2.5.5 Variable oxidation states

• Transition elements show variable oxidation states. Vanadium species in oxidation states IV, III and II are formed by the reduction of vanadate(V) ions by zinc in acidic solution.
• 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.
• • (VO2)+ Oxidation state +5 ( a yellow solution)
  • VO^2+ Oxidation state + 4 (a blue solution)
  • V3+ Oxidation state + 3 (a green solution)
  • V2+ Oxidation state + 2 (a violet solution)
• • [Ag(NH3 )2 ]+ is used in Tollens’ reagent to distinguish between aldehydes and ketones .
  • Aldehydes reduce the silver in the Tollens’ reagent to silver. Reduction half eq: [Ag(NH3 )2 ]+ + e- -> Ag +2NH3
  • Oxidation half eq: CH3CHO + H2O -> CH3CO2H + 2H+ + 2e
• Suggest one property of an impurity that would cause calculated mass of FeSO4¬7H2O in X to be greater than the actual mass of X. Explain your answer [2 marks]

  • Impurity is a reducing agent / reacts with dichromate / impurity is a version of FeSO4 with fewer than 7 waters not fully hydrated
  • Accept - a compound that converts Fe3+ into Fe2+
  • Such that for a given mass, the impurity would react with more dichromate than a similar mass of FeSO4.7H2O
  • OR for equal masses of the impurity and FeSO4.7H2O, the impurity would react with more dichromate.
  • Must compare mass of impurity with mass of FeSO4.7H2O