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Standard Hydrogen Electrode Presenta2on and Interac2ve Demonstra2on Department of Chemistry & Biochemistry University of Oregon Eugene, Oregon 97403 USA
Standard Hydrogen Electrode Presenta2on and Interac2ve Demonstra2on
Department of Chemistry & Biochemistry University of Oregon
Eugene, Oregon 97403 USA
E0cell = standard cell potential
• E0cell can be measured directly
• In this zinc-‐copper cell • E0
cell is measured to be +1.10 V
But, how do we determine E0half-cell??
• Can be calculated from standard electrode potential (E0half-cell)
for each half-reaction as given in the table
E0cell = E0
cathode (reduction) – E0anode (oxidation)
Standard Reduction Potential
• The SHE half-cell consists of a glass tube, H2 gas at 1.0 atm, platinum electrode (inert, not reacting, provides a surface for the H2 gas to react), 1.0 M HCl at 25°C.
1.0 M HCl(aq)
E° values are referenced to a standard hydrogen electrode (SHE).
Standard Hydrogen Electrode When coupled with a less active metal, the SHE can serve as an anode. The oxidation half-reaction is the oxidation of H2 to 2H+, the potential difference = 0.0 Volts
H2(g, 1 atm) ⎯→ 2 H+(aq, 1M) + 2e- E° = 0.0 Volts
an oxida2on reac2on
Best ox agent
hRps://www.google.com/search?biw=1146&bih=835&tbm=isch&sa=1&ei=-‐NVLWu_KEMqS0gKy74PwDA&q=Zinc+Standard+Hydrogen+Electrode+electrochemical+cell+diagram&oq=Zinc+Standard+Hydrogen+Electrode+electrochemical+cell+diagram&gs_l=psy-‐ab.3...1183528.1194595.0.1194887.40.37.3.0.0.0.162.3297.29j7.36.0....0...1c.1.64.psy-‐ab..3.0.0....0.jb22KMCv9zs#imgrc=X7CrnyG_XG2I_M:
Observe the computer anima2on
1. Sketch what occurs at the surface of the pla2num electrode
2. Write the half-‐reac2on that occurs at the surface of the pla2num electrode.
Standard Reduction Potentials
2H+(aq) + Cu(s) E° = +0.34 V H2(g) + Cu2+(aq)
Cu(s) E° = ? V Cu2+(aq) + 2e-‐
2H+(aq) + 2e-‐ E° = 0.0 V H2(g)
Overall cell reac2on:
Anode half-‐reac2on:
Cathode half-‐reac2on:
The standard poten2al of a cell is the difference of the standard half-‐cell poten2als as reduc2on reac2ons for the reduc2on at the cathode and the oxida2on at the anode:
The measured poten2al for this cell: E°cell = +0.34 V
Ecell ° = Ered ° (cathode) -‐ Ered ° (anode) +0.34 V = X Cu2+/Cu – {0.0 V} SHE
E°cell is a Potential Difference Between Two Half-Cells
The greater the difference between the two half-cell reactions, the greater the voltage of the cell.
0.00
+0.40 V Cu(s) Cu2+(aq) + 2e-‐
2H+(aq) + 2e-‐ H2(g)
+0.34 V
Observe the computer anima2on
1. Sketch what occurs at the surface of the pla2num electrode
2. Write the half-‐reac2on that occurs at the surface of the pla2num electrode.
3. Sketch what occurs at the surface of the zinc electrode
4. Write the half-‐reac2on that occurs at the surface of the zinc electrode.
Cell anima2on of a zinc SHE cell URL:
hRps://www.youtube.com/watch?v=CQ7gYg64Msc
Standard Hydrogen Electrode We select as a standard half-reaction the reduction of 2H+ to H2 under standard conditions, which we assign a potential difference = 0.0 Volts
2 H+(aq, 1M) + 2e- ⎯→ H2(g, 1 atm) E° = 0.0 Volts
a reduc2on reac2on
The SHE can serve as a cathode.
Observe the computer anima2on
1. Sketch what occurs at the surface of the pla2num electrode
2. Write the half-‐reac2on that occurs at the surface of the pla2num electrode.
How to Calculate a Standard Reduction Potential
Zn2+(aq) + H2(g) Zn(s) + 2H+(aq) Overall cell reac2on:
Anode half-‐reac2on:
Cathode half-‐reac2on:
Zn(s) Zn2+(aq) + 2e-‐ E° = -‐0.76 V As a standard reduc2on poten2al
Zn2+(aq) + 2e-‐ Zn(s) E° = 0.76 V oxida2on
H2(g) 2H+(aq) + 2e-‐
Zn2+(aq) + 2e-‐ Zn(s)
Ecell ° = Ered ° (cathode) -‐ Ered ° (anode)
+0.76 V = 0.0 V (SHE) -‐ { x } Zn/Zn2+
X = -‐ 0.76 V
E° = ? V
E° = 0.0 V
E° = +0.76 V
E°cell is a Potential Difference Between Two Half-Cells
The greater the difference between the two half-cell reactions, the greater the voltage of the cell.
0.00 V
-‐0.50 V
Zn(s) Zn2+(aq) + 2e-‐
2H+(aq) + 2e-‐ H2(g)
-‐0.76 V
E°cell = +2.37 V
Determine the iden2ty of the metal.
