When hydrogen chloride is bubbled into water, the resulting solution is acidic. When it is bubbled into the non-polar liquid, hexane, it dissolves but the resulting solution is not acidic. Explain these observations. 
What do I write to get the full four marks?
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- Thread Starter
- 19-02-2016 11:46
- 19-02-2016 21:10
I don't know if I can get 4 marks on it (since I'm not mark-scheme machine!), but I'll try and write to you what I think they are looking for in the question.
First thing is to clarify what definition of an acid to use:
1. There are three that I know of (Arrhenius, Bronsted-lowry and Lewis) - I'll take a stab in the dark and assume that the one we are interested in for this question is to do with Bronsted-lowry acids.
2. The definition of a Bronsted-lowry acid is a proton donor.
So essentially, we're just looking to explain why HCl dissolved in water acts as a proton donor, whereas HCl dissolved in hexane does not.
HCl in water:
1. Dissociates into H+(aq) and Cl-(aq) [Interesting side fact if you didn't know is that HCl is actually a covalent gas, but it doesn't matter, since it will still form H+ and Cl- ions in solution]
HCl in hexane:
1. Does not dissociate - remains as individual molecules of HCl just now it is HCl (aq) i.e. dissolved
The way I would therefore tackle this question is:
A Bronsted-lowry acid is defined as a proton donor . When HCl dissolves in water, it dissociates to form H+ ions and Cl- ions - this hence produces H+ i.e. protons as a result . However, when HCl dissolves in hexane, it does not dissociate . Instead, it remains as HCl (aq), so has not donated any protons/H+ ions into the solution . The resulting solutions for HCl in water would be more acidic, as pH = -log[H+] hence pH has decreased , whereas the solution for hexane will remain at the same pH as there has been no change in the [H+] .
I just put these  marks in where appropriate. It doesn't mean that these are at all on the mark scheme, but its my opinion of it anyway.
I hope I've helped a tiny bit. Just remember that I'm an A level student myself so take what I write with, well... a pinch of salt.Last edited by Spectral; 19-02-2016 at 21:11.