Acids and Bases
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As well as using the hard and soft acid and base description to discuss the interactions, we can look at the thermodynamic properties of the acid-base reaction. The aim is to include electronic, structural rearrangement, and steric effects in a small set of parameters.
When we consider general acids such as Al3+, Cr3+, and BF3, we find there are good correlations between the order of affinity to bases obtained with them and the order obtained when H+ is used as the acid. However, a different order is observed for an acid such as Hg2+.
A strong acid, with pKa<0, is almost completely ionized in aqueous solution. Similarly, a strong base is almost fully protonated in solution.
The strength of a bronsted acid, HA, in aqueous solution is given in term of the acidity constant, Ka (which is also known as the acid ionization constant).
Arrhenius: acids and bases dissolve in water releasing H+ and OH– respectively. This is a narrow categorization.