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Arrange the following in the order of property indicated for each set: (i). `F_2,Cl_2,Br_2I_2-` increasing bond dissociation enthaply. (ii). `HF,HCl,HBr,HI`- increasing acid strength. (iii). `NH_3,PH_3,AsH_3,SbH_3,BiH_3-` increasing base strength. |
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Answer» (i). Bond dissociation entalpy: `I_(1)ltF_(2)ltBr_(2)ltCl_(2)` Down the group `(darr)` i.e., `F_2` to `Cl_2`, bond dissociation enthalpy should decrease with the increase in bond distance or length for `F_2` to `I_2`. Howerver, the bond dissociation enthalpy of `F_2` is smaller than that of `Cl_2` and even that of `Br_2`. This is dye to the fact that F atojm is very small and hence the three lone pairs of electrons on each F repel the bond pair holding the F-atoms in `F_2` molecule. (ii). Acid sthrength: `HFltHClltHBrltHI` The relative acid strength of hydraacids depends on their bond dissociation entalpies As the size of atom increases from F to I the bond strength decreases from `H-F` to `H-I` and hence bond dissociation enthalpies decreases from `H-F` to `H-I`. Hence the acid strength increases from `H-F` to `H-I`. (iii). Base strength: `BiH_3ltSbH_3ltAsH_3ltPH_3ltNH_3` Due to presence of lone pair of electrons on the central atom in `NH_3`,`PH_3`,`AsH_3`,`SbH_3` and `BiH_3`, all behave as lewis bases. Down the group, i.e., from `NH_3` to `BiH_3`, the size of the central atom increases hence lone pair occupies a larger bolume. Consequently the electron density on the central atom decreases and Lewis base strength decreases from `NH_3` to `BiH_3`. |
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