Understanding Redox Reactions from Electrode Potentials

1. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Specification • Key terms • Standard electrode potential • Cell potential

2. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Specification • Key terms • Standard electrode potential • Cell potential

3. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Specification • Key terms • Standard electrode potential • Cell potential

4. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Specification • Key terms • Standard electrode potential • Cell potential

5. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Starter • Key terms • Standard electrode potential • Cell potential

6. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Predictions Predicting redox reactions: Predictions can be made about the feasibility of any potential redox reactions using standard electrode potentials. Table 1 shows three redox systems sorted with the most negative standard electrode potential at the top. • Key terms • Standard electrode potential • Cell potential • The most negative system has the greatesttendency to be oxidised and loseelectrons. • The most positive system has the opposite

7. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Predictions Predicting redox reactions: We can predict the feasibility of a redox reaction from Eθ values. In table 1 a reaction should take place between an oxidising agent (species being reduced) and a reducing agent (species being oxidised) provided that the redox system of the oxidising agent has a morepositive E θ value than the redox system of the reducing agent. • Key terms • Standard electrode potential • Cell potential

8. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Predictions Comparing redox reactions: Redox system C has a more positive E θ value and will have a greater tendency to be reduced than redox system A and B. The oxidising agent Ag+ (aq) on the left of C should react with reducing agents on the right in redox systems A and B (with Cr(s) and Cu(s). • Key terms • Standard electrode potential • Cell potential

9. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Predictions Comparing redox reactions: Redox system B has a more positive Eθ value and will have a greater tendency to be reduced than redox system A. The oxidising agent Cu2+ (aq) on the left of B should react with reducing agents on the right in redox systems A (with Cr(s)). • Key terms • Standard electrode potential • Cell potential The oxidising agent Cr3+ (aq) on the left of A will not react as there are no redox systems with a less positive Eθ value.

10. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Predictions • Overall equation: • Combining the possible redox reactions above gives the following equations: • Cr(s) + 3Ag + (aq)  Cr3+ (aq) + 3Ag(s) • Cu(s) + 2Ag + (aq)  Cu2+ (aq) + 2Ag • 2Cr(s) + 3Cu2+ (aq)  2Cr3+ (aq) + 3Cu (s) • You can apply this same principle to any pair of redox systems. • Remember: • The redox system with the more positive Eθ value will gain electrons. • The redox system with the less positive Eθ value will lose electrons. • Key terms • Standard electrode potential • Cell potential

11. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Limitations Limitations of predictions using Eθvalues: Reaction rate: If there is a very large activation energy, it will result in a very slow rate. Electrode potentials may indicate the thermodynamic feasibility of a reaction but they give no indication of the rate of a reaction. • Key terms • Standard electrode potential • Cell potential

12. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Limitations Limitations of predictions using Eθvalues: Concentration: Standard electrode potentials are measured using concentrations of 1 moldm-3. If the concentration of a solution is not 1 moldm-3 then the value of the electrode potential will be different from the standard value. Eg. The redox equilibrium and standard electrode potential for the Zn2+ l Zn redox system is shown below: Zn2+ (aq) + 2e-⇌Zn(s) Eθ = -0.76V If the concentration is greater than 1 moldm-3the equilibrium will shift to the right, removing electrons from the system and making the electrode potential less negative. (If the concentration is less than 1 moldm-3then the opposite will happen). • Key terms • Standard electrode potential • Cell potential

13. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Limitations Limitations of predictions using Eθvalues: Other factors: The actual conditions used for the reaction may be different from the standard conditions used to record Eθ values. Standard electrode potentials apply to aqueous equilibria but many reactions take place that are not aqueous. • Key terms • Standard electrode potential • Cell potential

14. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Limitations Limitations of predictions using Eθvalues: • Key terms • Standard electrode potential • Cell potential

15. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Plenary • Key terms • Standard electrode potential • Cell potential

16. Module 5: Predictions from electrode potentials L.O. to demonstrate knowledge, understanding and application of feasilbility of reactions Plenary • Key terms • Standard electrode potential • Cell potential