Download Electrolytic Cells Objective You will be able to describe an

Electrolytic Cells
lesson 5
chapter 13
Objective
You will be able to describe an
electrolytic cell
Consider a voltaic cell as follows.
Cu(s)
Zn(s)
ZnSO4(aq)
CuSO4 (aq)
Label the cathode and anode.
Identify the direction of electron movement.
Identify the direction of ion movement.
Calculate the cell potential.
Now consider adding a battery to force the reverse
reaction.
Cu(s)
Zn(s)
ZnSO4(aq)
CuSO4 (aq)
Label the cathode and anode.
Identify the direction of electron movement.
Identify the direction of ion movement.
Calculate the cell potential.
Electrolysis
Using electricity to force a nonspontaneous redox reaction to take place.
What is needed?
A power source (an electron pump, such
as a battery)
Two electrodes
The cathode is hooked up to the
negative terminal of the power source
and the anode is hooked up to the
positive terminal.
An electrolyte
A porous boundary isn't required, but
the products of the reaction need to be
separated or they will react
spontaneously.
Summary of electrodes
Calculating the cell potential
E˚cell = E˚R(cathode) - E˚R(anode)
Cell potentials are negative for electrolytic cells
indicating that the reaction is non-spontaneous
The cell potential represents the voltage needed to
make the reaction ʻgoʼ.
Example: CuSO4 (aq) electrolysis
Entities present?
SOA: Cathode reaction?
SRA: Anode reaction?
Net Cell reaction
Cell Potential
Evidence of Reaction
Metal forming on cathode
Solution turns blue as ions go into it.
Chlorine Anolmoly
Sometimes the predicted reaction doesn't always
take place in an electrolytic cell.
Example: Chloride Ions in Solution
water is a stronger SRA than Cl-, but in an electrolytic
cell Cl- react at the anode.
Predicted
Entities in solution?
SRA: water
2H2O 4e- + 4H+(aq) + O2(g)
Actual Reaction
Actual Reaction : 2Cl- 2e- + Cl2(g)
Reason: the actual voltage required for water to
react is higher than the theoretical voltage. This
difference is known as overvoltage.
Example: electrolysis of NaCl
Entities?
SOA,
SRA
Cathode reaction
Anode reaction
Net Reaction
Evidence of Reaction
One of the main uses of electrolysis is the production of
pure elements from compounds.
In 1808 Sir Humphrey Davy used electrolysis to discover
Mg, Ca, Sr, and Ba.
When trying to purify elements from solution, water can
interfere with the reaction because often it is the SOA or
SRA.
To avoid interference from water, the compound can be
melted, and the molten compound can act as the
electrolyte (but this is difficult).
Example: molten CaCl (l)
2
Species:
Ions move to opposed charged electrode.
SOA(cathode):
SRA(anode):
Net:
Assignment
Read p.
Write the half cell reactions for the electrolysis of
sodium sulfate solution and molten potassium
chloride.