1. TOPIC 6 ELECTROCHEMISTRY
By:
Chemistry Lecturer
School of Allied Health Sciences
City University College of Science and Technology

2. Electrolytes and Non-electrolytes
Electrolytes: Chemical compound that can conduct electricity in the molten state or in aqueous solution
Electricity conducted by free moving ions
Non electrolytes: Chemical compound that cannot conduct electricity in any state

3. ELECTROLYTES
NON-ELECTROLYTES

4. Electrolytic of a molten compound
Electrolysis is a process whereby a compound is decomposed by electric current
Electrolytic cell consists of two electrodes
Anode – Positive terminal
Anions (-ve charged ions) attracted to anode
Cathode – Negative terminal
Cations (+ve charged ions) attracted to cathode

6. Electrolytic process
Generally a molten compound electrolytes AnBm produces Am+ cations and Bn- anions
AnBm  Am+ Bn-
Examples:
PbBr2  Pb Br-
NaCl  Na Cl-

7. LET’S TRY! Gives the anion and cation for the electrolytes: PbCl2 -
AgCl -
CuCl2 -
CuBr2 -

8. ANSWER Gives the anion and cation for the electrolytes:
PbCl2 - Pb2+ , Cl-
AgCl - Ag+ , Cl-
CuCl2 - Cu2+ , Cl-
CuBr Cu2+ , Br-

9. Two steps occur during electrolysis Movement of ions to the electrodes
Cations (+ve) move towards the cathode
Anions (-ve) move towards the anode
Discharge of ions at the electrodes
Cations discharged by receiving electrons (losing positive charge to become neutral)
An ne-  A
Anions discharged by releasing electrons (losing negative charge to become neutral)
Bn-  B + ne-

10. Steps in writing half equation
Anode
Cathode
Step 1
Ion to neutral
Br-  Br2
Cu2+  Cu
Step 2
Balance number of atoms
2Br-  Br2
Cu2+  Cu
Step 3
Balance the charge by adding the electrons
2Br-  Br2 + 2e
Cu2+ + 2e Cu

11. Overall equation
Balance the number of electrons from the half equation
Example:
Half equation
At the Anode: 2Cl-  Cl2 + 2e
At the cathode: Na+ + e  Na
Overall equation
2Cl-  Cl2 + 2e
(Na+ + e  Na) x 2 = 2Na+ + 2e  2Na
= 2Cl Na+  Cl Na

12. Predicting the products
In the electrolysis of a molten compound,
The metal component of the compound is formed at the cathode
The non-metal component is formed at the anode

13. Example: NaCl molten electrolytes
NaCl  Na+ + Cl-
Anode: Cl-
Cathode: Na+
Half equation
At the Anode: 2Cl-  Cl2 + 2e
At the cathode: Na+ + e  Na
Overall equation
2Cl-  Cl2 + 2e
(Na+ + e  Na) x 2 = 2Na+ + 2e  2Na
2Cl Na+  Cl Na
Product
Anode: Cl2 gas
Cathode: Na metal

14. Example: PbBr2 molten electrolytes
Solution
Equation PbBr2  Pb Br-
Anode: Br-
Cathode: Pb2+
Half equation
Anode: 2Br-  Br2 + 2e
Cathode: Pb2+ +2e  Pb
Overall equation
2Br-  Br2 + 2e
Pb2+ +2e  Pb
2Br- + Pb2+  Br2 +Pb
Products
Anode: Br2 gas
Cathode: Pb metal

15. Electrolysis of aqueos solution
Aqueos solution consists of TWO types of cations and anions; H+ and OH-
Example: Aqueos sodium chloride (NaCl) solution
Cations: Na+ and H+
Anions: Cl- and OH-
However only ONE type of cation and anion will be discharged at each electrode

16. Factors that determine the products formed
Positions of ions in the electrochemical series
The tendency of ions to be selectively discharged depends on their positions in a series known as electrochemical series (ES).
The lower the position of the ion in the ES, the easier the ion will be discharged.

17. ELECTROCHEMICAL SERIES

18. Example: Aqueos sodium chloride (NaCl) solution electrolytes
Cations: Na+ and H+
Anions: Cl- and OH-
Half equation
Anode: 4OH-  2H2O + O e
Cathode: 2H e  H2
Overall equation
4OH-  2H2O + O e
(2H e  H2 ) x 2 = 4H+ + 4e  2H2
4OH H+  2H2O + O2 + 2H2
Products
Anode: oxygen gas
Cathode: hydrogen gas

19. Example 2: Aqueos sodium sulphate (Na2SO4) solution electrolytes
Cations: Na+ and H+
Anions: SO42- and OH-
Half equation
Anode: 4OH-  2H2O + O e
Cathode: 2H e  H2
Overall equation
4OH-  2H2O + O e
(2H e  H2 ) x 2 = 4H+ + 4e  2H2
4OH H+  2H2O + O2 + 2H2
Products
Anode: oxygen gas
Cathode: hydrogen gas

20. Factors that determine the products formed
Effect of ion concentration
When the concentration of a particular type of ion is higher, ion will more likely to be discharged in electrolysis
Usually concentrated halide (Cl- / Br- / I- ions)

21. Factors that determine the products formed
Effect of types of electrodes used
The types of electrodes used can determined the type of ions discharged in electrolysis
Using metal electrodes at anode, ions are not discharged instead the metal dissolves by releasing electrons to form metal ions.
Example: using copper (Cu) electrodes.
Cu  Cu e
Hence mass of anode decrease.

22. Let’s Try!!
Gives the diagram, anion, cation, half equation, overall equation, products and observation using electrolytes
PbBr2 molten
PbCl2 molten
CuSO4 solution

23. THANK YOU…