Is Atomic Size the Key Factor in Determining Acidity Levels?

[hivesaeed4]

Suppose I have two acids HX and HZ. Say X is smaller then Z in atomic size. So then one would think that HX would be a more stable molecule then HZ since X has a higher charge density then Z (Both have same electronegativity) . HZ being a weaker molecule would dissociate more rapidly. Now my TA says that due to this HZ is more acidic then HX. Is this correct?

 

[Jorriss]

Pretty much. Compare the acidity of HF, HCl, HBr and HI.

 

[aroc91]

Correct. Acidity is a related to the stability of the conjugate base. A larger anion is more stable on its own than a small one, so the equilibrium favors the dissociated H+ and Z-.