Question

The reaction of which halogens with water is endothermic:
(A) ${F_2}$
(B) $B{r_2}$
(C) $C{l_2}$
(D) ${I_2}$

Answer 1

In general halogens occur in the diatomic state due to the van Waals force of attraction. To answer the above question we need to know which diatomic gas has energy in positive that is the sum of bond dissociation energy and hydration energy of the gas.

Complete step by step answer:
To answer the question we need to know two terms that are hydration energy and bond dissociation energy;
Bond dissociation energy: Bond dissociation energy is understood as the energy required to break a particular bond between diatomic molecules. If a compound has greater bond dissociation energy it means it has a strong diatomic bonding. . If a compound has lesser bond dissociation energy means it has a weak diatomic bonding. That is stronger the bond, more stable is the compound, weaker the bond less stable is the compound.
Hydration energy: The amount of energy released when one mole of ions dissolve in water, it can be also stated as solvation energy that is obtained by adding salt (solute) into the water (solvent) makes a solution (salt consists of anion and cation).
Halogens react with water to produce hydrogen halide $(HX)$ and a weak acid $(HOX)$ . This acid contains bleaching properties and is also an oxidizing agent. Now in the question, we are asked which halogen gas on dissolving with water gives an endothermic reaction that is the product formed absorbs energy from the surrounding.
Now in the series of halogen on going down the group the energy of dissociation becomes less negative and hence the reaction of iodine with water is an endothermic reaction.

Note:
Ongoing down the group the size of the halogen atoms decreases, therefore the first halogen that is fluorine has the smallest size and highest dissociation energy, the last halogen that is iodine has the largest size and lowest dissociation energy.