Chemistry Question #2377

Ian Hancock, a 16 year old male from Calgary asks on November 24, 2004,

In Chemistry today, we learned that when HCl is ionized in water, the H2O becomes H3O+, because of the coordinate covalent bond. Yet, when H2SO4 is ionized in water, it does not become H4O(2+) (at least that's what the teacher says.) Can you explain why? (if the O in H2O can carry 1 coordinate covalent bond on one free pair of e- s, should it not be able to carry a second coordinate covalent bond on the other pair?

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The answer

Reg Mitchell answered on January 13, 2005

Two positive charges on a small molecule is too much for it. It's energetically unfavorable, so instead, there is always plenty of water so H2O + H2SO4 gives H3O+ and HSO4- then H2O + HSO4- can give H3O+ and SO4 2- but this is not as favorable (i.e. energetically profitable) as the first, so if there is plenty of water, mostly it is HSO4- but there will be some SO4 2- ions.

Charge, whether it is positive or negative, has to be spread out over the molecule to reduce the electric field energy. The bigger the molecule the easier it is to spread out the charge, (reduce the electric field strength). It is easier to generate SO4 2- than H4O 2+ because sulfate is much bigger than water.

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