Explain why and how fluorine differs from rest of family members?
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Fluorine differs from rest of the elements of its family due to (i) its small size (ii) highest electronegativity, (iii) low bond dissociation energy and (iv) absence of d-orbitals in the valence shell. The main points of difference are :
(1) Fluorine is most reactive of all the halogens due to lower value of F — F bond dissociation energy (F2 = 158, Cl2 = 243, bromine = 192 and iodine = 151 kJ mol–1) .
(2) Being the most electronegative element, it shows only an oxidation state of –1 and does not show positive oxidation states due to absence of d-orbitals in its valence shell. Other halogens show positive oxidation states of +1, +3, +5 and +7.
(3) Due to small atomic size and high electronegativity of H, HF undergoes strong H-bonding while other halogen acids do not. As a result,
(i) HF is a liquid (boiling point 292.5K), while other halogen acids are gases at room temperature (boiling point of HCl = 189 K, HBr = 206 K, HI = 238 K).
(ii) HF is weakest of all the halogen acids due to high strength of H — F bond.
(iii) Due to H-bonding, HF can form acid salts of the type KHF2, i.e., K+[H – F ... F–] while HCl, HBr and HI do not form such salts (i.e., no KHCl, KHBr2 and KHl2 are known).
(4) Fluorides have the maximum ionic character. For example AlF3 is ionic while other halides of are covalent.
(5) Of all the halogens, fluorine has the highest positive electrode potential (F2 = 2.87, Cl2 = 1.36, Br2 = 1.09 and 12 = 0.53 volt) i.e., it is most easily reduced and hence acts as the strongest oxidising agent. It brings about the highest oxidation of other elements with which it combines. For example with S, it gives SF6, with I2 it gives IF1. Other halogens do not always bring about the highest oxidation state. For example, with sulphur gives Cl2 gives SCl4, Br2 while SBr2 does I2 not react at all. F2 is so powerful oxidising agent that it can even oxidise inert-gases.
(6) HF cannot be stored in glass bottles since it reacts with silicates to form fluorosilicates.
Na2SiO3 + 6 HF ———> Na2SiF6 + 3 H2O
While other halogen acids (HCl, HBr and HI) do not react with silicates and hence can be stored in glass bottles.
(7) AgF is soluble in H2O while all other silver halides i.e., AgCl, AgBr and Agl are insoluble in water. In constant, CaF2 is insoluble while other calcium halides i.e., CaCl2, CaBr2, Cal2 are soluble inH2O.
(8) Due to absence of -orbitals, fluorine, does not form polyhalide ions while other halogens form polyhalides of the type I3–, Br3–, I5– etc
(1) Fluorine is most reactive of all the halogens due to lower value of F — F bond dissociation energy (F2 = 158, Cl2 = 243, bromine = 192 and iodine = 151 kJ mol–1) .
(2) Being the most electronegative element, it shows only an oxidation state of –1 and does not show positive oxidation states due to absence of d-orbitals in its valence shell. Other halogens show positive oxidation states of +1, +3, +5 and +7.
(3) Due to small atomic size and high electronegativity of H, HF undergoes strong H-bonding while other halogen acids do not. As a result,
(i) HF is a liquid (boiling point 292.5K), while other halogen acids are gases at room temperature (boiling point of HCl = 189 K, HBr = 206 K, HI = 238 K).
(ii) HF is weakest of all the halogen acids due to high strength of H — F bond.
(iii) Due to H-bonding, HF can form acid salts of the type KHF2, i.e., K+[H – F ... F–] while HCl, HBr and HI do not form such salts (i.e., no KHCl, KHBr2 and KHl2 are known).
(4) Fluorides have the maximum ionic character. For example AlF3 is ionic while other halides of are covalent.
(5) Of all the halogens, fluorine has the highest positive electrode potential (F2 = 2.87, Cl2 = 1.36, Br2 = 1.09 and 12 = 0.53 volt) i.e., it is most easily reduced and hence acts as the strongest oxidising agent. It brings about the highest oxidation of other elements with which it combines. For example with S, it gives SF6, with I2 it gives IF1. Other halogens do not always bring about the highest oxidation state. For example, with sulphur gives Cl2 gives SCl4, Br2 while SBr2 does I2 not react at all. F2 is so powerful oxidising agent that it can even oxidise inert-gases.
(6) HF cannot be stored in glass bottles since it reacts with silicates to form fluorosilicates.
Na2SiO3 + 6 HF ———> Na2SiF6 + 3 H2O
While other halogen acids (HCl, HBr and HI) do not react with silicates and hence can be stored in glass bottles.
(7) AgF is soluble in H2O while all other silver halides i.e., AgCl, AgBr and Agl are insoluble in water. In constant, CaF2 is insoluble while other calcium halides i.e., CaCl2, CaBr2, Cal2 are soluble inH2O.
(8) Due to absence of -orbitals, fluorine, does not form polyhalide ions while other halogens form polyhalides of the type I3–, Br3–, I5– etc
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