Electrolysis of ionic compounds
Putting electrical energy in to force
the nonspontaneous change
electrolysis
• Opposite process to electrochemical (voltaic)
cells
• Electrical energy is put into the reaction
• The nonspontaneous reactions occur
• Useful industrial process for the production of
“reduced metals”
• electroplating
Molten ionic compounds
• Molten = melted (NOT dissolved!)
• Only species present are the cation and anion
• The cation (metal ion) will be reduced into a
neutral metal atom at the cathode
– Ex: Fe2+ + 2e- → Fe
• The anion (nonmetal ion) will be oxidized to
neutral at the anode
– Ex: 2 Cl- → Cl2 + 2e-
Aqueous ionic compounds
• Besides the cation and anion , water is present
as well
• Water can be reduced instead of the metal ion
at the cathode:
– Ex: 2H2O(l) + 2e- → H2(g) + 2 OH-(aq) Eo = -0.83V
• Water can be oxidized instead of the nonmetal
ion at the anode:
– Ex: 2H2O(l) → O2(g) + 4e- + 4 H+(aq)
Eo = 1.23V
In an aqueous solution of an ionic
compound, the half reaction with the
greater (more positive) reduction
potential (Eo) will be reduced.
• Ex: electrolysis of aqueous CoBr2
• Present and able to be reduced: Co2+, H2O
Co2+ + 2e- → Co
Eo = -0.28V
2H2O(l) + 2e- → H2(g) + 2 OH-(aq) Eo = -0.83V
Co2+ will be reduced instead of water
In an aqueous solution of an ionic
compound, the half reaction with the
greater (more positive) reduction
potential (Eo) will be reduced.
• Ex: electrolysis of aqueous BaBr2
• Present and able to be reduced: Ba2+, H2O
Ba2+ + 2e- → Ba
Eo = -2.90V
2H2O(l) + 2e- → H2(g) + 2 OH-(aq) Eo = -0.83V
H2O will be reduced instead of Ba2+
In an aqueous solution of an ionic
compound, the half reaction with the
lower (more negative) reduction
potential (Eo) will be oxidized.
• Ex: electrolysis of aqueous CoBr2
• Present and able to be oxidized: Br1-, H2O
2 Br1- → Br2 + 2eEo = 1.07V
2H2O(l) → O2(g) + 4e- + 4 H+(aq)
Eo = 1.23V
Br1- will be oxidized instead of water
In an aqueous solution of an ionic
compound, the half reaction with the
lower (more negative) reduction
potential (Eo) will be oxidized.
• Ex: electrolysis of aqueous CoF2
• Present and able to be oxidized: F1-, H2O
2 F1- → F2 + 2eEo = 2.87V
2H2O(l) → O2(g) + 4e- + 4 H+(aq)
Eo = 1.23V
H2O will be oxidized instead of F1-