Entropy and Precipitation

A large value for -TDS is characteristic only for precipitation reactions of acidic cations with basic anions.

This corresponds to an increase in the disorder of the system which would not be expected to occur on forming a precipitate!

The entropy change in precipitation is independent of the type of crystalline product formed. but is related to Z2/r for the cation.

Most ions release enough water molecules upon precipitation to give a net disordering effect. These ions are referred to as electrostatic structure makers.

All but the non acidic cations and non basic anions make negative contributions to the entropy term for precipitation reactions. The non acidic and non basic ions are termed electronic structure breakers. Although it might be expected that large cations with their greater maximum coordination numbers would attach and order water molecules more effectively than small cations, this is not what is observed. The greater the charge or the smaller the radius (the more acidic or basic the ion), the greater its structure-making properties.

Ions that are at all acidic or basic pull the electrons in the hydrated water molecules strongly enough to enhance the hydrogen bonding capability of the waters in the primary hydration sphere toward other water molecules.
The stronger the attraction of the bare ion for water molecules, the greater will be the number of layers of water molecules.

The reason that most acidic cations and basic anions react to give precipitates is due to the disorder that results from the release of numerous waters from the hydration spheres.

Mg(H2O)362+ + CO3(H2O)282-Mg(CO3) (s) + 64 H2O

The large size of non acidic cations and anions may cause disruption of "iceberg" structures in the nearby liquid water. This would result in positive entropies of solution. Hence the name electrostatic structure breakers.

Enthalpy and Precipitation

The insolubility of salts from nonacidic cations and nonbasic anions as well as cross-combination salts is due to the enthalpy term, DH.

The enthalpy of precipitation can be related to the properties of the cation and anion if the process is broken into 3 steps (Hess' Law says that the enthalpy change for the overall reaction equals the sum of the enthalpy changes for each of the component steps.)
  1. hydrated cation is dehydrated and converted to a gaseous bare cation
  2. hydrated anion is dehydrated and converted to a gaseous bare anion
  3. gaseous cation and anion form the ionic solid

DHppt (MyXm) = -yDHhyd(Mm+) - mDH(Xy-) + U(MyXm)

Calculate the lattice energy for Ba(ClO4)2

DHppt = -DHhyd(Ba2+) - 2DH(ClO4-) + U(Ba(ClO4)2)

(+12kJ/mol) = -(-1304kJ/mol)(1 mol)-(-227 kJ/mol)(2 mol) + U

U = -1746 kJ/mol

Calculate the lattice energy for Mg(ClO4)2.

The absolute magnitude of the lattice energy is not important. The key is whether the lattice energy is larger than the combined hydration enthalpies.

U (kJ)
Combined Enthalpies