Electrochemistry (Ch 17) - AP Chemistry

4/1/2011
GALVANIC CELLS
• Consider a galvanic cell based on the reaction
Al 3+ (aq) + Mg(s) Al(s) + Mg 2+ (aq)
– The half-reactions are
• Al 3+ + 3 e - Al
E° = -1.66 V
• Mg 2+ + 2 e - Mg
E° = -2.37 V
– Give the balanced cell reaction and calculate E° for the cell.
• 2 Al 3+ (aq) + 3 Mg(s) 2 Al(s) + 3 Mg 2+ (aq)
• E° cell = 0.71V
LINE NOTATION
• Line notation is handy for representing electrochemical cells.
– The anode components are listed on the left and the cathode components are
listed on the right, separated by double vertical lines (indicating the salt bridge
or porous disk).
• For example, the cell on the previous slide would be:
Mg(s) | Mg 2+ (aq) || Al 3+ (aq) | Al(s)
– Phase difference (boundary) is indicated by a single vertical line
• Notice that the substance constituting the anode is far left and cathode is far right
– Commas are used to separate ionic species in the same solution
Pt(s)|ClO 3- (aq), ClO 4- (aq), H + (aq) ||H + (aq), MnO 4- (aq), Mn 2+ (aq)|Pt(s)
Ch 17 - Electrochemistry 19
Ch 17 - Electrochemistry 20
COMPLETE DESCRIPTION OF A GALVANIC CELL
• NOTE: A cell will always run spontaneously in the direction that
produces a positive cell potential (E° cell )
• A complete description of a galvanic cell includes:
– The cell potential (always positive for a galvanic cell where E° cell = E° cat – E° an )
and the balanced cell reaction.
– The direction of electron flow, obtained by inspecting the half-reactions and
using the direction that gives a positive E cell .
– Designation of the anode and cathode
– The nature of each electrode and the ions present in each compartment. A
chemically inert conductor is required if none of the substances participating in
the half-reaction is a conducting solid.
DESCRIPTION OF A GALVANIC CELL
• Describe completely the galvanic cell based on the following halfreactions
under standard conditions:
Ag + + e - Ag
Fe 3+ + e - Fe 2+
E° = 0.80 V
E° = 0.77 V
– Ag + (aq) + Fe 2+ (aq) Fe 3+ (aq) + Ag(s) E° cell = 0.03 V
– Electrons flow from Fe 2+ to Ag +
– Anode: Fe 2+ compartment Cathode: Ag + compartment
– Pt(s)|Fe 2+ (aq), Fe 3+ (aq)||Ag + (aq)|Ag(s)
Ch 17 - Electrochemistry 21
Ch 17 - Electrochemistry 22
CELL POTENTIAL, ELECTRICAL WORK, AND
FREE ENERGY
Section 17.3
CELL POTENTIAL AND WORK
• The work that can be done by the electrons transferred
through a wire depends on the “push” (thermodynamics)
behind the electrons.
– This electromotive force (emf) is defined in terms of a potential
difference (in volts) between two points in the circuit (1 V = 1 J/C)
– Cell potential (E ) and work (w) have opposite signs:
w max = –qE max
• In any real, spontaneous process some energy is always wasted – the
actual work realized is always less than the calculated maximum
Ch 17 - Electrochemistry 23
Ch 17 - Electrochemistry 24
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