Justify giving reactions that among halogens, fluorine is the best oxidant and among hydrohalic compounds, hydroiodic acid is the best reductant.

Explanation

Among halogens oxidation power decreases from fluorine to iodine due to decrease in electronegativities and electron gain enthalpies. This can be explaoned as follows.
Fluorine can displace `Cl_(2),Br_(2)andI_(2)` from the corresponding halides.
`2KCl+F_(2)to2KF+Cl_(2)`
`2KBr+F_(2)to2KF+Br_(2)`
`2KI+F_(2)to2KF+I_(2)`
Chlorine can displace `Br_(2)andI_(2)` from bromides and iodides respectively but cannot displace fluorine fluorides
`2KBr+Cl_(2)to2KCl+Br_(2)`
`2KI+Cl_(2)to2KCl+I_(2)`
Bromine can displace `I_(2)` from iodine but cannot displace `F_(2)` from fluorides or `Cl_(2)` from chlorides.
`2KI+Br_(2)to2KBr+I_(2)`
Iodine cannot displace any other halogen from their halides.
In the hydrogen halides the reduction power increases from HF to HI. This is because of the decrease in thermal stability of hydrogen halides with increase in bond length. Further the tendency to hold the electron decreases from HF to HI. So HF cannot be oxidised but HI can be easily oxidised. Hence HI is the best reductant.