Electrochem Eng L02-23 Pure redox reactions in Pourbaix diagram

Zhe Cheng (成哲)

4 Sept 202109:26

Summary

TLDRThe video explains two key pure redox reactions of iron, illustrated as horizontal lines on a Pourbaix diagram where pH has no effect. Line one represents the Fe²⁺/Fe(s) couple, showing how the electrode potential shifts with concentration and calculating a potential of –0.528 V versus SHE for a 1 mM Fe²⁺ solution. Line two represents the Fe³⁺/Fe²⁺ couple, with a standard potential of 0.770 V versus SHE, indicating stability regions for Fe²⁺ below this potential and Fe³⁺ above it. The video emphasizes using the Nernst equation to relate activity and potential, clarifying how these horizontal lines define the electrochemical stability of iron species.

Takeaways

🔋 Horizontal lines in a Pourbaix diagram represent pure redox reactions that do not involve protons (H+) or hydroxyl ions (OH−).
⚙️ Line 1 separates solid iron (Fe) from iron(II) ions (Fe2+) based on electrode potential.
🧪 The half-cell reaction for Line 1 is: Fe2+ + 2e− → Fe(s).
📘 The Nernst equation is used to calculate the electrode potential for the Fe2+/Fe redox couple.
📉 The standard electrode potential for the Fe2+/Fe reaction is −0.440 V versus the standard hydrogen electrode (SHE).
🧱 The activity of pure solid iron is assumed to be 1 because pure solids and liquids have unit activity.
➗ Since two electrons are transferred in the Fe2+/Fe reaction, the value of n in the Nernst equation is 2.
📊 For a Fe2+ concentration of 0.001 M, the activity is approximated as 0.001 because the solution is dilute.
🔻 Using the Nernst equation, the calculated electrode potential for the Fe2+/Fe system at 0.001 M is −0.528 V versus SHE.
📍 The calculated potential determines the exact position of Line 1 on the Pourbaix diagram.
🔄 Changing the concentration of Fe2+ would shift the horizontal redox line upward or downward on the diagram.
⚡ Line 2 represents the redox equilibrium between Fe3+ and Fe2+ ions.
🧪 The half-cell reaction for Line 2 is: Fe3+ + e− → Fe2+.
📘 The standard electrode potential for the Fe3+/Fe2+ couple is +0.770 V versus SHE.
➕ Only one electron is transferred in the Fe3+/Fe2+ reaction, so n = 1 in the Nernst equation.
⚖️ When both Fe2+ and Fe3+ activities are equal at 0.001, the logarithmic term in the Nernst equation becomes zero.
📈 The electrode potential for the Fe3+/Fe2+ couple remains +0.770 V under the given conditions.
🛡️ At potentials lower than +0.770 V, Fe2+ is the more stable species, while at higher potentials Fe3+ becomes more stable.
🌡️ All calculations in the transcript assume a temperature of 25°C.
🧭 Pourbaix diagrams help determine which iron species are thermodynamically stable under different pH and potential conditions.

Q & A

What does a horizontal line in a potential diagram represent in this transcript?
-A horizontal line represents a pure redox reaction that does not involve protons (H⁺) or hydroxyl ions (OH⁻). It shows the equilibrium potential between two oxidation states of an element.
What is the redox half-cell reaction for the Fe²⁺ / Fe solid couple described in line 1?
-The half-cell reaction is Fe²⁺ + 2e⁻ → Fe (solid).
What is the standard electrode potential (E⁰) for the Fe²⁺ / Fe solid reaction?
-The standard electrode potential is -0.440 V versus the standard hydrogen electrode (SHE).
How is the Nernst equation applied to calculate the electrode potential for Fe²⁺ / Fe solid?
-The Nernst equation is used as E = E⁰ - (0.0592/n) * log(activity of product / activity of reactant). For Fe²⁺ / Fe, this becomes E = -0.440 + 0.0296 * log(a_Fe²⁺), where n = 2 and the activity of solid Fe is 1.
How is the activity of ions approximated in dilute solutions?
-For dilute solutions, the activity of an ion can be approximated as the concentration divided by the standard concentration (1 M). For example, [Fe²⁺] = 0.001 M gives activity a = 0.001 (unitless).
What is the calculated electrode potential for Fe²⁺ / Fe solid at 0.001 M Fe²⁺?
-The electrode potential is approximately -0.528 V versus SHE.
What is the half-cell reaction for the Fe³⁺ / Fe²⁺ couple described in line 2?
-The half-cell reaction is Fe³⁺ + e⁻ → Fe²⁺.
What is the standard electrode potential (E⁰) for the Fe³⁺ / Fe²⁺ reaction?
-The standard electrode potential is 0.770 V versus SHE.
Why does the electrode potential for Fe³⁺ / Fe²⁺ remain 0.770 V even at low concentrations of 0.001 M?
-Because both Fe²⁺ and Fe³⁺ have the same low concentration, the ratio of activities in the Nernst equation is 1, and log(1) = 0, so the potential equals the standard potential.
How can the stability of Fe²⁺ and Fe³⁺ be predicted using electrode potential?
-For a given pH, if the electrode potential is below 0.770 V, Fe²⁺ is more stable. If the potential is above 0.770 V, Fe³⁺ is more stable.
Why is the activity of pure solids like Fe taken as 1 in these calculations?
-In thermodynamics and electrochemistry, the activity of a pure solid or pure liquid is defined as 1 because its chemical potential does not change with concentration.
How does changing the concentration of ions affect the position of horizontal lines in a potential diagram?
-Changing the concentration changes the activities in the Nernst equation, which shifts the electrode potential up or down. This moves the corresponding horizontal line in the diagram.