Pblock Elementsii Result Question 4
4. Among the following reactions of hydrogen with halogens, the one that requires a catalyst is
(2019 Main, 10 Jan II)
(a) $\mathrm{H}_2+\mathrm{Cl}_2 \longrightarrow 2 \mathrm{HCl}$
(b) $\mathrm{H}_2+\mathrm{I}_2 \longrightarrow 2 \mathrm{HI}$
(c) $\mathrm{H}_2+\mathrm{F}_2 \longrightarrow 2 \mathrm{HF}$
(d) $\mathrm{H}_2+\mathrm{Br}_2 \longrightarrow 2 \mathrm{HBr}$
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Answer:
Correct Answer: 4. ( b )
Solution:
- Chemical reactivity of halogens decreases down the group. The chemical reactivity follows the order.
$\therefore \quad \mathrm{F}_2 > \mathrm{Cl}_2 > \mathrm{Br}_2 > \mathrm{I}_2$.
The highest reactivity of fluorine is attributed to two factors:
(i) The low dissociation energy of $\mathrm{F}-\mathrm{F}$ bond (which results in low attraction energy for the reaction).
(ii) Very strong bonds which are formed. Both properties arise from, small size of fluorine. $\mathrm{I}_2$ is being the least reactive halogen, it requires a catalyst for the reaction.
$\mathrm{H}_2+\mathrm{I}_2 \longrightarrow 2 \mathrm{HI}$
BETA
