Acid-base equilibria
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Is HCl a stronger acid than H2SO4? If possible could you please tell me their dissociation constants too.
Dissociation constants for several acids and bases are given in Table 6.5 of the Nuffield Book of Data.
In this table you will find that sulphuric acid’s dissociation constant as a monoprotic acid is given as “very large”. Thus the process:
H2SO4 -> H+ + HSO4– is virtually complete. Notice that the second dissociation constant, i.e. for the process HSO4 -> H+ + SO42– does have a quoted value of 1.0 x 10–2, so the ionization of one mole of original molecules gives just over 1 mol of H+ ions, and not 2 mol, as is often thought.
In water the first dissociation constant for both acids is obviously “very large”. In water both acids are, in effect, equally strong. Chemists sometimes call this the levelling effect. Note that any acid that is a stronger acid than the H3O+ ions is always fully ionised in water.
The only way to discover which acid is the stronger is to dissolve them in a solvent that is less easy to protonate than water and to see which acid is then ionised to a greater extent. Hydrogen chloride, for example, is only partially ionised in pure ethanoic acid. In this solvent it behaves as a weak acid. This is how it is possible to show that the order of acid strength for hydrogen halides is HI > HBr > HCl. It is also possible to show that HCl is a stronger acid that H2SO4.
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updated: 21 August 2003
