- Explain electromotive force.

- Construct the reference hydrogen electrode and explain why it is a reference.

- Distinguish reduction potentials from oxidation potentials.

- Calculate the standard potential from the reduction potentials.

The standard cell potential, D*E*^{o},
of the a galvanic cell can be evaluated from the
standard reduction potentials
of the two half cells *E*^{o}.
The reduction potentials are measured against the standard hydrogen
electrode (SHE):

The reduction potentials of all other half-cells measured in volts against the SHE are the difference in electrical potential energy per coulomb of charge.

Note that the unit for energy J = Coulomb volt, and the Gibbs free energy
*G* is the product of charge *q* and potential difference *E*:

consists of the oxidation and reduction reactions:

Zn

Note that the above cell is in reverse order compared to that given
in many textbooks, but this arrangement gives the
standard reduction potentials directly, because the Zn half
cell is a reduction half-cell. The negative voltage indicates
that the reverse chemical reaction is spontaneous.
This corresponds to the fact that
Zn metal reacts with an acid to produce H_{2} gas.

As another example, the cell

consists of an oxidation and a reduction reaction:

Cu

but the potential is so small that the reaction is too slow to be observed.

*Solution*

From a table of
standard reduction potentials we have
the following values

Zn ® Zn

**Discussion**

The positive potential confirms your observation that zinc metal reacts
with cupric ions in solution to produce copper metal.

*Solution*

From the table of standard reduction potentials, you find

Ag = Ag

*Discussion*

The negative potential indicates that the reverse reaction should be
spontaneous.

Some calculators use a lithium battery. The atomic weight of Li is 6.94, much lighter than Zn (65.4).

- The
**electromotive force (EMF)**is the maximum potential difference between two electrodes of a galvanic or voltaic cell. -
The standard reduction potential of
*M*^{n+}, 1 M /*M*couple is the standard cell potential of the galvanic cell:Pt | H _{2}, 1 atm | H^{+}, 1 M ||*M*^{n+}, 1 M |*M* -
The standarde oxidation potential of
*M*|*M*^{n+}, 1 M couple is the standard cell potential of the galvanic cell:*M*|*M*^{n+}, 1 M || H^{+}, 1 M | H_{2}, 1 atm | Pt - If the cell potential is negative, the reaction is reversed. In this case, the electrode of the galvanic cell should be written in a reversed order.

**In which cell does reduction takes place? The right-hand cell or the left-hand cell in the notation**| left | left ^{+}|| right^{+}| right |?*Answer... Right*

Consider...

Oxidation takes place in the left hand cell.

Reduction takes place in the Right hand cell or cathode.**Reduction potentials of half cells are measured against what?**- The zinc half cell Zn | Zn
^{2+}1 M. - The hydrogen half cell Pt | H
_{2}| H^{+}1 M. - The hydrogen half cell H
^{+}1 M | H_{2}| Pt. - The copper half cell Cu
^{2+}1 M | Cu. - The hydrogen half cell Pt | H
_{2}| H^{+}10^{-7}M.

*Answer... B.*

Consider...

Pt | H gives the reduction potential._{2}| H^{+}1 M || right^{+}| right- The zinc half cell Zn | Zn
**Is the potential for the battery**

Pt | H positive or negative?_{2}| H^{+}|| Cl_{2}| Cl^{-}| Pt*Answer... Positive*

Consider...

Cl The reaction is spontaneous._{2}+ 2 e ® 2 Cl^{-}. . .*E*° = 1.36

H_{2}® 2 H^{+}+ 2 e . . .*E*° = 0.00

----------------------------------

Cl_{2}+ H_{2}® 2 HCl . . . D*E*° = 1.36 V

© cchieh@uwaterloo.ca