![Page 1: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/1.jpg)
Lecture 12: Cell Potentials
• Reading: Zumdahl 11.2
• Outline– What is a cell potential?– SHE, the electrochemical zero.– Using standard reduction potentials.
![Page 2: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/2.jpg)
Cell Potentials• In our galvanic cell, we had a species being oxidized at the anode, a species being reduced at the cathode, and electrons flowing from anode to cathode.
• The force on the electrons causing them to full is referred to as the electromotive force (EMF). The unit used to quantify this force is the volt (V)
• 1 volt = 1 Joule/Coulomb of charge V = J/C
![Page 3: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/3.jpg)
Cell Potentials (cont.)• We can measure the magnitude of the
EMF causing electron (i.e., current) flow by measuring the voltage.
Anode Cathode
e-
![Page 4: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/4.jpg)
1/2 Cell Potentials• What we seek is a way to predict what the voltage will be between two 1/2 cells without having to measure every possible combination.
• To accomplish this, what we need to is to know what the inherent potential for each 1/2 cell is.
• The above statement requires that we have a reference to use in comparing 1/2 cells. That reference is the standard hydrogen electrode (SHE)
![Page 5: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/5.jpg)
1/2 Cell Potentials (cont.)
• Consider the following galvanic cell
• Electrons are spontaneously flowing from the Zn/Zn+2 half cell (anode) to the H2/H+ half cell (cathode)
![Page 6: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/6.jpg)
1/2 Cell Potentials (cont.)• We define the 1/2 cell potential of
the hydrogen 1/2 cell as zero.
SHE
P(H2) = 1 atm
[H+] = 1 M
2H+ + 2e- H2 E°1/2(SHE) = 0 V
![Page 7: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/7.jpg)
1/2 Cell Potentials (cont.)• With our “zero” we can then measure the
voltages of other 1/2 cells.
Zn Zn+2 + 2e-
E° SHE = 0 V
• In our example, Zn/Zn+2 is the anode: oxidation
2H+ + 2e- H2
Zn + 2H+ Zn+2 + H2
E°cell = E°SHE + E°Zn/Zn+2 = 0.76 V
0
E°Zn/Zn+2 = 0.76 V
![Page 8: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/8.jpg)
Standard Reduction Potentials
• Standard Reduction Potentials: The 1/2 cell potentials that are determined by reference to the SHE.
• These potentials are always defined with respect to reduction.
Zn+2 + 2e- Zn E° = -0.76 V
Cu+2 + 2e- Cu E° = +0.34 V
Fe+3 + e- Fe+2 E° = 0.77 V
![Page 9: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/9.jpg)
Standard Potentials (cont.)
• If in constructing an electrochemical cell, you need to write the reaction as a oxidation instead of a reduction, the sign of the 1/2 cell potential changes.
Zn+2 + 2e- Zn E° = -0.76 V
Zn Zn+2 + 2e- E° = +0.76 V
• 1/2 cell potentials are intensive variables. As such, you do NOT multiply them by any coefficients when balancing reactions.
![Page 10: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/10.jpg)
Writing Galvanic Cells
For galvanic cells, Ecell > 0
In this example:
Zn/Zn+2 is the anode
Cu/Cu+2 is the cathode
Zn Zn+2 + 2e- E° = +0.76 V
Cu+2 + 2e- Cu E° = 0.34 V
![Page 11: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/11.jpg)
Writing Galvanic Cells (cont.)Zn Zn+2 + 2e- E° = +0.76 V
Cu+2 + 2e- Cu E° = 0.34 V
Cu+2 + Zn Cu + Zn+2
E°cell = 1.10 V
Notice, we “reverse” the potential for the anode.
E°cell = E°cathode - E°anode
![Page 12: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/12.jpg)
Writing Galvanic Cells (cont.)
Shorthand Notation
Zn|Zn+2||Cu+2|Cu
Anode Cathode
Salt bridge
![Page 13: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/13.jpg)
Predicting Galvanic Cells
• Given two 1/2 cell reactions, how can one construct a galvanic cell?
• Need to compare the reduction potentials of the two half cells.
• Turn the reaction for the weaker reduction (smaller E°1/2) and turn it into an oxidation. This reaction will be the anode, the other the cathode.
![Page 14: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/14.jpg)
Predicting Galvanic Cells (cont.)
• Example. Describe a galvanic cell based on the following:
Ag+ + e- Ag E°1/2 = 0.80 VFe+3 + e- ----> Fe+2 E°1/2 = 0.77 V Fe+2 ----> Fe+3 + e- E°1/2 = -0.77 V
Ag+ + Fe+2 Ag + Fe+3 E°cell = 0.03 V
E°cell > 0….cell is galvanic
![Page 15: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/15.jpg)
Another Example• For the following reaction, identify the two half cells, and use these half cells to construct a galvanic cell
3Fe+2(aq) Fe(s) + 2Fe+3(aq)+2 0 +3oxidation
reduction
Fe+2(aq) + 2e- Fe(s)E° = -0.44 V
Fe+3(aq) + e- Fe+2(aq)E° = +0.77 V
![Page 16: Lecture 12: Cell Potentials Reading: Zumdahl 11.2 Outline –What is a cell potential? –SHE, the electrochemical zero. –Using standard reduction potentials](https://reader030.vdocuments.mx/reader030/viewer/2022032702/56649cc15503460f949889d6/html5/thumbnails/16.jpg)
Another Example (cont.)
Fe+2(aq) + 2e- Fe(s)E° = -0.44 V
Fe+3(aq) + e- Fe+2(aq)E° = +0.77 V
weaker reduction
Fe(s) Fe+2(aq) + 2e-E° = +0.44 V
2 x
2Fe+3(aq) + Fe(s) 3Fe+2(aq)E°cell = 1.21 V