Variable oxidation states

• Transition metals can form a number of stable ions each with metal in a different oxidation state. This is because the 4s and 3d sub levels are very close in energy so easier to remove electrons
• Vanadium ions

  • +5 oxidation state would be the V(O2)+ ion with its colour being yellow
  • +4 oxidation state would be the (VO)2+ ion with its colour being blue
  • +3 oxidation state would be the V3+ ion with its colour being green
  • +2 oxidation state would be the V2+ ion with its colour being violet
• In presence of H2SO4 acid (where the H+ comes from), when Zn is added to vanadium ions its reduced.
• Reaction of Vanadium ions with Zinc

  First,

  2V(O2)+ (aq) + Zn(s) + 4H+(aq) --> 2(VO)2+ (aq) + Zn2+ (aq) + 2H2O(l)

  • Solution turns from yellow to blue and Vanadium is reduced from +5 to +4
• Reaction of Vanadium ions with Zinc

  Second reaction,

  2(VO)2+ (aq) + Zn(s) + 4H+ (aq) ---> 2(V) 3+(aq) + Zn2+ (aq) + 2H2O (l)

  • The solution changes colour from blue to green
• Reaction of Vanadium ions with Zinc

  Third reaction,

  2(V)3+ (aq) + Zn(s) ---> 2(V)2+(aq) + Zn2+

  • Solution changes from green to violet
• At lower oxidation states (+3 ,+2) , transition metals form ionic bonds. At higher oxidation states, transition metals can't form compounds or molecular ions. E.g. manganese forms ionic bonds in manganese (||) chloride but only forms covalent bonds in Mn(O4)- oxoanion
• Transition metals are often in their highest oxidation state when bonded to very electronegative elements like oxygen and fluorine
• The more positive the electrode potential, the less stable the ion and so the more likely it is to be reduced (gain more electrons) .
• Electrode potential is measured in standard conditions and for aqueous solutions therefore, different ligands could make the redox potential larger or smaller depending on how well they bind to the metal ion
• pH can also affect electrode potential

  • Some ions need H+ to be present in order to be reduced
  • Others release OH- ions into the solution when they are reduced
  • In general, redox potentials will be more positive in acidic conditions so more likely to be reduced
• Tollens's reagent

  • Silver would be reduced to Ag
  • You prepare it by adding enough NH3 to AgNO3 to form colourless [Ag(NH3)2]+

  Equation:

  RCHO + 2[Ag(NH3)2]+ + 3OH- ---> RCOO- + 2Ag + 4NH3 +2H2O
• • In a titration, the concentration of a solution is determined by titrating with a known concentration.
  • In redox titrations, an oxidising agent is titrated against a reducing agent.
  • Indicators would normally be used but most transition metal ions naturally change colour when changing its oxidation state
• Method of titrating

  1. Reducing agent (either (C2O4)2- or Fe2+ ) is put into the conical flask with a pipette
  2. Then an excess of dilute H2SO4 is added so there is enough H+ ions to allow the oxidising agent to be reduced
  3. Oxidising agent KMnO4 is added to the burette (its colour is deep purple.
  4. KMnO4 is added into the conical flask until the colour changes from colourless to pink
• Titration equation of Manganate(V||)

  Titration of Fe2+ with (MnO4)-

  • (MnO4)- (aq) + 8H+ + 5 Fe2+ ---> Mn2+ (aq) + 5 Fe3+ (aq) + 4H2O(l)

  Titration of (C2O4)2- with (MnO4)-

  • 2(MnO4)- (aq) + 16H+ 5(C2O4)2- (aq) --> 2Mn2+ (aq) + 8H2O (l) + 10CO2(g)
• Variable oxidation states refer to the ability of an element to form ions with different charges by losing or gaining electrons
• To remember the colours of colour change when Vanadium reacts with Zn remember the acronym You Better Get Vanadium so the colours in order for +5 to +2 is Yellow, Blue, Green and Violet