Transcript ELECTROCHEMISTRY Voltaic and Electrolytic Cells

ELECTROCHEMISTRY
Voltaic and Electrolytic Cells
EDHS Fall 2010
IB CHEM
Review…
Redox reaction are those involving the
oxidation and reduction of species.
LEO says GER…
Losing electrons  Oxidation
Gaining Electrons  Reduction
NOTE: Oxidation and reduction must occur
together.
They cannot exist alone.
Let’s Look at the
REDOX Reaction
between Zinc and
Copper
Oxidation
Half-Reaction:
Zn(s)  Zn2+(aq) + 2 e–.
Reduction
Half-Reaction:
Cu2+(aq) + 2 e–  Cu(s)
Some Vocabulary…
► Electrodes:
are usually metal strips/wires
► connected by an electrically conducting wire.
► Salt Bridge: is a U-shaped tube that contains a
gel permeated with a solution of an inert
electrolyte.
► Anode: is the electrode where oxidation takes
place.
► Cathode: is the electrode where reduction takes
place.
Voltaic Cell
Voltaic Cells
► In
a spontaneous redox reaction, electrons
flow from the oxidizing reactant (reducing
agent) to the reducing reactant (oxidizing
agent).
► If
the two half–reactions can be separated,
this flow of electrons, instead of occurring
at the surface of the metal, occurs through
an external wire and an electric current is
generated.
Voltaic Cells Continued…
►A
battery, like the ones found in a flashlight
or calculator, contains oxidizing and
reducing substances.
► As the electrons are transferred they are
“tapped” in order to provide the voltage
necessary to power the flashlight or
calculator.
Analogy for Voltaic Cells….
►A
good analogy for the flow of electrons is the
flow of water. Water flows spontaneously
downhill.
► Dams
and waterwheels are examples of ways that
the energy of flowing water is tapped to generate
power.
► Sometimes
we want water to flow uphill. In this
case we need to supply energy, in the form of a
pump, to make this happen.
Redox Reactions as a Source of
Power
► In
order for a redox reaction to serve as a
source of power, the reaction must be
spontaneous.
► What
► In
is the reaction is not spontaneous?
this case, we can use electricity to make
the reaction “go”.
Electrolytic Cells
An electrolytic cell is a device which uses
electricity to drive a non–spontaneous redox
reaction.
For example, water can be separated into
hydrogen and oxygen gas (a non–
spontaneous reaction) using electricity.
► Negative
electrode (cathode):
Na+ + e-  Na
► Positive electrode (anode):
2 Cl-  Cl2 + 2 e-