Electrochem Eng L02-21 Hydrogen evolution line in Pourbaix diagram

Summary

The video explains the hydrogen evolution reaction in the context of a Pourbaix diagram for iron in water, with iron ions at 1.0 millimolar concentration. It details the electrochemical half-cell reaction where two protons combine with two electrons to form hydrogen gas, and derives the electrode potential using the Nernst equation. The calculation incorporates proton activity, the standard hydrogen electrode reference, and the pH of the solution. By breaking down each step, the video clarifies how the dashed line on the Pourbaix diagram represents the potential at which hydrogen evolution occurs, linking fundamental electrochemistry concepts to practical diagram interpretation.

Takeaways

• 😀 The video focuses on a Pourbaix diagram for iron in water, specifically for Fe³⁺ and Fe²⁺ ions at 1.0 mM concentration.
• 😀 The lower dashed line on the diagram represents the hydrogen evolution reaction (HER).
• 😀 The electrochemical half-reaction for hydrogen evolution is: 2H⁺ + 2e⁻ → H₂(g).
• 😀 The standard electrode potential for the hydrogen half-cell reaction is 0 V versus the standard hydrogen electrode (SHE).
• 😀 The Nernst equation is used to calculate the electrode potential for the hydrogen evolution reaction.
• 😀 The Nernst equation for HER accounts for the number of electrons transferred (n = 2) and the activities of protons and hydrogen gas.
• 😀 The activity of hydrogen gas is assumed to be 1 under 1 atm conditions for simplification.
• 😀 The logarithm of the reciprocal of proton activity is equivalent to the pH of the solution.
• 😀 Substituting pH into the Nernst equation gives the electrode potential for HER as E = -0.0592 × pH volts versus SHE.
• 😀 This negative slope line on the Pourbaix diagram defines the boundary below which hydrogen gas evolves and water is reduced.

Q & A

What is the main focus of the transcript provided?

The transcript focuses on explaining the hydrogen evolution reaction (HER) on a Pourbaix diagram for iron in water, and how the electrode potential for this reaction is calculated using the Nernst equation.

What is the electrochemical half-cell reaction for hydrogen evolution?

The electrochemical half-cell reaction for hydrogen evolution is: 2 H⁺ + 2 e⁻ → H₂.

Which equation is used to determine the electrode potential for the hydrogen evolution reaction?

The Nernst equation is used to determine the electrode potential: E = E⁰ - (0.0592/n) * log([products]/[reactants]).

What does 'n' represent in the Nernst equation for this reaction?

'n' represents the number of electrons transferred in the reaction. For the hydrogen evolution reaction, n = 2.

How is the activity of hydrogen gas accounted for in the calculation?

The activity of hydrogen gas is assumed to be 1 (corresponding to 1 atmosphere of ideal gas), simplifying the Nernst equation for the reaction.

How is the proton activity related to pH in this context?

Proton activity (aH⁺) is related to pH by the equation: pH = -log(aH⁺). Therefore, log(1/aH⁺) in the Nernst equation directly equals pH.

What is the standard electrode potential (E⁰) for the hydrogen evolution reaction versus the standard hydrogen electrode (SHE)?

The standard electrode potential E⁰ for the hydrogen evolution reaction versus SHE is 0 V.

How is the final electrode potential for hydrogen evolution expressed in terms of pH?

The electrode potential for hydrogen evolution is expressed as: E = -0.0592 × pH volts vs. SHE.

What does the dashed line on the Pourbaix diagram represent?

The dashed line represents the potential at which hydrogen evolution occurs. It shows the boundary below which hydrogen gas will form from protons.

Why is the stoichiometry factor of 2 important in the Nernst equation for this reaction?

The stoichiometry factor of 2 accounts for the two protons involved in the reaction. It affects the exponent in the Nernst equation and ensures the electrode potential is correctly calculated.