Predicting the Degree of Hydrolysis of Cations
It is useful to be able to gauge the approximate acidity of a given metal cation because it allows us to anticipate if a compound will react violently when exposed to water or atmospheric humidity, whether its hydroxide or oxide will precipitate on forming an aqueous solution or determine the form in which a metal ion pollutant exists in a lake, i.e., will it be an insoluble sludge at the bottom or will it remain in solution to be taken up by living organisms.
Although equations have been derived which accurately predict the extent of hydrolysis of metal ions, these are too complicated to be used for quick estimations of approximate acidity. Acidity of metal ions (their ability to produce hydronium ions in aqueous solution) is a function of their charge and size.
|pKa = 15.14 - 88.16(Z2/r)
Charge and Radius Effects
A plot of pKa versus Z2/r shows a linear relationship. On the graph below, metals with electronegativities less than 1.5 are plotted as purple circles. Recall that the larger the value of pKa, the less acidic the cation.
Click on the graph to see the details.
Cations of metals having electronegativities greater than 1.5 have smaller pKas (are more acidic) than other metal ions of similar charge and size.
The equation can be modified to include the effect of "excess" electronegativity of a cation and it "excess" acidity.
|pKa = 15.14 - 88.16[(Z2/r) + 0. 096(EN-1.50)]
This equation is only valid if the Pauling electronegativity of metal is greater than 1.50
A plot of pKa versus [(Z2/r) + 0.096 (EN-1.50)] shows
a linear relationship. On the graph below, metals with electronegativities
less than 1.5 are plotted as open circles. Click
to see the details.
Taken from Wulfsberg, Gary. Principles of Descriptive Chemistry; Brooks/Cole Publishing:Monterey CA, 1987; p.29.
Classify the following cations and describe their reactions with water.