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 Acid-base equilibria

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Why is it that when an acid is dissolved in water, the hydrogen ion attaches to a H2O to make H3O? It seems unnecessary, because the H+ has its full outer e-shell and so do the atoms that make up H2O.

Ulex replies
But H+ doesn’t have a full outer shell, does it? H+ has no electrons at all; it is a bare proton and therefore intensely positively charged. It will be attracted very strongly to pairs of electrons which are not already involved in forming bonds – hence H3O+. In fact, the question really should be ‘Why does the acid molecule part with its H+ at all?’. The answer is that it does whatever results in the lowest energy state, i.e. the most stable state.

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updated: 25 January 2005

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