23.5 - predictions from electrode potentials

• What can standard electrode potentials be used to predict?


  The feasibility of any potential redox reaction.
• What does the most negative redox system have a tendency to do?


  Lose electrons and be oxidised.

  
• What does the most positive redox system have a tendency to do?


  Gain electrons and be reduced.

  
• What must the redox system of an oxidising agent (reduced itself) have?


  A more positive standard electrode potential value than the redox system of the reducing agent.
• How do you write an overall redox equation?


  By combining the two half-equations.
• Limitations of predictions using standard electrode potential values


  - Reaction rates.

  - Concentration.
• Why is reaction rates a limitation when predicting feasibility using standard electrode potential values?


  As with predicting feasibility using Gibbs free energy, a limitation of standard electrode potential values lies with reactions that have a very large activation energy, resulting in a very slow rate. So electrode potentials may indiciate the thermodynamic feasibility of a reaction, but they give no indication of the rate of a reaction.
• Why is concentration a limitation when predicting feasibility using standard electrode potential values?


  Standard electrode potentials are measured using concentrations of 1 mol dm⁻³. Many reactions take place using concentrated or dilute solutions. If the concentration of a solution is not 1 mol dm⁻³, then the value of the electrode potential will be different from the standard value.
• Why is the use of standard conditions a limitation when predicting feasibility using standard electrode potential values?


  The actual conditions used for a reaction may be different from the standard conditions used to record standard electrode potentials. This will affect the value of the electrode potential.
• Why is the use aqueous reagents a limitation when predicting feasibility using standard electrode potential values?


  Standard electrode potentials apply to aqueous equilibria. Many reactions take place that are not aqueous.
• If the standard cell potential is positive, what type of value of Gibbs free energy will we get and how does this affect feasibility?


  A negative value of Gibbs free energy and thus the reaction is feasible.