The hydration energies (strictly, hydration enthalpies) fall, as
expected, as we descend either Group, and are larger for Group II
than for Group I ions. The solubilities of the salts of Groups I and II
are determined by a balance between lattice energy, hydration
energy and the entropy change in going from solid to solution, and
only a few generalisations are possible. Thus high charge and low
ionic radii tend to produce insolubility (for example salts of lithium,
beryllium and magnesium, especially those with doubly charged
anions such as carbonate COa~). At the other end of the scale, low
charge and large radii also produce low solubility (for example salts
of potassium, rubidium and caesium containing large anions such
as the tetraphenylborate anion (p. 136). In between, solubility is the
rule for all Group I salts, and for most Group II salts containing
singly-charged negative ions; for many Group II salts with doublyor
triply-charged anions (for example COj", SOj", PO^ ) insolubility
is often observed.
The hydration energies (strictly, hydration enthalpies) fall, asexpected, as we descend either Group, and are larger for Group IIthan for Group I ions. The solubilities of the salts of Groups I and IIare determined by a balance between lattice energy, hydrationenergy and the entropy change in going from solid to solution, andonly a few generalisations are possible. Thus high charge and lowionic radii tend to produce insolubility (for example salts of lithium,beryllium and magnesium, especially those with doubly chargedanions such as carbonate COa~). At the other end of the scale, lowcharge and large radii also produce low solubility (for example saltsof potassium, rubidium and caesium containing large anions suchas the tetraphenylborate anion (p. 136). In between, solubility is therule for all Group I salts, and for most Group II salts containingsingly-charged negative ions; for many Group II salts with doublyortriply-charged anions (for example COj", SOj", PO^ ) insolubilityis often observed.
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