Chem 1011 – Intersession 2011
Class #33
15-Jun-11
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Class 33: Voltaic Cells
•
Sec 18.3 – Voltaic (or Galvanic) Cells:
Generating Electricity from Spontaneous
Chemical Reactions
▫
Electrochemical Cell Notation
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Announcements
• Review Sessions:
▫ Thursday, June 16th: 12pm – 2pm in C-3033
▫ Monday, June 20th: 12pm – 2pm in C-4002
• On the Web
▫ Term Grades (tests only right now)
▫ Answers to Tests #1 and #2
• Final Exam
▫ Tuesday, June 21st: 9am – 11:30am in SN-4063
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Voltaic (Galvanic) Cells
• Electrical Current
▫ flow of electrical charge.
• Example: Electrons flowing through a wire.
• Since redox reactions involve the transfer of electrons
from one substance to another, they have the potential
to generate electrical current.
• Electrical current is measured in units of ampere (A),
where:
1 A = 1 C/s
• And the charge on a single electron is 1.602 x 10-19 C, so
1 A corresponds to the flow of 6.424 x 1018 electrons per
second!
Voltaic Cells
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Chem 1011 – Intersession 2011
Class #33
15-Jun-11
4
Voltaic (Galvanic) Cells
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Voltaic (Galvanic) Cells
• Voltaic* Cell
▫ an electrochemical cell that produces electrical
current from a spontaneous chemical reaction.
• Electrolytic Cell
▫ consumes electrical current to drive a
nonspontaneous chemical reaction.
• *Voltaic cells are also called Galvanic cells.
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Voltaic (Galvanic) Cells
Oxidation half-cell
Oxidation half-reaction
Voltaic Cells
Reduction half-cell
Reduction half-reaction
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Chem 1011 – Intersession 2011
Class #33
15-Jun-11
7
Voltaic (Galvanic) Cells
Zn electrode
(anode)
Cu electrode
(cathode)
Electrode: conductive surfaces though which electrons can enter or leave half-cells
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Voltaic (Galvanic) Cells
• Anode
▫ electrode where oxidation occurs. Since electrons flow out
of the anode and into the external circuit, the anode is
designated as negative (–).
• Cathode
▫ electrode where reduction occurs. Since electrons flow
from the external circuit into the cathode, the cathode is
designated as positive (+).
• Electrons flow from the anode, to the cathode (from
negative to positive) through the wires connecting the
electrodes.
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Voltaic (Galvanic) Cells
Anions migrate
toward anode
Voltaic Cells
Cations migrate
toward cathode
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Chem 1011 – Intersession 2011
Class #33
15-Jun-11
10
Voltaic (Galvanic) Cells
• Salt Bridge
▫ The function of the salt bridge is to allow the
migration of positive and negative ions between the
half-cells, thus completing the electrical circuit..
• An example is an inverted, U-shaped tube,
containing a strong electrolyte, suspended in gel,
held in by permeable stoppers.
• The salt bridge serves to complete the circuit,
allowing electrical current to flow.
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Voltaic (Galvanic) Cells
• The flow of electrons provides a usable source of
electrical energy. If it were feasible to use this
galvanic cell commercially, we would call it a
battery.
• As the galvanic cell operates, the redox reaction of
the cell approaches equilibrium and the capacity of
the cell to deliver useful electrical energy decreases.
• At equilibrium, the cell ceases to function (think
"dead battery"!)
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Shorthand Cell Notation
• Shorthand description of Voltaic cell:
electrode | electrolyte || electrolyte | electrode
Oxidation (Anode)
Reduction (Cathode)
• oxidation half-cell on the left; reduction half-cell on the
right
• single | = phase barrier
▫ if multiple electrolytes in same phase, a comma is used
rather than |
▫ can use an inert electrode
• double line || = salt bridge
Voltaic Cells
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Chem 1011 – Intersession 2011
Class #33
15-Jun-11
13
Shorthand Cell Notation
Anode = Zn(s)
The anode is
oxidized to Zn2+
Cathode = Cu(s)
Cu2+ ions are
reduced at the
cathode
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
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Inert Electrodes
• Some electrochemical cells do not use metal strips as
electrodes; instead, they use gaseous or aqueous
reactants and platinum or graphite rods which function
as inert electrodes.
• When the participants in a half-reaction are all in the
aqueous phase, a conductive surface is needed for
electron transfer to take place. In such cases an inert
electrode of graphite or platinum is often used.
▫ In the following electrochemical cell, an iron strip acts as
the anode and a platinum strip acts as the cathode. Iron is
oxidized at the anode and MnO4– is reduced at the cathode.
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Inert Electrode
Voltaic Cells
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Chem 1011 – Intersession 2011
Class #33
15-Jun-11
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Problem
• For the following voltaic cell, label each electrode by
metal type, indicate whether it is the anode or cathode,
and show the sign of each electrode. Within each halfcell write the correct balanced half-reaction. Indicate
the direction of electron flow through the wire. Label the
ions of the salt bridge (KNO3) and indicate their
direction of movement.
Cr(s) | Cr3+(aq) || Ag+(aq) | Ag(s)
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Another Shocking Class at 11!
•
Sec 18.4 – Standard Electrode Potentials
▫
▫
•
Predicting the Spontaneous Direction of an
Oxidation Reduction Reaction
Predicting Whether a Metal will Dissolve in Acid
Sec 18.8 – Electrolysis: Driving Nonspontaneous
Chemical Reactions with Electricity
▫
Voltaic Cells
Stoichiometry of Electrolysis
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