Question

Assertion: Lithium salts are mostly hydrated.
Reason: The hydration enthalpies of alkali metal ions decrease with increase in ionic sizes.
A) Both assertion and reason is correct and reason is the correct explanation of assertion
B) Both assertion and reason is correct but reason is not the correct explanation of assertion
C) Assertion is correct but reason is incorrect
D) Assertion is incorrect but reason is correct

Answer 1

Lithium is the smallest in size among the alkali metals. Hence, $\mathrm{Li}^{+}$ ion can polarize water molecules more easily than other alkali metals. As a result, water molecules get attached to lithium salts as water of crystallization.

Complete step-by-step answer:
- The hydration enthalpy decreases from $\mathrm{Li}^{+}$ to $\mathrm{Cs}^{+}$ ions. The smaller the ions, the stronger the attractions are between the molecules of water. It also depends on the charge/size ratio.
- From this we can conclude that hydration enthalpy is directly proportional to the charge of ions and inversely proportional to the size of the ion. This is called an ionic potential.
- Lithium ion has the greatest reducing nature in aqueous medium as it possesses the maximum hydration enthalpy and higher polarising power.
- For group 1 elements, charge remains constant that is, +1 and so the hydration energy depends only on the size. Size of alkali metal ions increases on going down the group as due to the addition of shells. Thus, the hydration enthalpies decrease down the group of alkali metals.
Clearly, the answer is A, since both the reason and assertion are true and the reason is the correct explanation of the assertion provided.

Note: It can also be understood by surface charge density on which the magnitude of hydration enthalpy depends. The surface charge is more for the smaller ions as they have higher force of attraction between the ion and the polar end of water. Thus, the smaller ions like lithium form lithium salts which are so hydrated and stable.