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

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F2,HF

Cl2,HCl

Br2,HBr

I2,HI

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Detailed Solution

NO ANSWER In Hydrogen halidesDown 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>HIIn halogensDown the group as the size of halogen increases X – X bond enthalpy must decrease.∴ Expected trend F2 > Cl2 > Br2 > I2But due to very small size of fluorine atoms, :F· ·· · − F· ·· ·: the repulsions between the lone pairs of adjacent atoms is very high. ∴ F – F bond enthalpy is abnormally low. Experimental Trend Cl2 > Br2 > F2 > I2

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