CHM 1046
General Chemistry II
Dr. Michael Blaber


Electrochemistry

Cell EMF


 Here is a summary of a voltaic cell:

A key question is: what causes the electrons to flow from the anode to the cathode?

The potential difference between two electrodes is measured in Volts (V)

1 V = 1 J/C (i.e. energy per electron)

The cell voltage will depend upon the specific half-reactions, concentrations of ions, and temperature

E0cell = E0red(cathode) - E0red(anode)

If E0red(cathode) > E0red(anode) then the electron flow from anode to cathode is spontaneous

Standard Reduction Potentials

The cell potential of a voltaic cell depends upon the two redox half-reactions that comprise the functioning cell

There is one problem, however: we have seen that an isolated half-reaction doesn't result in reduction or oxidation or electron flow. Therefore, how are we going to assign a standard potential to a half-reaction?

2H+(aq, 1M) + 2e- ® H2(g, 1 atm) E0red = 0.0V

This is a Standard Hydrogen Electrode (SHE)

How do we use this SHE to determine the standard reduction potential, E0red, for some redox half-reaction?

Zn(s) ® Zn2+(aq) + 2e- (Zinc electrode)

2H+(aq) + 2e- ® H2(g) (SHE)

Zn(s) + 2H+(aq) ® Zn2+(aq) + H2(g)

E0cell = E0red(cathode) - E0red(anode)

0.76V = E0red(cathode) - E0red(anode)

0.76V = 0 - E0red(anode)

E0red(anode) = -0.76V

This may seem a little strange because the cell reads +0.76V and yet the determined reduction potential for the zinc electrode is -0.76V.

Zn2+(aq) + 2e- ® Zn(s) E0red = -0.76V

Whenever we assign a potential to a half-reaction using the equation for the standard cell potential, it is always done in reference to a reduction reaction

The standard cell potential (Voltage) describes the potential per unit charge that the reaction produces

Some standard reduction potentials at 25C:

Standard Potential, (E0red) in Volts

Reduction Half-Reaction

0.80

Ag+(aq) + e- ® Ag(s)

0.34

Cu2+(aq) + 2e- ® Cu(s)

0

2H+(aq) + 2e- ® H2(g)

-0.76

Zn2+(aq) + 2e- ® Zn(s)

-3.05

Li+(aq) + e- ® Li(s)

 


A silver-lithium battery is a voltaic cell with a silver cathode and a lithium anode. What is the voltage of this voltaic cell?

E0cell = E0red(cathode) - E0red(anode)

E0cell = 0.8V - (-3.05V) = 3.85 Volts

What will happen if a science project on batteries suggests that a cathode be made out of Zinc and the anode of Copper?

E0cell = E0red(cathode) - E0red(anode)

E0cell = -0.76V - (0.34V) = -1.10 Volts

The battery will run "backwards". In other words, the greater reduction potential of Copper will actually drive oxidation of the Zinc. If a volt meter is connected to the cell with the assumption that the Zinc is the cathode, then the voltage will read negative 1.10 volts


Oxidizing and Reducing Agents

In the above discussion of reduction potentials and their relationship to one another in half-reactions, we can predict which direction current will flow in a voltaic cell. In other words, we can predict which reactant will be oxidized and which will be reduced

The more positive the E0red value of a half-reaction, the greater the tendency of the compound to be reduced - and therefore to oxidize another compound (i.e. the greater the tendency to act as an oxidizing agent)


2000 Dr. Michael Blaber