Vanadium Chemistry (Edexcel International A Level Chemistry)

• Vanadium is a transition metal which has variable oxidation states
• The table below shows the important ones you need to be aware of

• The variation in oxidation states of transition metal ions is illustrated by the reaction of zinc with ammonium vanadate(V) (also known as ammonium metavanadate) under acidic conditions
• Vanadium had four common oxidation states, from +2 to +5
• Zinc is a reducing agent that is capable to reducing vanadium(V) to vanadium(II) in a sequence of steps accompanied by vibrant colour changes

The reduction of vanadate(V) ions by zinc in acidic conditions is one of the most colourful reactions in chemistry

• For vanadium, we need to consider the following standard electrode potential values

• The half equations are arranged from the most negative E^θ at the top to the most positive E^θ at the bottom
  • The strongest oxidising agent is the reactant from the electrode reaction with the most positive standard electrode potential
    • In this case, the most positive value is + 1.00 V, which means that VO₂⁺ is the strongest oxidising agent
  • The strongest reducing agent is the product from the electrode reaction with the most negative standard electrode potential
    • In this case, the most negative value is – 1.18 V, which means that V is the strongest reducing agent 
• For all of the following reactions, we will use zinc as the reducing agent

Reduction of V from +5 to +4 by Zn

• Vanadium is being reduced from an oxidation number of +5 to +4 in half equation 5
• So, the two half equations we need to consider are:
  • 2. Zn²⁺ (aq) + 2e^– Zn (s)     E^θ = – 0.76 V
  • 5. VO₂⁺ (aq) + 2H⁺ (aq) + e^– VO²⁺ (aq) + H₂O (l)     E^θ = + 1.00 V
• Half equation 5 has the most positive standard electrode potential, E^θ, value
  • Therefore, this is the reduction reaction 
  • VO₂⁺ (aq) + 2H⁺ (aq) + e^– VO²⁺ (aq) + H₂O (l) 
• Half equation 2 has the least positive standard electrode potential, E^θ, value
  • Therefore, this is the oxidation reaction and needs to be reversed
  • Zn (s)  Zn²⁺ (aq) + 2e^– 
• We can obtain the overall equation by combining the reduction equation (half equation 5) and the oxidation equation (the reversed half equation 2)
  • Remember: When combining half equations, they must have the same number of electrons in both equations
    

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

  •  The equal number of electrons on both sides of the equation cancel out to give the overall equation: 

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

Reduction of V from +4 to +3 by Zn

• Vanadium is being reduced from an oxidation number of +4 to +3 in half equation 4
• So, the two half equations we need to consider are:
  • 2. Zn²⁺ (aq) + 2e^– Zn (s)     E^θ = – 0.76 V
  • 4. VO²⁺ (aq) + 2H⁺ (aq) + e^– V³⁺ (aq) + H₂O (l)     E^θ = + 0.34 V
• Half equation 4 has the most positive standard electrode potential, E^θ, value
  • Therefore, this is the reduction reaction 
  • VO²⁺ (aq) + 2H⁺ (aq) + e^– V³⁺ (aq) + H₂O (l) 
• Half equation 2 has the least positive standard electrode potential, E^θ, value
  • Therefore, this is the oxidation reaction and needs to be reversed
  • Zn (s)  Zn²⁺ (aq) + 2e^– 
• We can obtain the overall equation by combining the reduction equation (half equation 5) and the oxidation equation (the reversed half equation 2)
  • Remember: When combining half equations, they must have the same number of electrons in both equations
    

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

  •  The equal number of electrons on both sides of the equation cancel out to give the overall equation: 

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

Reduction of V from +3 to +2 by Zn

• Vanadium is being reduced from an oxidation number of +3 to +2 in half equation 3
• So, the two half equations we need to consider are:
  • 2. Zn²⁺ (aq) + 2e^– Zn (s)     E^θ = – 0.76 V
  • 3. V³⁺ (aq) + e^– V²⁺ (aq)      E^θ = – 0.26 V
• Half equation 3 has the most positive / least negative standard electrode potential, E^θ, value
  • Therefore, this is the reduction reaction 
  • V³⁺ (aq) + e^– V²⁺ (aq)
• Half equation 2 has the least positive standard electrode potential, E^θ, value
  • Therefore, this is the oxidation reaction and needs to be reversed
  • Zn (s)  Zn²⁺ (aq) + 2e^– 
• We can obtain the overall equation by combining the reduction equation (half equation 5) and the oxidation equation (the reversed half equation 2)
  • Remember: When combining half equations, they must have the same number of electrons in both equations
    

    2V3+ (aq) + 2e–		2V2+ (aq)
    Zn (s)		Zn2+ (aq) + 2e–

  • The equal number of electrons on both sides of the equation cancel out to give the overall equation: 

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

Reduction of V from +2 to 0 by Zn

• Vanadium is being reduced from an oxidation number of +3 to +2 in half equation 1
• So, the two half equations we need to consider are:
  • 2. Zn²⁺ (aq) + 2e^– Zn (s)     E^θ = – 0.76 V
  • 1. V²⁺ (aq) + 2e^– V (s)      E^θ = – 1.18 V
• Half equation 1 has the most negative standard electrode potential, E^θ, value
  • Therefore, this is the oxidation reaction and needs to be reversed 
  • V (s)  V²⁺ (aq) + 2e^– 
• Half equation 2 has the most positive / least negative standard electrode potential, E^θ, value
  • Therefore, this is the reduction reaction
  • Zn²⁺ (aq) + 2e^–  Zn (s) 
• We can obtain the overall equation by combining the reduction equation (half equation 5) and the oxidation equation (the reversed half equation 2)
  • When combining these half equations, there is no need to change them as they have the same number of electrons in both equations
    

    V (s)		V2+ (aq) + 2e–
    Zn2+ (aq) + 2e–		Zn (s)

  •  The equal number of electrons on both sides of the equation cancel out to give the overall equation: 

V (s) + Zn²⁺ (aq) V²⁺ (aq) + Zn (s) 

• • Zn is not electron releasing with respect to V²⁺
  • This means this reaction is not thermodynamically feasible

Predicting oxidation reactions

• The same method can be used to predict whether a given oxidising agent will oxidise a vanadium species to one with a higher oxidation number