8 Variable oxidation states

General trends •Relative stability of +2 state with respect to +3 state increases across the period •Compounds with high oxidation states tend to be oxidising agents e.g MnO4 – •Compounds with low oxidation states are often reducing agents e.g V2+ & Fe2+. Transition elements show variable oxidation states. When transition metals form ions they lose the 4s electrons before the 3d.The redox potential for a transition metal ion changing from a higher to a lower oxidation state is influenced by pH and by the ligand. Vanadium has four main oxidation states VO2+ Oxidation state +5 ( a yellow solution) VO2+ Oxidation state + 4 (a blue solution) V3+ Oxidation state + 3 (a green solution) V2+ Oxidation state + 2 (a violet solution) Vanadium The ion with the V at oxidation state +5 exists as a solid compound in the form of a VO3 – ion, usually as NH4VO3 (ammonium vanadate (V). It is a reasonably strong oxidising agent. Addition of acid to the solid will turn into the yellow solution containing the VO2+ ion. Addition of zinc to the vanadium (V) in acidic solution will reduce the vanadium down through each successive oxidation state, and the colour would successively change from yellow to blue to green to violet [Ag(NH3 )2 ]+ is used in Tollen’s reagent to distinguish between aldehydes and ketones . Aldehydes reduce the silver in the Tollen’s reagent to silver. Red ½ eq: [Ag(NH3 )2 ]+ + e-  Ag +2NH3 Ox ½ eq: CH3CHO + H2O  CH3CO2H + 2H+ + 2e