[AlwaysQuestioning]

Hi guys.

 I've been reading pages on dissociation constant, and noticed that apparently , when the number of ions ( ionic strength) in the solution in which an acid dissociates in increases, the dissociation constant increases (pKa values drop). Let's say carbonic acid's dissociation constant into hydrogen carbonate ions and hydrogen ions is apparently higher when it dissolves in a solution of water with let's say Na+ and Cl- ions in it as compared to pure water? Why is this so? Isn't there less water molecules to interact with the carbonic acid molecules to cause it to dissociate?

Thanks!!!!

[Babcock_Hall]

If I understand your explanation correctly, it would tend to make the pK_a larger.  Your explanation may become more important at high concentrations of solute.  Here is a link to the Debye-Huckel theory, which itself is only an approximation:  http://chemwiki.ucdavis.edu/Physical_Chemistry/Physical_Properties_of_Matter/Solutions/Nonideal_Ssolutions/Debye-Hückel/Debye-Hückel_Theory_of_Electrolytes

[Borek]

It is no about water only, it is also about electrostatic interactions between ions. There is no simple explanation, more like "we observe this and this".

[AlwaysQuestioning]

So there isn't any current theoretical explanation for this observation?  >:
Edit: I've also been reading about free radicals lately, and wikipedia said that free radicals can be considered to be only fairly stable in vacuum and inert conditions. So is this to say, are free radicals metastable then? Like stable but only as long as without disturbance from any other atom/molecule ? (So as to say as long as it doesn't meet any other atoms or molecules it should be fine right?) Or for those atoms in the atmosphere (let's say chlorine atom) that is part of a larger multiple -step reaction (decomposition of ozone), is it better to call them transient instead of metastable?


Thanks so much!

[Borek]

So there isn't any current theoretical explanation for this observation?

Yes, there is. It is called Debye-Huckel theory. The equations it uses are derived from the first principles (electrostatic interactions of the ions). But just because we can derive these equations doesn't mean there exist a simple explanation. Besides, DH theory fails for higher concentrations, where we replace it with other theories - and these other theories are not based on first principles, they are mostly fitted to the observed behavior of the solutions.