Predicting Feasibility of Redox Reactions (CIE A Level Chemistry)

• Transition elements can form ions with various oxidation states
• The change in their oxidation states involves the transfer of electrons
• Transition elements are often involved in redox reactions
• A redox reaction is a reaction in which one species is oxidised (loses electrons) and another is reduced (gains electrons)
• The standard electrode potentials (E^θ) of the two species can be used to predict the feasibility of redox reactions involving transition elements and their ions

Predicting the feasibility of redox reactions

• The standard electrode potential (E^θ) of a species gives an indication of how well it can be reduced
• In the exam, you will be provided with a half equation and the standard electrode potential (E^θ)
• The half equations are always written as a reduction equation
  • They are equilibrium reactions, as they demonstrate the equilibrium reached when the species in the equation gains electrons at the same rate as it loses electrons
• The more positive the standard electrode potential (E^θ) of a species is, the more readily that element will be reduced (gain electrons)
  • This is always when compared to the standard hydrogen electrode
  • The opposite is of course true; the more negative the standard electrode potential (E^θ) of a species is, the more readily that element will be oxidised (lose electrons)
• The feasibility of a reaction can be predicted using these values
• For example, the feasibility of Fe³⁺ being reduced to Fe²⁺ when reacted with Cu²⁺ can be predicted using their standard electrode potentials

Standard electrode potentials of Fe(III) & Cu(II) table

Half equation	Standard electrode potential, Eθ (V)
Fe3+ + e–  Fe2+	+0.77
Cu2+ + e–  Cu+	+0.15

• The table above shows that yes, the reaction is feasible and Fe³⁺ is more likely to get reduced to Fe²⁺
  • Fe³⁺ has a more positive standard electrode potential
  • Fe³⁺ will gain electrons more readily than Cu²⁺
  • Therefore, Fe³⁺ is the better oxidising agent
  • The reaction for this half equation will therefore proceed in the forward direction (reduction)

Fe³⁺ + e^–  Fe²⁺    E^θ = +0.77 V

• Since it is feasible that the Fe³⁺ will be reduced and this half equation will move in the forward direction, this means that the half equation for copper will move in the backward direction (oxidation)
  • Cu²⁺ equation has a less positive (or more negative) standard electrode potential
  • The Cu⁺ will therefore be oxidised to Cu²⁺
  • The reaction for this half equation will therefore be in the reverse direction

Cu²⁺ + e^–   Cu⁺    E^θ = +0.15 V

• Combining these two half-equations to get the overall equation gives (after cancelling the electrons on both sides):

Fe³⁺ + Cu⁺  Fe²⁺ + Cu²⁺

• The standard cell potential is:
  • E^θ = E^θ_reduction – E^θ_oxidation 
  • E^θ = (+0.77) – (+0.15) 
  • E^θ = +0.62 V

• The positive value of E_cell^θ (+0.62 V) suggests that the reaction is likely to proceed
• The changes in the transition element ions’ oxidation states are therefore feasible
• Standard electrode potentials (E^θ) are only predictions about the feasibility of a reaction; they do not guarantee that a reaction will definitely occur
  • For example, a reaction may be feasible according to these rules but have a very large activation energy barrier meaning that, in reality, it will not occur