Acid-base equilibria
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Why is formic acid is a stronger acid than acetic acid?
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Corrie writes .....
If HA is a weak acid, then the equilibrium that is established when it partially dissociates in solution is:
HA(aq) <=> H+(aq) + A-(aq)
For formic acid this equilibrium lies further over to the right than for ethanoic acid. One reason for this is that the formate ion (HCOO-) is a little more stable than the ethanoate ion (CH3COO-) in solution.
Why should this be? The CH3- group in ethanoic acid is more 'electron donating' (called a positive inductive effect) than the H- group in formic acid it replaces. This makes the ethanoate ion less stable in solution compared to ethanoic acid than is the case with the formate/formic acid system. So the ethanoic acid equilibrium lies more to the left, i.e. ethanoic acid behaves as a weaker acid.
Another way of looking at this is to consider the basic properties of the two negative ions. The extra electron donating ability of the CH3- group will make CH3COO- a slightly stronger base than than HCOO-. Thus the equilibrium for ethanoic acid lies further to the left, as before.
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updated: 12 February 2008
