ELECTROCHEMISTRY
Electrochemistry is the study of the relationship between chemical reactions
and electricity.
Electrochemical Cells
An electrochemical cell (sometimes called a voltaic or galvanic cell) is a device
used to produce an electric current from a spontaneous redox reaction. How do
we know that a redox reaction is spontaneous? We must consider something
called the standard electrode potential for both the oxidation and reduction
half-reactions. (“Standard” means the solutions are 1 M, and the gases are at l
atm.) The standard electrode potential (symbolized by E^0 ) measures, in volts, the
electric potential difference (which is like potential energy) of a given half-
reaction relative to the following half-reaction:
2H+(aq) + 2e− → H 2 (g)If we’re measuring E^0 red (reduction potential), it’s measured relative to the above
reduction. If we’re measuring E^0 ox (oxidation potential), it’s measured relative to
the reverse of the above reaction. To determine if a redox reaction is
spontaneous, simply add E^0 ox for the oxidation half-reaction and E^0 red for the
reduction half-reaction. The sum of E^0 ox and E^0 red is E^0 cell, which is the electric
potential difference for the overall redox reaction. If E^0 cell is greater than 0, then
the redox reaction is spontaneous. One other thing about E^0 ox and E^0 red values:
Even if a half-reaction is multiplied by some coefficient, never, ever multiply
E^0 ox by that coefficient. The test writers will supply you with all the E^0 values
you’ll need. Now consider the following redox reaction:
Important
Never multiply E^0 values
by anything!Cu2+(aq) + Zn(s) → Cu(s) + Zn2+(aq)Here are the half-reactions and E^0 values.