TOPIC 6 ELECTROCHEMISTRY

1
TOPIC 6ELECTROCHEMISTRY

• 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

5
(No Transcript)
6
Electrolytic process

• Generally a molten compound electrolytes AnBm
  produces Am cations and Bn- anions
• AnBm ? Am Bn-
• Examples
• PbBr2 ? Pb2 2Br-
• NaCl ? Na Cl-

7
LETS 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-
• CuBr2 - 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
Writing half equation
Steps in writing half equation 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- 2Na ? Cl2 2Na

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- 2Na ? Cl2 2Na
• Product
• Anode Cl2 gas
• Cathode Na metal

14

• Example PbBr2 molten electrolytes
• Solution
• Equation PbBr2 ? Pb2 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 O2 4e
• Cathode 2H 2e ? H2
• Overall equation
• 4OH- ? 2H2O O2 4e
• (2H 2e ? H2 ) x 2 4H 4e ? 2H2
• 4OH- 4H ? 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 O2 4e
• Cathode 2H 2e ? H2
• Overall equation
• 4OH- ? 2H2O O2 4e
• (2H 2e ? H2 ) x 2 4H 4e ? 2H2
• 4OH- 4H ? 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 ? Cu2 2e
• Hence mass of anode decrease.

22
Lets Try!!

• Gives the diagram, anion, cation, half equation,
  overall equation, products and observation using
  electrolytes
• PbBr2 molten
• PbCl2 molten
• CuSO4 solution

23

• THANK YOU