[qwerty12321]

I have been researching extensively, but am unable to find if the extent of the common ion effect is influenced by the concentration of the common ion already in the solution? For example, if there are two solutions of weak acid which have dissociated to a different extent, would the addition of more H+ affect the one that is less dissociated more substantially than the one with many H+ already in solution?

[Borek]

Once you mix them ions are indistinguishable.

[qwerty12321]

So that's a no?

I was wondering if it was like the entropy change at various temperature - if the temperature is high, the change in entropy will have less of an effect.

[jerryshizzle123]

if there are two solutions of weak acid which have dissociated to a different extent, would the addition of more H+ affect the one that is less dissociated more substantially than the one with many H+ already in solution?

Well, considering neither are strong acids, the addition of H+ would have a greater effect the one that is less dissociated. Consider the dissociation of the following weak acids:
HF <---> H+  +  F-     Ka = 6.8 x 10^-4
HCN <---> H+  +  CN-    Ka = 1.8 x 10^-5
As you can see, the HCN dissociates to a lesser extent than the HF (i.e., HCN produces fewer H+ ions in solution than HF). Thus, a given amount of added H+ would much more significantly drive the equilibrium of HCN to the left than the equilibrium of HF.

[qwerty12321]

Thanks so much, this is exactly what I was after! Do you know of any sites that would explain this at all?

[Borek]

HCN <---> H+  +  CN-    Ka = 1.8 x 10^-5

Check your Ka value, it is wrong.

As you can see, the HCN dissociates to a lesser extent than the HF (i.e., HCN produces fewer H+ ions in solution than HF). Thus, a given amount of added H+ would much more significantly drive the equilibrium of HCN to the left than the equilibrium of HF.

You are comparing apples and oranges. HCN was almost not dissociated, so - all other things identical - the amount of CN^- protonated was much lower than the amount if F^- protonated.

Question is poorly defined, so the answer is ambiguous.

[jerryshizzle123]

Check your Ka value, it is wrong.

Could be, but it doesn't affect my point.

You are comparing apples and oranges. HCN was almost not dissociated, so - all other things identical - the amount of CN^- protonated was much lower than the amount if F^- protonated.

Question is poorly defined, so the answer is ambiguous.

It's not poorly defined and you're looking at this from an absolute, not a relative, standpoint. Since the HF produced more H+ ions in solution, a given amount of added H+ doesn't shift the equilibrium as significantly as it does for HCN, which produced fewer H+ ions. He was right to compare this situation to entropy: just as a given quantity of heat produces a much greater percent change in the entropy of lower temperature surroundings than in the entropy of higher temperature surroundings, a given quantity of added H+ shifts the equilibrium to the left much more profoundly in a reaction that produces fewer dissociated H+ ions than in one which produces many. Since the weaker acid (in this case, the HCN) produces fewer dissociated H+ ions, it is the one more affected by the addition of H+.

[Borek]

It's not poorly defined

added H+ shifts the equilibrium to the left much more profoundly

Define "shift of the equilibrium" in a quantitative way. So far you are just hand waving.