Question

The low solubility of $LiF$ and that of $CsI$ in water are respectively due to which of the properties of the alkali metal ions?

• A. Higher hydration enthalpy of $Li^+$, higher lattice enthalpy of $Cs^+$
• B. Smaller hydration enthalpy of $Li^{+}$, higher lattice enthalpy of $Cs^{+}$
• C. Smaller lattice enthalpy of $Li^+$, higher hydration enthalpy of $Cs^+$
• D. Smaller hydration enthalpy of $Li^+$, smaller lattice enthalpy of $Cs^+$
• E. Higher lattice enthalpy of $Li^+$, higher hydration enthalpy of $Cs^+$

Answer 1

Answer: (E)

If hydration energy is greater as compare to lattice energy then the salt is soluble, and if hydration energy is lesser as compared to lattice energy, then salt is insoluble.

LiF is almost insoluble due to high lattice energy. Both ions in LiF are very small. In this case, hydration energy is very less.

In case of CsI both ions are larger in size. As a result both the ions in CsI are less hydrated therefore, its hydration energy will decrease. Hence, it is insoluble in water