WHY Eu2+ ,Yb2+, are strong reducing agent whereas Tb4+ is an oxidising agent?
Most stable oxidation state of lanthanides is +3 . But some of the lanthanides such as Eu ,Yb and Tb exist in +2 and +4 oxidation state.
Thus to revert to thye stable +3 oxidation state.Thus Eu2+ gets oxidised to Eu+3 oxidation state and Yb2+ gets oxidised to Yb+3 oxidation state and thus act as reducing agent.
Similarly Tb+4 gets reduced to Tb+3 in aqueous solution and thus acts as a good oxidising agent.
Thus to revert to thye stable +3 oxidation state.Thus Eu2+ gets oxidised to Eu+3 oxidation state and Yb2+ gets oxidised to Yb+3 oxidation state and thus act as reducing agent.
Similarly Tb+4 gets reduced to Tb+3 in aqueous solution and thus acts as a good oxidising agent.