Question

Why is LiF almost insoluble in water whereas LiCl is soluble not only in water but also in acetone?
A. For a substance to dissolve in water, its hydration energy must be greater than its lattice energy.
B. Due to the small size of \[L{i^ + }\] ion and F- ions, the lattice energy of LiF is very high. LiF is covalent in nature. Therefore, LiF is insoluble in water.
C. Due to the larger size of \[C{I^ - }\] ion, the lattice energy of LiCl is less than its hydration energy. Hence, LiCl is soluble in water and in acetone.
D. Due to the large size of \[{F^ - }\] ion and high hydration energy, the LiF is insoluble in water.

Answer 1

Hint: To understand the solubility of the compounds in water, there are two important terms which should be known. They are lattice energy or enthalpy and hydration enthalpy. When the hydration energy of the compound is greater than that of lattice energy of that compound ,that compound becomes soluble.

Complete step by step answer:
-Lattice enthalpy is an amount of energy which is required to break a bond. On the other hand we can say that the energy released when two atoms make bonds. Higher the lattice energy higher will be the bond energy of that bond as well as the bond strength.
-Hydration enthalpy is an amount of energy which is released on hydration of one mole number of ions. When the solvent is not water is known as a solvation energy.
-Now if the hydration energy of any particular ion overcomes the amount of lattice energy the bond dissociates and solubility occurs.
-In case of LiF due to the small size of fluoride it forms a strong bond with lithium . As a result, it shows high lattice enthalpy but lower hydration enthalpy. Due to this LiF is insoluble in water.
-On the other hand, due to the large size of chloride atom the bond is weaker with lithium as a result it shows lower lattice energy. Due to this it is soluble in water. And also, LiCl shows partial covalent in nature due to the large size of chloride atom. So, for this covalent nature it is also soluble in acetone.

So, the correct options are A and C.

Note:
Lattice energy is the force of attraction between the ions in a solid.lattice energy is directly proportional with the charge of the ions, higher the charge of the ions higher will be the lattice energy and vice-versa.Lattice energy also depends upon the size of the ions, lower the size of ions higher will be the lattice energy and vice-versa.