Question

Explain the following with proper reason :
(i)Fluorine cannot be prepared from fluorides by chemical oxidation.
(ii)Anhydrous HCl is a bad conductor of electricity while aqueous HCl is a good conductor.
(iii)Halogens are strong oxidising agents.
(iv)Bleaching of flowers by chlorine is permanent while after bleaching with `SO_(2)`, the colour returns.
(v)Iodine dissolves more in KI solution than in water.
(vi)`KHF_(2)` is well known whereas `KHCI_(2)` or `KHBr_(2)` does not exist.
(vii)Ferric iodide is very unstable but ferric chloride is stable.
(viii)Fluorine does not form `F_(3)^(-)` (polyhalide) ion.
(ix)HF is not stored in glass bottles but kept in wax lined bottles.
(x)HF has a greater electronegativity difference and more ionic character than HCl, HBr and Hl but it is the weakest acid.

Answer 1

Correct Answer - (i)The standard reduction potential of fluorine is maximum.
`:ddotunderset(..)F.+e^(-)to:ddotunderset(..)F.^(-)`
Thus, it cannot be oxidised by any other reagent. `F^(-)` ion is very stable due to small size and high electronegativity of fluorine atom.
(ii)In anhydrous state, `HCI` is a covalent molecule. In aqueous solution, `HCI` combines with water molecule to form `H_(3)O^(+)` and `CI^(-)` ions.
`HCl (g)+H_(2)O(l)toH_(3)O^(+) (aq)+Cl^(-)(aq)`
(iii)Halogens act as a strong oxidising agents because they have high tendency to accept electron, i.e., they have high electron affinity values.Their reduction potentials are high (positive) and decrease from `F_(2)` to `I_(2)`.Thus, oxidising nature decreases from `F_(2)` to `I_(2)`.
(iv)Chlorine bleaching action is due to oxidation while that of sulphur dioxide is due to reduction.Hence, the substance bleached by `SO_(2)` is reoxidised by the oxygen of the air to its original state. `Cl_(2)` bleaches coloured material by oxidation and thus bleaching is permanent.
`CI_(2) + H_(2)Oto2HCI + [O]`
Coloured material +`[O] to` colourless
on the other hand beaching by `SO_(2)` is by reduction and thus temporary because colourless articles are further oxidised by air.
`SO_(2)+2H_(2)Oto2H_(2)SO_(4) + 2[H]`
Coloured material + `[H] to` Colourless `overset([O])to` coloured
(v)`I_(2)` is a covalent molecule.Thus, its solubility is less in polar solvent, i.e., water.Potassium iodide combines with iodine and forms a polyhalide with an ionic compound.Being ionic, `KI_(3)` is more soluble.
`KI+I_(2)toKI_(3)(K^(+)I_(3)^(-))`
(vi)Hdrofluoric acid exists as dimeric molecule `(H_(2)F_(2))` due to hydrogen bonding.It, thus, exhibits dibasic nature and forms two series of salts, `KHF_(2)` as `[K^(+)` and `F^(-)---H--F]` and `KF[K^(+)` and `F^(-)]`.`HCl` and `HBr` exists as monomeric molecules as hydrogen bonding is not present.
(vii)`I^(-)` ions is a strong reducing agent.Iodide thus reduces ferric into ferrous, `Cl^(-)` ion is a weak reducing agent and does not reduce `Fe^(3+)` to `Fe^(2+)`.
(viii)`F`-does not have empty `d`-orbital liked other halogen. The formation of `X_(3)^(-)` ions involves `sp^(3)d`-hydridization.
(ix)`HF` attacks glass bottles. The sodium and potassium silicates are converted into flurosilicates
`Na_(2)SiO_(3)+6HFtoNa_(2) SiF_(6) + 3H_(2)O`
(x)The weakest acidic nature of `HF` is due to following three factors:
(i)Strong `H-F` bond as the dissociation energy is high.
(ii)Large heat of dehydration due to hydrogen bonding.
(iii)Low value of electron affinity.