Download Voltaic Cells - Ms. Mogck`s Classroom

BATTERIES
AND CELLS
Batteries
• A battery is a group of cells,
connected together in a
series (to form more energy)
ELECTRIC CELL
• Continuously converts
chemical energy into
electrical energy
• Real life electrochemistry!
• Each cell is composed of 2
electrodes (solid electrical
conductors – usually 2 metals or
graphite and metal)
• Each cell also contains 1
electrolyte (aqueous electrical
conductor)
• 1 Positive electrode = CATHODE
• Reduction occurs at the cathode
(GERC)
• 1 Negative electrode = ANODE
• Oxidation occurs at the anode
(LEOA)
Voltaic Cells
• A voltaic cell is an
arrangement of 2 half cells
separated by a porous
boundary
Half Cells
• A half cell consists of 1
electrode and 1 electrolyte
Half cell Notation
• A half cell can be represented through the
following shorthand
Zn(s) ZnSO4(aq)
CuSO4(aq) Cu(s)
Porous Boundary
• A porous boundary separates
the 2 electrolytes, while still
permitting ions to move
between the 2 solutions
(through tiny openings in a
salt bridge)
External Circuit
• The connection between the
anode and the cathode
through which the electrons
travel (metal wire)
• Often hooked to an
voltmeter
ELECTRICITY
• Electricity is
the flow of
electrons
from the
anode to the
cathode!!
Voltmeter
• A device that is used to measure
the energy difference between
any 2 points in an electric circuit
• Energy is measured in VOLTS (V)
Energy Potential
Difference
• Fancy way of describing the
voltage (difference in energy)
• Voltage depends on the
chemical composition of the
reactants within the cell
Cell Potential = Voltage
• The theoretical voltage can be calculated using
the formula:
Ecell = SOA – SRA
Or……
Ecell = Cathode - Anode
Where Did We Get These #s???
• The standard Hydrogen electrode is a redox
electrode which forms the basis of the
thermodynamic scale of oxidation-reduction
potentials.
• It is used to form a basis for comparison with all
other electrode reactions, therefore hydrogen’s
standard electrode potential is declared to be
zero at all temperatures
• Potentials of any other electrodes are compared
with that of the standard hydrogen electrode at
the same temperature.
Challenging Diploma Example
• If the Ni2+(aq) + 2e-  Ni(s) half
reaction is defignated as the
reference half reaction with an
electrode potential of 0.00V, then
what is the electrical potential for
the Fe3+(aq) + e-  Fe2+(aq) half
reaction?
Electric Current
• A measure of the rate of flow
of charge past a point in an
electrical circuit
• Measured in Amperes (A)
Example:
• Write the equations for the half-reactions and the
overall reaction that occurs in the following cell:
C(s) Fe2+(aq), Fe3+(aq)
C(s)
Cr2O72-(aq), H+(aq)
• Step 1: Label the ALL oxidizing and reducing agents.
• Step 2: Find the STRONGEST OXIDIZING AGENT and
the STRONGEST REDUCING AGENT
C(s) Fe2+(aq), Fe3+(aq)
C(s)
Cr2O72-(aq), H+(aq)
• Remember the SRA gets oxidized at the
ANODE!
• Remember the SOA gets reduced at the
CATHODE!
• Step 3: Write the ½ reactions (from chart or using
acid method)
• cathode
Cr2O72-(aq) + 14H+(aq) + 6e-  2Cr3+ (aq) + 7H2O(l)
• anode
6 [ Fe2+(aq)  Fe3+(aq) + e- ]
• Step 4: Balance electrons and cross out products
and reactants to combine reactions
Cr2O72-(aq) + 14H+(aq) + 6e-  2Cr3+ (aq) + 7H2O(l)
6 [ Fe2+(aq)  Fe3+(aq) + e- ]
Cr2O72-(aq) + 14H+(aq) + 6Fe2+(aq)  2Cr3+ (aq) + 7H2O(l) + Fe3+(aq)
• Step 5: draw the cell representation of what is
going on, including electron movement
Example 2:
• A silver copper voltaic cell consists of a copper
half cell with a Cu(s) electrode and a 1.0M
Cu(NO3)2 electrolyte, as well as a silver halfcell with an Ag(s) electrode and a 1.0M AgNO3
electrolyte. The 2 half cells are connected by a
salt bridge containing KNO3. Write the half
reactions and the net reaction.
• SRA = Cu(s) gets oxidized at the ANODE
• SOA = Ag+(aq) gets reduced at the CATHODE
• cathode
2 [ Ag+(aq) + e-  Ag(s) ]
• anode
Cu(s)  Cu2+(aq) + 2e-
• Net reaction
Cu(s) + 2Ag+(aq)  Cu2+(aq) + 2Ag(s)
Is this a spontaneous reaction????