Electrochemistry
• Electrochemical Cell – an apparatus that
uses redox reactions to produce electrical
energy.
• Voltaic Cell – a type of electrochemical cell
that converts chemical energy to electrical
energy with a spontaneous redox reaction.
Set Up
• We will use the reaction:
Zn(s) + Cu2+  Zn2+ + Cu(s)
• We will place 1M ZnSO4
and a strip of zinc on one
beaker.
• We will place 1 M CuSO4
and a strip of copper in the
other beaker.
Set Up
• In order for electrons to be
exchanged, a wire is
connected to each metal
piece.
• Since zinc in the strip is
being oxidized, there will
be a build up of positive
ions (Zn+2) in solution as
electrons leave.
• The zinc strip loses mass
over time.
Set Up
• In the copper beaker,
since electrons are
entering, there will be
a build up of negative
ions in the solution.
(Cu+2 in solution
becomes Cu)
• The copper strip gains
mass over time.
Set Up
• A salt bridge is used to
allow ions to flow from
one side to the other.
• It contains a soluble salt
that is contained by a
“plug” such as agar gel.
• Ions can move through
plug, but solutions do not
mix.
• Anions enter the cell
where oxidation occurs.
• Cations enter the cell
where reduction occurs.
Voltaic Cell
• The electrode where oxidation takes place is the anode.
• The electrode where reduction takes place is the cathode.
• Which metal becomes oxidized or reduced is determined
by it’s reduction potential.
• Reduction potential is the tendency of an substance to gain
electrons. This is measured in the units of volts.
• The more negative the number the more likely it will be
oxidized, the more positive the more likely it will be
reduced.
Reduction Potentials
Calculating Cell Potential
• All reduction
potentials are
measured against
hydrogen (which can
be oxidized or reduced
by the substance.)
• To measure the cell we
created we first look at
the copper.
Calculating Cell Potential
• Now to measure what
is happening at the
zinc electrode.
• Since copper has the
more positive value, it
will be reduced and
the zinc will be
oxidized.
Calculating Cell Potential
• To calculate the overall potential:
E0cell = E0red – E0ox
• Therefore for our cell:
0.342V-(-0.762V) = +1.104 V
• A positive number indicates it occurs
spontaneously
Practice Problems
• Calculate the cell
potential of the
following:
• A. Chromium in Cr+3
solution and copper in
Cu+2 solution.
• B. Tin in Sn+2
solution and iodine in
I- solution.
• A. E° = +1.086 V
• B. E° = +0.6730 V
Battery
• Battery- can contain a single cell or
packages of several cells
• Small batteries (household) contain a single
cell.
• Large batteries (car) contains many cells
that can conduct more current. These are
lead-acid batteries.
Types of Batteries
• Primary batteries – produce electricity by using a
redox reaction that is not reversible.
• Once reactants are gone, battery is thrown out.
• Typical alkaline batteries oxidize powdered zinc.
• Secondary batteries – are rechargeable because
redox reactions are reversible.
• These batteries are usually made of nickel and
cadmium (NiCad).
Types of Batteries
• Lithium batteries – use lithium because it is
lightest in mass of all metals and has lowest
standard reduction potential.
• This allows the battery to last much longer.
Some Vocabulary
• Electroplating: using electrical current to cause
ions in solution to form a metal layer on a surface.
• One Ampere: (A) amount of electrical current
equal to 1 coulomb of charge per sec.
• One Coulomb: (C) amount of charge that passes a
point when 1 amp of current flows for 1 sec.
(1A = 1C/sec)
• One Faraday: (F) amount of electricity that passes
per 1 mole of electrons (1F=96485C or
1 mole e- =96485C)
Let’s try some math
• A current of 2.50 amps is passed through a
solution of Ni(NO3)2 for 2.00 hours. What
mass of nickel is deposited at the cathode?
• Ans: 5.48 g
Another Problem
• If you wish to convert 1.00 grams of Au+3
ions to solid Au, how long must you
electrolyze the solution if the current
passing through the circuit is 2.00 amps?
• Ans: 735 sec
Let’s do one more!
• After 9.50 minutes at 5.50 amps, all of the
Fe+3 ions were plated out of 600.0 mL of
solution. What was the original
concentration of Fe+3 in solution?
• Ans: 0.0181 M