First slide

Chemical properties

Question

Which of the following pairs shows highest bond dissociation enthalpy among halogens and lowest bond dissociation enthalpy among hydrogen halides?

Moderate

A

$F_{2}, HF$
B

${Cl}_{2}, HCl$
C

${Br}_{2}, HBr$
D

$I_{2}, HI$

Solution

NO ANSWER
In Hydrogen halides
Down the group affinity of hydrogen towards the halogen atom decreases as the size of the halogen atom increases
$∴$ Thermal stability & BDE of $HF > HCl > HBr > HI$
In halogens
Down the group as the size of halogen increases X – X bond enthalpy must decrease.
$∴$ Expected trend
$F_{2} > {Cl}_{2} > {Br}_{2} > I_{2}$
But due to very small size of fluorine atoms, $: F_{\cdot \cdot}^{\cdot \cdot} - F_{\cdot \cdot}^{\cdot \cdot} :$ the repulsions between the lone pairs of adjacent atoms is very high.
$∴$ F – F bond enthalpy is abnormally low.
Experimental Trend ${Cl}_{2} > {Br}_{2} > F_{2} > I_{2}$

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