Voltaic Cell: Standard Hydrogen Electrode Demonstration
A H2(g) tank serves as the source of H2(g) being bubbled into the Standard Hydrogen Electrode (SHE) in 1.00 M HCl(aq) at 25°C ,1.00 atm pressure, near sea level. The reference half-cell is the reduction of hydrogen ions to hydrogen gas
2H+(aq) +2e- -> H2(g, 1 atm) E° = 0.0 V
This occurs when the SHE is acting as a cathode, when it is attached to a more active metal such as zinc. When the SHE is attached to a less active metal such as copper, the SHE acts as an anode and the oxidation half-reaction is
H2(g, 1 atm) -> 2H+(aq) +2e- E° = 0.0 V
A Standard Hydrogen Electrode (SHE) is used to show the E°cell generated in order to determine the standard reduction potentials of two half-reactions:
Zn2++ 2e- -> Zn E° = -0.76 V for Zn|Zn2+||SHE , the zinc electrode is the site of oxidation (anode)
E°cell = E°reduction (Cathode) - E° reduction (Anode)
+0.76 V = 0.0 V - E° reduction (Anode)
E° reduction (Anode) = -0.76 V
Cu2++ 2e- -> Cu E° = +0.34 V for SHE||Cu2+|Cu electrochemical cells. the copper electrode is the site of reduction (cathode)
E°cell = E°reduction (Cathode) - E° reduction (Anode)
+0.34 V = E°reduction (Cathode) - 0.0 V
+0.34 V = E°reduction (Cathode)
After the individual half-cell emfs are determined, combine the half-cells to give the E° cell for Zn + Cu2+(aq) -> Zn2+(aq) + Cu
Cu2++ 2e- -> Cu E° = +0.34 V
2H+(aq) +2e- -> H2(g, 1 atm) E° = 0.0 V
Zn2++ 2e- -> Zn E° = -0.76 V
E°cell = E°reduction (Cathode) - E° reduction (Anode)
E°cell = +0.34 V - (-0.76 V) = +1.10 V
A Standard Hydrogen Electrode is used to show the E°cell generated: SHE||Cu2+|Cu and Zn|Zn2+||SHE. The effectiveness of this demonstration can be increased by showing computer animations representing what occurs at the anode and cathode of each cell at the particle or atom level.
Computer animations for the SHE electrode (drafts):
Zn|Zn2+||SHE electrochemical cells oxidation half-reaction at the zinc electrode https://vimeo.com/220550690 Zn -> Zn2++ 2e-
reduction half-reaction at SHE https://vimeo.com/220550539 2H+(aq) +2e- -> H2(g, 1 atm) E° = 0.0 V
SHE||Cu2+|Cu electrochemical cell oxidation half-reaction at SHE https://vimeo.com/220550498 H2(g, 1 atm) -> 2H+(aq) +2e- E° = 0.0 V
reduction half-reaction at the copper electrode https://vimeo.com/220550267 Cu2++ 2e- -> Cu E° = +0.34 V
including the computer simulations & animations provides an excellent opportunity for students to connect the macroscopic, microscopic (particle/atom) and symbolic levels of representation on Johnstone's Triangle.
Zinc - standard hydrogen electrode electrochemical cell computer animation
https://www.youtube.com/watch?v=CQ7gYg64Msc
Active Learning A POGIl Activity can accompany this demonstration to involve students in active learning. Rather than lecturing to students, have the students use the SHE Metal electrode block diagrams to take notes on what they observe from the demonstration and from the computer animation of what occurs at the surface of the SHE electrode and the metal electrode for the ZnShe cell and the SHECu cell. Have students identify where oxidation and reduction occurs and have the students write the half-reactions. Clicker questions are available to support and enhance the demonstration and the computer simulations & animations.
Learning Objectives
1. Describe and identify the components of a Standard Hydrogen Electrode.
2. Write the half-reactions that occurs in a Standard Hydrogen Electrode when it is acting as a cathode. Write the half-reactions that occurs in a Standard Hydrogen Electrode when it is acting as an anode.
3. Explain how a Standard Hydrogen Electrode is used as a reference electrode (E° = 0.0 V) to measure E° half-cell values.
4. Show how a Standard Hydrogen Electrode can be used to rank half-cell reactions and determine an emf series.
5. Show how E° half-cells are combined to give the E° cell.
6. Show that ∆E° is the difference in emf between two half-cell potentials. Use a number line to emphasize this point.
References
1. Greenbowe, T.J. An Interactive Multimedia Software Program for Exploring Electrochemical Cells. J. Chem. Educ., 1994, 71 (7), p 555.
