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Why fluorine cannot be obtained by the electrolysis of HF though Bromine can be obtained by the electrolysis of HBr.
The acidic strength of hydrogen halides varies in the order HF<HCl<HBr<HI. Because bond (H–X) dissociation enthalpy decrease in the order: H–F > H–Cl >H–Br > H–I.
HF is the weakest acid among hydrohalo-acids inspite of the fact that fluorine is most electronegative.
F2 cannot be formed due to its small size, highest electronegativity, low F-F bond dissociation enthalpy, and non availability of d orbitals in valence shell.
Since H-Br bond dissociation energy is less , and since Br2 has low electroonegativity and high Br-Br bond dissociation enthalpy, Br2 is obtained by electrolysis
HBr electrolysis:
At anode: 2HBr ---> Br2 +2H+ +2e-
At cathode: 2H+ +2e- -----> H2
But for HF, Fluorine formed at anode in electrolysis of HF, reacts violently with water forming hydrogen fluoride, and liberates oxygen which is highly charged with ozone. 2 F2 + 2 H2O ==> 4 HF + O2
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