Transcript ELECTROLYSIS AND ELECTROLYTIC CELLS [NON spontaneous

ELECTROLYSIS AND ELECTROLYTIC
CELLS
[NON spontaneous cells]
Purpose: To produce purified forms of
elements
Electrolytic cells
•Nonspontaneous
•Usually one cell
•Two of the same electrodes (graphite is a good
choice)
•Must have a power source
•Use energy to create a chemical change
• Voltaic cells are
spontaneous
• Separated into two cells
• Is a battery
• Anode is negative
• Cathode is positive
• AnOx
• Red Cat
• Fat Cat
• Electrolytic cells are
forced to work by using
a power source
• Single container
• NEEDS a battery
• Anode is positive
• Cathode is negative
• AnOx
• Red Cat
• Fat Cat
• EPA (electrolytic
positive anode)
If there is no water present and
you have a pure molten ionic
compound, then…
The cation will be reduced (gain
electrons/go down in charge).
The anion will be oxidized (lose
electrons/go up in charge).
Diagram an electrolytic cell of a molten solution
of KI. Be sure to include anode, cathode, and
overall cell reactions. Clearly state any
observations and label electron flow.
• To predict products in aqueous solutions:
• No IA or IIA metal will ever be oxidized
• No polyatomic ion will ever oxidize
Instead– use water
Memorize these equations:
Oxidation
Reduction
2H2O  O2 + 4H+ + 4e2H2O + 2e-  H2 + 2OH-
If water is present and you
have an aqueous solution of the
ionic compound, then…
You’ll need to figure out if the ions
are reacting, or the water is reacting.
You can always look at a reduction
potential table to figure it out.
Faraday’s Law
• The amount of substance being oxidized or reduced at each
electrode during electrolysis is directly proportional to the
amount of electricity that passes through the cell.
• Use dimensional analysis
• #coulombs= It
I is current (ampheres)
(q)
q is charge (coulombs), t = time
Conversions given:
• 1 volt = 1 Joule/Coulomb
• 1 Amp = 1 Coulomb/second (current)
• Faraday = 96,500 Coulombs/mole of e• mole of e- come from balanced redox equation
Free Energy
G = -nFE
G = Gibb’s Free Energy
[Reaction is spontaneous if ΔG is negative]
n = number of mole of electrons
F = 96,500 coulomb/mol eE = cell potential 1 volt = Joule/coulomb
Summary of Gibb’s Free Energy and Cells
 -Eo implies NONspontaneous
 +Eo implies spontaneous (would be a good
battery!)
 E = 0, equilibrium reached (dead
battery)
 the larger the voltage, the more spontaneous
the reaction
 G will be negative in spontaneous reactions
 K>1 are favored
Two important equations
G = - nFE [“minus nunfe”]
G = - RTlnK [“ratlink”]
G = Gibbs free energy n = number of moles of electrons.
F = Faraday constant 9.6485309 x 104 J/V (1 mol of
electrons carries 96,500C )
E = cell potential
R = 8.31 J/molK
T = Kelvin temperature
K = equilibrium constant [products]coeff/[reactants]coeff
Favored conditions
Ecell > 0
G < 0
K>1