Re: Why is sulphuric acid a stronger acid than sulphurous acid and why?

Area: Chemistry
Posted By: Jason Hajinakitas, Post-Doctoral Fellow,CSIRO
Date: Tue Aug 12 18:18:33 1997
Area of science: Chemistry
ID: 870610222.Ch

Message:

Irene, I hope this answers your question.
Sulfurous acid is properly defined as SO2 dissolved in water.  It is not 
the commonly call H2SO3 in solution. Hence the equilibrium  of SO2 in H2O 
is represented by:

SO2  + xH2O  =  SO2.xH20
SO2xH2O = HSO3-(aq) + H30+ + (x-2)H2O

Hence the FIRST acid dissociation constant for "sulfurous acid" is:

K1= ([HSO3-][H+])/([total dissolved SO2]-[HSO3-]-[SO32-])
  =1.3x10-2 giving a pKa of 1.9
If you compare this to the pKa1 for Sulfuric acid (which exists as H2SO4) 
you find values of <0.  Hence sulphuric acid is much stronger than 
sulphurous.

Current Queue | Current Queue for Chemistry | Chemistry archives

Try the links in the MadSci Library for more information on Chemistry.