15
Electrolytic Cells Lesson 9 Electrolytic Cells
Electrolytic CellsLesson 9
Electrolytic Cells
Notes on Electrolytic Cells
An electrolytic cell is a system of two inert (nonreactive) electrodes (C or Pt) and an electrolyte connected to a power supply. It has the following characteristics
1. Nonspontaneous redox reaction 2. Produces chemicals from electricity 3. Forces electrolysis to occur
Electrolytic Cell The electrode that is connected to the -ve terminal of the power supply will gain electrons and therefore be the site of reduction. Oxidation always occurs at the anode and reduction at the cathodeElectrons flow through the wire and go from anode to cathodeAnions (- ions) migrate to the anode and cations (+ions) migrate towards the cathode.
1. Draw and completely analyze a molten NaBr electrolytic cell.
PtPt
DC Power Source
- +DC Power Source
- +
Electrons go from anode to cathode.
Na+
Br-
_reductioncathode2Na+ + 2e- → 2Na(l)
-2.71 v
+oxidationanode2Br- → Br2(g)+ 2e-
-1.09 v
2Na+ + 2Br- → Br2(g) + 2Na(l) E0 = -3.80 v
e-
e-
MTV = +3.80 v
The negative is reduction
The positive is oxidation
The MTV is the minimum theoretical voltage required to start a reaction
Molten or (l) means ions but no water
cations to cathodeanions to anode
What do you do if there is water in the cell?
Reduction of water
Water will reduce before anything below this line. Nothing below this line can reduce in aqueoussolution
Pb2+ reduces in water
Water reduces in a K+ solution
Treat as if it were here
Reduction
If you have water present (aq) 1.0 M solution
Take the higher reaction The strongest oxidizing agent Consider the overpotential effect
Oxidation of water
Treat as if here
Br- (lower) oxidizes before water
Water (lower) oxidizes before F-
Oxidation
If you have water present (aq) 1.0 M solution
Take the lower reaction The strongest reducing agent Consider the overpotential effect
1. Draw and completely analyze an aqueous KI electrolytic cell.
PtPt
DC Power Source
- +DC Power Source
- +
K+
H2OI-
The negative is reduction
The positive is oxidation
Cation or water
Reduction
Cathode
Consonants
highest
For Reduction take the highest
1. Draw and completely analyze an aqueous KI electrolytic cell.
PtPt
DC Power Source
- +DC Power Source
- +
K+
H2OI-
The negative is reduction
The positive is oxidation
Anion or water
Oxidation
Anode
Vowels
lowest
For Oxidation take the lowest
1. Draw and completely analyze a molten KI electrolytic cell.
PtPt
DC Power Source
- +DC Power Source
- +
Electrons go from anode to cathode.
K+
H2OI-
_reductioncathode2H2O+2e- → H2(g) + 2OH-
-0.41 v
+oxidationanode2I- → I2(s) + 2e-
-0.54 v
2H2O + 2I- → H2 + I2(s) + 2OH- E0 = -0.95 v
e-
e-
MTV = +0.95 v
The negative is reduction
The positive is oxidation
cations to cathode
anions to anode
The overpotential effect is a higher than normal voltage required for the half reaction.
This is often due to extra voltage required to produce a gas bubble in solution.
