Acid-base equilibria
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Hi, just wondering how you predict whether a base is strong or weak, can you predict with only the formulae? thanks
Jenkin writes ...
Not really. We can say that metal hydroxides are strong bases because they are fully dissociated in aqueous solution, although many of them, such as calcium hydroxide, are only slightly soluble so the concentration of OH- ions is quite low. Remember that following the Bronsted-Lowry theory, bases are proton acceptors, so here it is the OH- ion that is the proton acceptor.
The other important bases are nitrogen compounds, starting with ammonia, NH3, and including many organic compounds, especially amines.
Ammonia is a weak base because although in aqueous solution it produces OH- ions
NH3(aq) + H2O(l) -> NH4+(aq) + OH-(aq)
equilibrium lies well to the left so the concentration of OH- ions is low.
Amines can be thought of as ammonia which has had hydrogen replaced by one or more organic groups; the more there are of these the stronger the base, so trimethylamine, (CH3)3N, is a stronger base than methylamine, CH3NH2. This is a consequence of the alkyl groups ‘pushing’ electrons towards the N atom so there is an increase in polarity and it accepts protons more readily. This does not happen with aromatic rings, so phenylamine, C6H5NH2, is a weaker base than methylamine.
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updated: 25 January 2008
