Sciencemadness Discussion Board

The Stronger Acid

RedsAreRaw - 30-1-2017 at 10:43

Came across this question: "(i) What is the acid in an aqueous solution of HBr? In dry HBr? (ii) Which is the stronger acid? (iii) Which can transfer a hydrogen ion to an alkene?"

For (i) I got H3O+ and HBr
For (ii) I got HBr
For (iii) I got HBr

I was just wondering if this was correct. I was guessing HBr for (iii) because of bond dissociation energy.

Thanks.

DraconicAcid - 30-1-2017 at 11:24

For i), there won't be any HBr molecules in an aqueous solution, at least not to any measurable concentration.

ave369 - 31-1-2017 at 08:46

In aqueous solution, the acid is always H3O+. In dry HBr, there is no acid: the only hydrogen halide that is acidic when completely dry is HF. It self-ionizes into H2F+ and HF2-, and H2F+ is very acidic. The other hydrogen halides do not self-ionize.

[Edited on 31-1-2017 by ave369]

DraconicAcid - 31-1-2017 at 10:12

Quote: Originally posted by ave369  
In aqueous solution, the acid is always H3O+. In dry HBr, there is no acid: the only hydrogen halide that is acidic when completely dry is HF. It self-ionizes into H2F+ and HF2-, and H2F+ is very acidic. The other hydrogen halides do not self-ionize.

[Edited on 31-1-2017 by ave369]


Dry HBr does not necessarily mean pure HBr. It can be dissolved in any non-aqueous solvent.

nezza - 1-2-2017 at 01:36

Dry HBr is purely covalent and there is no acid species present.

RedsAreRaw - 1-2-2017 at 08:08

Thanks for the input!
Quote:

Dry HBr is purely covalent and there is no acid species present.


By definition, an acid is a substance that gives up a proton, or an electron pair acceptor. Does this mean that it has to have given up this proton to be considered an acid, or just that it has the ability to give up a proton?

And I messed up the question for (iii). It's supposed to say "Which can better transfer a hydrogen ion to an alkene?"

Zyklon-A - 2-2-2017 at 03:30

It just needs the ability. If it has already given up its proton then its no longer an acid. HBr gas will protonate water completely so that virtually no HBr molecules remain. You'll now have H3O+ and Br- desolved in water. The hydronium ions are now what make the solution acidic, they'll protonate any weaker acid or or stronger base. The first answer should be hydronium ions or perhaps just hydrobromic acid.
II should be hydrogen bromide.
III should be hydrogen bromide