Anybody good with acids and bases?

Question

HCl and HI are equally strong acids in water but in pure acetic acid, HI is a stronger acid than HCl. Why?
and
Reactions requiring either an extremely strong acid or an extremely strong base are carried out in solvents other than water. Why is this necessary?

Answer 1

To set the record straight, HI is always stronger than HCl, even in water. It is just that both are overwhelmingly strong in water and thus you cannot really distinguish their strength in water.

The strength of acids depends on several factors.
One of them is electonegativity. The more electronegative the atom that is bound to H, the stronger the acid will be (because it pulls the electrons more and stabilizes the negative charge of the ion). If that were the only factor you would expect HCl to be stronger.
However, for binary acids, the size of the anion is much more important: the bigger the anion the more the charge will be spread (or smaller charge density if you prefer) making the conjugate base weaker/less reactive and thus the acid stronger.
I- is bigger than Cl- so HI is a stronger acid.

The general rule for binary acids is that acidity increases as you go from left to right in a period of the periodic table (electronegativity effect) and from top to bottom in a group (due to the size of the anion).

The strength of acids is expressed by pKa. The lower the pKa the stronger the acid. As you know Ka is the dissociation constant of the acid and for a monoprotic acid we can write

HA <=> H+ +A-
Ka= [H+][A-]/[HA]

However H+ are NEVER free in solution. They are always solvated. E.g. in water you have H+ +H2O -> H3O+
If you look carefully, the solvation reaction is actually a Brosted-Lowry acid-base reaction with water acting as a base.
When you use pure acetic acid you will have a similar solvation reaction.
But water has pKa=pKw=14 and acetic acid has pKa=4.75. This means that water is approximately 10^10 times more basic than acetic acid and thus it will react much more readily with H+ than acetic acid will. This in turn means that the acids in acetic acid will have a much harder time giving their H+ in solution than in water. So in that environment the difference in the acidity of HI and HCl can become apparent.(Think of it for simplicity as a case where in water you have such a huge signal that it is out of scale and by changing solvents you change the baseline so that you can actually measure the strength)

Reactions requiring extremely strong acids are with compounds that are extremely weak bases. These reactants would be less basic than water and thus if you had water as solvent, the extremely strong acid would react with water and not with the desired reagent.
Water can act also as an acid. Thus the same reasoning applies to reactions of extremely strong bases with extremely weak acids.

Answer 2

because the difference in electronegativity between H and I i greater and therefore more acidic

reatcion are not carried in water because water is a polar solvent

Answer 3

acids are those substance that changes the lythium's color from blue to red while bases turn the paper from red to blue