{3}^{-}$

What are all the possible A/B definitions? I know of three: Arrhenius, Bronsted-Lowry, and Lewis. And also "hard" and "soft," I suppose, so four.

11. Jan 25, 2014

### Staff: Mentor

Hard and soft AKA Pearson theory of acids and bases, yes, that would be four.

But there were more - Usanowich theory, Ebert & Konopik theory, and few others I don't remember. They just never gained any popularity.