standard voltages
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Standard Voltages. Reading: Masterson 18.2 Outline What is a standard voltage (cell potential) SHE, the electrochemical zero. Using standard reduction potentials to calculate standard voltage of a voltaic cell. Standard Voltage/Cell Potentials. - PowerPoint PPT PresentationTRANSCRIPT
Standard Voltages
• Reading: Masterson 18.2
• Outline– What is a standard voltage (cell potential)
– SHE, the electrochemical zero.– Using standard reduction potentials to calculate standard voltage of a voltaic cell
Standard Voltage/Cell Potentials
• In a voltaic cell, a species is oxidized at the anode, a species is reduced at the cathode, and electrons flow from anode to cathode.
• The force on the electrons causing them to flow is referred to as the electromotive force (EMF). The unit used to quantify this force is the volt (V)
Standard Voltage/Cell Potentials (cont.)
• We can measure the magnitude of the EMF causing electron (i.e., current) flow by measuring the voltage.
Anode Cathode
e-
Standard Voltage/Cell Potentials (cont.)
Eo = Eored + Eo
ox
In the case below we have experimentally determined that 1.06 volts = Eo
red + Eoox
Anode Cathode
e-
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. (i.e. knowing the Eo
red and Eoox values
• The above statement requires that we have a reference to use in comparing 1/2 cells. That reference is the standard hydrogen electrode (SHE)
1/2 Cell Potentials• Consider the following galvanic cell
• Electrons are spontaneously flowing from the Zn/Zn+2 half cell (anode) to the H2/H+ half cell (cathode)
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
1/2 Cell Potentials• 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
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
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.
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
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.
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.
• The stronger reducing agent will become the anode and get oxidized (flip this equation) while the stronger oxidizing agent will become the cathode and get reduced.
ALL OLD CARS RUST= Anode (oxidation); Cathode (reduction)
What about our copperplating lab?
Was it an example of galvanic/voltaic cell?The anode was the copper metal and the iron nail was the cathode ….. Yet copper is a weaker reducing agent than iron so you would expect their roles to be reversed?
The answer is NO! It is not a galvanic cell. We accomplished this feat by using the battery as an external electron pump. In an electrolytic cell, a nonspontaneous redox reaction is made by pumping electrical energy into the system … electrolysis. (Section 18.5 of your textbook)