Question

The purpose of addition of KF to HF in the preparation of ${{\text{F}}_{\text{2}}}$ by electrolysis is:
(A) To increase the conductance of HF
(B) To decrease the oxidation potential of HF
(C) To increase the F concentration
(D) To decrease the KF concentration

Answer 1

Oxidation Potential is the energy change, measured in volts, required to add or remove electrons to or from an element or compound. Obtaining ${{\text{F}}_{\text{2}}}$ from HF by electrolysis is not easy so KF is added.

Complete Step By Step Solution
The acidic strength of hydrogen halides varies in the order $\text{HF}<\text{HCl}<\text{HBr}<\text{HI}$ .Because bond $\left( \text{H}-\text{X} \right)$ dissociation enthalpy decreases in the order $\text{ H}-\text{F}>\text{H}-\text{Cl}>\text{H}-\text{Br}>\text{ H}-\text{I}$ . HF is the weakest acid among hydro halo-acids in spite of the fact that fluorine is most electronegative. ${{\text{F}}_{2}}$ cannot be formed due to its small size, highest electronegativity, low $\text{F}-\text{F}$ bond dissociation enthalpy and non- availability of d orbitals in valence shell. Fluorine formed at anode in electrolysis of HF, reacts violently with water forming hydrogen fluoride, and liberates oxygen.
So, to prepare ${{\text{F}}_{2}}$ by electrolysis KF is added to HF which decreases its oxidation potential and the preparation becomes easy.
Therefore, the correct option is (B).

Additional Information
Electrolysis: It is defined as a process of decomposing ionic compounds into their elements by passing a direct electric current through the compound in a fluid form. The cations are reduced at cathode and anions are oxidized at the anode.
HF: Hydrogen fluoride is a chemical compound with the chemical formula HF. This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. Hydrogen fluoride is a highly dangerous gas, forming corrosive and penetrating hydrofluoric acid upon contact with moisture. The gas can also cause blindness by rapid destruction of the corneas. Hydrogen fluoride does not boil until $20{}^\circ \text{C}$ in contrast to the heavier hydrogen halides, which boil between $-85~{}^\circ \text{C }\left( -120~{}^\circ \text{F} \right)\text{ }$ and $-35{}^\circ \text{C}\left( -30{}^\circ \text{F} \right)$ .

Note
Study the concepts of Electrolysis, dissociation into ions, Oxidation potential. Read about HF, its bond dissociation energy and preparation of fluoride ions from it, properties of KF, its influencing factors on HF which leads to easy formation of fluoride ions.