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Grade: 12


Why fluorine cannot be obtained by the electrolysis of HF though Bromine can be obtained by the electrolysis of HBr.

11 years ago

Answers : (1)

Ramesh V
70 Points

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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Naga Ramesh
IIT Kgp - 2005 batch

11 years ago
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