[orgoclear]

I was revising the acid-base concepts when i came upon this question:

Which solution is more acidic? (a) HCl (unionized) dissolved in benzene or (b) HCl in water.

I presumed that since there is ionisation in water, the HCl is more effective in giving H+ ion hence that should be more acidic. But the answer was given the opposite. Can anyone help me in this regard?

[Borek]

I don't get it too. No idea what they mean by 'acidic' here.

[StarvinMarvin]

Maybe what they are referring to is the fact, that in benzene H+ would not be stabilized anyhow, therefore its rate of formation and decomposition into HCl is very high. So, due to the lack of stabilization, reaction of freshly formed H+ in benzene would be much more rapid than in a polar solvent. I don't know whether I think correctly, it's the way I see it.

[AWK]

Degree of HCl dissociation benzene solution is below 10 %. Is means HCl is a weak acid in this solution.
Comparison of acidity is rather impossible since there is no H₃O⁺ ions in benzene solution.

[orgoclear]

i dont get it either. It is from Morrison and Boyd. in the exercises of the first chapter

[StarvinMarvin]

I heard Morrison and Boyd is full of bugs. Maybe that's the reason

[joelhall]

makes sense to me:

HCl + H2O -> hydrochloric acid, HCl acts as a strong acid.

according to 'advanced chemistry' by clugston & flemming...

'hydrogen chloride dissolves readily in benzene C6H6.  benzene is an example of a non-aqueous solvent.  hydrogen chloride does not act as an acid when it is dissolved in benzene...'

therefore obviously the answer should be b) in water.

[orgoclear]

can anyone see this and then answer my query?

http://targetiit.com/iit_jee_forum/posts/better_acid_8744.html

[orgopete]

Let me see if I an explain this. In the following equilibrium, the reaction goes completely to the right.

HCl + H₂O  H₃O⁺ + Cl^-

That means Cl^- is a weaker base than H₂O. If we repeated the reaction below, the equilibrium goes to the left.

NH₄Cl + H₂O  H₃O⁺ + Cl^- + NH₃

The explanation is the same. The equilibrium favors the weakest base (least available electrons).

If we took hydrochloric acid and added another compound to it, the equilibrium would favor formation of the weakest base. If the compound being added is more basic than water, the conjugate acid of the compound will be the major product of the equilibrium.

The question at hand does not give us the other compound specifically, but the principle is the same. If we add a compound that is a weaker base than water, but a stronger base than benzene, in which system will the greater proportion of the protonated product form? (I didn't mean to give away that the electrons of benzene are less available than water.)