Variable Oxidation States (Oxford AQA International A Level Chemistry)

Variable Oxidation States

• When transition elements forms ions they lose electrons from the 4s subshell first

• This is because when the orbitals are occupied, the repulsion between electrons pushes the 4s into a higher energy state so that it now becomes slightly higher in energy than the 3d subshell

  • The 4s is now the outer shell and loses electrons first

• The loss of the 4s electrons means that +2 is a common oxidation state in transition metals

• The reason why the transition metals have variable oxidation states all comes down to energy

Common oxidation states of transition elements

Ti and V successive ionisation energy graph

• We can see from the graph that the first few ionisation energies are relatively small and relatively close together

• This means that the energy difference associated with removing a small number of electrons enables transition metals to vary their oxidation state with ease

• The +2 and +3 oxidation states are commonly shown by the transition elements

  • The +3 oxidation state is more stable up to chromium

  • The +2 oxidation state is more stable in the later elements

• Transition metal ions with oxidation state +3 and above tend to be polarising and have a degree of covalent character in the bonds they form.

  • The ions have a high charge density and pull electrons towards themselves

• The maximum oxidation state possible corresponds the total number of electrons in the 4s and 3d which reaches a maximum at manganese

  • An example you may be familiar with is the manganate(VII) ion, MnO₄^- which is a powerful oxidising agent

Redox Potential & Oxidation State

• The redox potential for a transition metal ion changing from a higher to a lower oxidation state is influenced by:

  • pH

  • The ligand involved in the complex

Influence of pH

• When aqueous transition metal ions undergo a change in oxidation state, the reactions frequently involve hydrogen ions

• By analysing the reaction equations we can see the redox processes taking place

• The reduction of manganese in the manganate(VII) ion takes place readily in acidic solutions

MnO₄^- (aq) + 8H⁺ (aq) + 5e^- ⇌  Mn²⁺ (aq) + 4H₂O (l)      E^ꝋ = +1.51 V  

• The manganese changes oxidation state from +7 to +2

• The positive value of E^ꝋ indicates the equilibrium position is well over to the right

• An excess of acid drops the pH and provides the hydrogen ions to ensure this reaction usually goes to completion

• However, in a neutral solution the reduction is limited to the +4 oxidation state for the manganese

MnO₄^- (aq) + 2H₂O (l) + 3e^- ⇌  MnO₂ (s) + 4OH^- (aq)   E^ꝋ = +0.59 V  

• Notice that the value of E^ꝋ  has changed as a result of changing the pH of the solution

Changing the ligand

• Changing the ligand also influences the redox potential

• If you compare the hexaaquanickel(II) ion with the hexaamminenickel(II) ion, the E^ꝋ becomes more negative when ammonia ligands replace water ligands

[Ni(H₂O)₆]²⁺ (aq) + 2e^- ⇌ Ni(s) + 6H₂O (l)           E^ꝋ = -0.26

[Ni(NH₃)₆]²⁺ (aq) + 2e⁻  ⇌  Ni(s) + 6NH₃ (aq)    E^ꝋ = -0.49

• Ammonia is a stronger ligand than water meaning that it binds better to the nickel(II) ion

• The position of the second equilibrium is slightly more to the left than the first one (hence the more negative E^ꝋ value)

Oxidation States of Vanadium

• Vanadium is a transition metal which has variable oxidation states

Important vanadium oxidation states

• Vanadium also exists in a +5 oxidation state as the vanadate ion, VO₃^- , in compounds such as ammonium vanadate,NH₄VO₃

  • It is a reasonably strong oxidising agent

  • Addition of acid to solid ammonium vanadate will produce a 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

  •  The colour would successively change from yellow to blue to green to violet 

Reduction from +5 to +4

• Zn is the reducing agent:

Zn²⁺ (aq) + 2e^- → Zn (s)

• VO₂⁺ is the oxidising agent:

VO₂⁺ (aq) + 2H⁺ (aq) + e^- → VO²⁺ (aq) + H₂O (l)

• To obtain the overall equation:

  • Reverse the Zn²⁺ half equation

  • Double the VO₂⁺ half equation (to match the number of electrons)

  • Combine the half equations

2VO₂⁺ (aq) + 4H⁺ (aq) + Zn (s) → 2VO²⁺ (aq) + Zn²⁺ (aq) + 2H₂O (l)

• The colour change for this reduction is from yellow to blue

Reduction from +4 to +3

• Zn is the reducing agent:

Zn²⁺ (aq) + 2e^- → Zn (s)

• VO²⁺ is the oxidising agent:

VO²⁺ (aq) + 2H⁺ (aq) + e^- → V³⁺ (aq) + H₂O (l)

• To obtain the overall equation:

  • Reverse the Zn²⁺ half equation

  • Double the VO₂⁺ half equation (to match the number of electrons)

  • Combine the half equations

2VO²⁺ (aq) + 4H⁺ (aq) + Zn (s) → 2V³⁺ (aq) + Zn²⁺ (aq) + 2H₂O (l) 

• The colour change for this reduction is from blue to green

Reduction from +3 to +2

• Zn is the reducing agent:

Zn²⁺ (aq) + 2e^- → Zn (s)

• V³⁺ is the oxidising agent:

V³⁺ (aq) + e^- → V²⁺ (aq)

• To obtain the overall equation:

  • Reverse the Zn²⁺ half equation

  • Double the VO₂⁺ half equation (to match the number of electrons)

  • Combine the half equations

2V³⁺ (aq) + Zn (s) → 2V²⁺ (aq) + Zn²⁺ (aq)

• The colour change for this reduction is from green to violet

• The reduction of vanadium can be combined with redox potentials

Reduction of Tollens' Reagent

• Tollens' reagent contains the transition metal complex ion called diamminesilver(I), [Ag(NH₃)₂]⁺

• It is not stable in solution so it is prepared when needed by:

  • Adding sodium hydroxide solution to silver nitrate solution

  • Then add concentrated ammonia solution

• The reaction briefly produces a brown precipitate which quickly re-dissolves to form a colourless solution containing the diamminesilver(I) ion

• Tollens' reagent is used to distinguish between aldehydes and ketones

  • It can also be used to detect reducing sugars such as glucose

• A few drops of an aldehyde are warmed with Tollens' reagent in a water bath

Tollens' test

• Aldehydes and ketones have the same carbonyl, C=O, functional group

  • The carbonyl group is at the end of the chain for aldehydes and within the chain for ketones

• This difference means that aldehydes are reducing agents and can reduce the silver ion to silver metal

• The result is a striking silver mirror coating on the inside of the test tube, hence it is known as the 'silver mirror' test

• The equation for the reaction is

[Ag(NH₃)₂]⁺  + e^- → Ag (s) + 2NH₃ (aq)  

• Apart from its usefulness in chemistry, this reaction was used in the past to create the coating on mirrors with silver