Question

The correct order of hydration enthalpies of alkali metal ions is:
(A) $\text{L}{{\text{i}}^{\text{+}}}\text{ N}{{\text{a}}^{\text{+}}}\text{ }{{\text{K}}^{\text{+}}}\text{ R}{{\text{b}}^{\text{+}}}\text{ C}{{\text{s}}^{\text{+}}}$
(B) $\text{L}{{\text{i}}^{\text{+}}}\text{ N}{{\text{a}}^{\text{+}}}\text{ }{{\text{K}}^{\text{+}}}\text{ C}{{\text{s}}^{\text{+}}}\text{ R}{{\text{b}}^{\text{+}}}$
(C) $\text{N}{{\text{a}}^{\text{+}}}\text{ L}{{\text{i}}^{\text{+}}}\text{ }{{\text{K}}^{\text{+}}}\text{ R}{{\text{b}}^{\text{+}}}\text{ C}{{\text{s}}^{\text{+}}}$
(D) $\text{N}{{\text{a}}^{\text{+}}}\text{ L}{{\text{i}}^{\text{+}}}\text{ }{{\text{K}}^{\text{+}}}\text{ C}{{\text{s}}^{\text{+}}}\text{ R}{{\text{b}}^{\text{+}}}$

Answer 1

Hint: Recollect what happens when table salt is dissolved in water and the type of bonding involved in this process. Hydration enthalpy is the amount of energy released to form a hydrate. Try to associate this data with atomic size and their reactivity to get the answer.


Complete step by step solution:
- The process of addition of water molecules to an ion or atom or molecule is known as Hydration or Solvation.
- Hydration enthalpy is the amount of heat energy released by one mole of ions to form hydrated species.
- When a bond is formed, energy is released. So, when ions interact with water molecules to form bonds, energy is released. This energy is known as hydration enthalpy.
- For example, table salt dissolves in water due to the solvation effect. When NaCl is added to water, it dissociates as sodium ion, $N{{a}^{+}}$ and chloride ion, $C{{l}^{-}}$. These ions are then surrounded by water molecules and solvated which leads to dissolution of salt in water. When the ion gets solvated, at that instance, heat energy is released which is hydration energy.
- In the question, we have been asked to order hydration enthalpies for alkali metal ions. We know that Caesium has the highest atomic size and it is also the most electropositive element in the periodic table.
- Hydration enthalpy depends upon charge-to-size ratio. If the atomic size increases and charge is the same, then charge to size ratio will keep decreasing. Therefore, caesium ion will have the least charge-to-size ratio and lithium ion will have the most charge-to-size ratio.
- Hydration enthalpy is directly proportional to charge-to-size ratio.
- Therefore, order of hydration enthalpy will be: $\text{L}{{\text{i}}^{\text{+}}}\text{ N}{{\text{a}}^{\text{+}}}\text{ }{{\text{K}}^{\text{+}}}\text{ R}{{\text{b}}^{\text{+}}}\text{ C}{{\text{s}}^{\text{+}}}$


- Therefore, the correct answer is option (A).

Note: Remember hydration enthalpy is the energy released during interaction of ions and water so, it will have negative signs. Don’t get confused that only because it has negative signs, the order will be reversed. Ignore the negative sign while writing the order of hydration enthalpy. Remember hydration enthalpy is directly proportional to charge-to-size ratio.