Barium sulfate (BaSO4) is insoluble in water because of water dipole strength is too weak to pull away the anions and cations of the BaSO4 as both Ba 2+ and SO4 2- ions are big and bigger cation stabilizes bigger anion strongly results in BaSO4 has higher lattice energy.
Why is BaSO4 insoluble in water?
Lattice Enthalpy (Lattice Energy)
In simple terms, Lattice enthalpy is the energy required to
split a compound into its constituent ions i.e. anions and cations. In other
words, it is a cohesive force that needs to bind ions of the compounds either in a gaseous state or in a solid-state and these are normally called formation
and dissolution lattice energy respectively.
Hydration Enthalpy (Hydration Energy)
Hydration enthalpy is simply defined as the amount of energy
released when one mole of ion combines with a water molecule. In a given
reaction, it has always a negative value as the energy is released during the
process which is also known as an exothermic reaction.
Explanation:
Higher the lattice energy, the molecule will be less soluble or
have less solubility. And the reverse of it, higher the hydration energy, the molecule
will be more soluble or have higher solubility.
Now in the case of BaSO4, it has higher lattice energy
because both Ba 2+ and SO4 2- ions are big and we know bigger cations
stabilize bigger anions more strongly. When it is dissolved in water, its hydration
energy decreases more rapidly than lattice energy.
For any compound to be soluble in water, its lattice
enthalpy should be lower and it should decrease more rapidly than hydration
energy. But this process is reversed when BaSO4 is placed in water and it is
insoluble.
Note: BaSO4 is not totally insoluble, it is sparingly soluble in water, and its
separated ions are quickly saturated with the undissolved precipitated
molecule. Generally, most of the insoluble compounds are sparingly soluble as
outermost ions are separated quickly but in very fewer amounts which are
negligible.
How is BaSO4 insoluble in Water?
As mentioned earlier, hydration energy when the water the molecule binds with ions are not so much stronger which is able to separate
the ions i.e. cations and anions of the BaSO4 molecule, as a result, BaSO4 is
insoluble in water.
We can also identify whether BaSO4 is soluble or insoluble
in water according to the solubility rules which states most of the sulfates
are soluble in water with exception of Ba 2+, Ca 2+, Pb 2+, Ag 2+, and Sr 2+.
BaSO4 Soluble or insoluble (Determining factors)
Solubility and Solubility Product
Higher the solubility and solubility product of the molecule, more soluble in water. But BaSO4 has very lower solubility (0.0002448 g/100 mL) and solubility product (1.0842 ×10-10) which makes it insoluble in water.
Solubility Principle
According to the solubility principle “like dissolves like”
which means similar type of solute is soluble in similar type solvent i.e.
polar molecules are most often soluble in polar solvents and nonpolar are soluble
in nonpolar solvents.
Temperature
Temperature has a very big role in the solubility of the
compounds. When the temperature of the solution increases, solubility also
increases and vice-versa. At room temperature BaSO4 is almost insoluble but when
temperature increases, its ions start separating more, and more and
solubility increases.
pH value
pH has reciprocal relation with the sparingly soluble bases
or basic salts solubility i.e. when pH increases, solubility decreases and while
pH decreases, solubility increases.
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