Uploaded by Cheuk Wang Lau

10.1 Galvanic Cells

advertisement
10.1 Galvanic Cells
Zn(s)
SRA
Cu2+(aq)
SOA
SO42-(aq)
H2O(l)
Zn(s)
SRA
Cu2+(aq)
SOA
SO42-(aq)
H2O(l)
▪ When Zn and Cu2+ are in direct contact, we
cannot harness the electrical energy of the
transferring electrons (it instead gets lost as
heat)
▪ In order to harness the electrical energy, the
oxidizing agent and the reducing agent have to
be separated in order to force the electron
transfer through a wire
▪ This electron flow/current can then be used
to power electrical devices!
▪ Electric cells adapted for scientific study are
often called galvanic cells or voltaic cells
▪ Electric cells adapted for scientific study are
often called galvanic cells or voltaic cells
▪ Two electrodes (a solid electrical conductor)
are in contact with electrolytes (ions in
solution)
▪ The electrolytes surrounding each electrode
are separated by a porous boundary that still
permits ions to move through tiny openings
between the two solutions
electrode a solid
electrical conductor
half-cell an
electrode and an
electrolyte
that form half of a
complete cell
Zn(s)
charge
into Zn2+
Cu2+ charge
into Cu(s)
- e- flow from Zn(s) to Cu(s)
creating a current
- This stops when there is a
buildup of charges in each
half cell
- Oxidation of Zn (into Zn2+) results
in buildup of Zn2+ (+)
- Reduction of Cu2+ will decrease
Cu2+ ions and only SO42- will be
left (-)
- The buildup prevents further etransfer → redox reaction stops
We can fix this by adding a salt bridge into the
system
Salt bridge: a u-shaped tube, whose ends are plugged with cotton balls, that is filled with a non-reactive
electrolyte. The cotton balls prevent the solution from pouring out, but they are porous to allow ion
movement. The flow of ions through the sodium sulfate salt bridge keeps the solution in each half-cell
electrically neutral.
Na2SO4
Na2SO4 is ideal since it’s soluble and ions do
not react with the electrolytes or electrodes
SRA
(Strong
reducing
agent)
SRA
(Strong
reducing
agent)
Na+ migrates to the cathode half cell to offset the loss
of Cu2+ ions in order to balance the buildup of Zn2+
Because of
the salt
bridge, the
solutions in
each half cell
remain
electrically
neutral
▪ A galvanic cell consists of two half-cells separated by a porous
boundary with solid electrodes connected by an external
circuit to produce an electrical current.
▪ The cathode is the positive electrode. Reduction of the
strongest oxidizing agent present in the cell occurs at the
cathode
▪ The anode is the negative electrode. Oxidation of the strongest
reducing agent present in the cell occurs at the anode
▪ Electrons travel in the external circuit from the anode to the
cathode
▪ Internally, anions in the salt bridge move toward the anode and
cations move toward the cathode as the cell operates. The
solution remains electrically neutral.
Cell Notation
▪ A cell can be represented with a shorthand
called line notation:
▪ The single line (|) indicates a phase boundary
such as the interface of an electrode and an
electrolyte in a half-cell
▪ The double line (||) to indicate a salt bridge
and represents a physical boundary such as a
porous boundary between half-cells
Zn(s) Zn2+(aq)
Cu2+(aq)
SO42-(aq)
Na+(aq)
Cu(s)
Cell Potential the electric potential difference
(voltage) between the two half-cells in a galvanic
cell; the SI unit is the volt, and the unit symbol is V
(1 V 5 1 J/C)
Ultimately, can electricity potentially be produced
How do we calculate for it?
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
∆E°r (cell) = E°r (cathode) – E°r (anode)
standard cell potential (∆E°r (cell) the electric
potential difference of a galvanic cell that is
operating under standard conditions
standard reduction potential (E°r) the ability of a
half-cell to attract electrons in a cell that is
operating under standard conditions
Standard Cells and Standard Cell Potential
Because cell potential varies depending on the
concentration of the chemicals in a cell. To
study cells more easily, chemists have defined
standard conditions under which cells operate. A
standard cell is a galvanic cell in which all the
entities are at SATP, with a concentration of 1.0
mol/L for solutions.
Standard Hydrogen Half Cell
How was this developed
Everything is according to H+
ΔE°r(cell) and Spontaneity
Spontaneous Reaction, ΔE°r(cell)> 0
Positive value means that cell reaction occurs
spontaneously (on its own)
Reaction at Equilibrium, ΔE°r(cell)= 0
Cell has been used up and no more electrons can be
transferred. To continue functioning, cell must be
recharged.
Non-spontaneous reaction, ΔE°r(cell)<0
Negative value means that cell reaction does not occur
spontaneously and need an external source to apply
energy for cell reaction to occur
Example: Determine the standard cell potential
and the net ionic equation for a redox reaction that
involves silver and zinc half-cells.
Download