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.

Writing Galvanic Cells (cont.)

Shorthand Notation

Zn|Zn+2||Cu+2|Cu

Anode Cathode

Salt bridge

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)

Homework #116

DUE MONDAY#19, 21b, 23