Question

Chlorine, Bromine and Iodine all react with hydrogen.
$(i)$ State the trend in the reactivities of the halogens with hydrogen.
$(ii)$ Explain the trend in terms of bond energies?

Answer 1

The element like chlorine, bromine and iodine reacts with hydrogen gas to form their respective halides. All these elements present in dimer form in nature. Thus the element which has more bond dissociation energy will be less reactive towards hydrogen gas. The bond dissociation energy depends upon the bond length and attraction of the nucleus with its valence electrons.

Complete Step By Step Answer:
$(i)$ In group seventeen as we go down the group the tendency of elements to react with hydrogen gas decreases accordingly. This is because of the increased size of elements while moving down the group. Thus we can write the order of reactivity of chlorine, bromine and iodine as:
$Cl > Br > I$ Decreasing order of reactivity.
Since they form halides with hydrogen as:
${H_2}{\text{ + }}C{l_2}{\text{ }} \to {\text{ }}2HCl$
${H_2}{\text{ + B}}{{\text{r}}_2}{\text{ }} \to {\text{ }}2HBr$
${H_2}{\text{ + }}{{\text{I}}_2}{\text{ }} \to {\text{ }}2HI$
Since chlorine is small in size therefore $C{l^{ - 1}}$ will be more stable as the force of attraction between the nucleus and its valence electron is greater as compared to other. Therefore chloride ions will be most stable and thus its reactivity towards hydrogen will be greater than others.
$(ii)$ When we go down the group the bond dissociation enthalpy decreases as well since the bond length increases due to increase in size of atom. Hence its halide will be less stable. Thus on moving down due to increase in size the stability of halides decreases.

Note:
As we go down the group the size of the element increases but the size of hydrogen remains constant. The size of hydrogen is small, having only one shell, therefore it forms weak halides with bromine and iodine as compared to chlorine. Also the bond dissociation energy decreases down the group as the distance of electrons from the nucleus increases which reduces its attraction.