pBlock ElementsII 2 Question 4
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4. Among the following reactions of hydrogen with halogens, the one that requires a catalyst is (2019 Main, 10 Jan II)
======= ####4. Among the following reactions of hydrogen with halogens, the one that requires a catalyst is (2019 Main, 10 Jan II)
3e0f7ab6f6a50373c3f2dbda6ca2533482a77bed (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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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.
$$ \mathrm{H}{2}+\mathrm{I}{2} \longrightarrow 2 \mathrm{HI} $$
