The document summarizes electrolysis of aqueous solutions. It defines electrolysis as the process of using an electric current to cause a non-spontaneous chemical reaction. It describes that during electrolysis, cations move to the cathode and are reduced, while anions move to the anode and are oxidized. Factors like the electrochemical series, ion concentration, and electrode type determine which ions are discharged. Products are identified based on half-reactions and observations like gas evolution and metal deposition.

1. 4S4 6th & 8th July 20
Electrolysis of
an aqueous solution

2. Learning outcomes
At the end of the lesson, you should able to:
• identify cations and anions in an aqueous
solution,
• describe the electrolysis of an aqueous solution,
• write half equations for the discharge of ions at
the anode and the cathode,
• predict the products of electrolysis of aqueous
solution.
• explain using examples factors affecting
electrolysis of an aqueous solution

3. Dilute
solution
Electrolysis of an aqueous solution

6. Electrolyte: Molten NaCl Electrolyte: NaCl solution
Carbon
electrodes
NaCl
solution
Cations Anions
Cations Anions
Na+ Cl-
Na+ Cl- H+ OH-

7. Sodium chloride solution
Cations Anions
+ Cl-
Na
+ OH -
H

8. Exercise:
Solution Cations Anions
Dilute sulphuric acid H+ SO42- , OH-
H2SO4
Silver bromide solution Ag+, H+ Br- , OH-
AgBr
Sodium nitrate solution Na+, H+ NO3-, OH-
NaNO3

9. Electrochemical series
a series to show the tendency of the ions to donate/ receive electrons.

10. Cations Anions
K+
Na+ F-
Ca2+ 2- Electrochemical
SO4 series
Easier to discharge
Mg2+
Al3+ NO3-
Zn2+ Cl-
Fe 2+
Sn2+ Br-
Pb2+ I-
H+
Cu2+ OH-
Ag+
Au+

11. Homework for this week
Copy & Memorise
the ELECTROCHEMICAL SERIES
Quiz on next week

12. Your
Electrochemical series own
K+ Potassium Pork Potential Pretty Pop
Na+ Sodium Sometimes Scientists Sally Stars
Ca2+ Calcium Can Can Could Can
Mg2+ Magnesium Make Make Marry Make
Al3+ Aluminium A A A Any
Zn2+ Zinc Zulu Zoo Zulu Zombie
Fe 2+ Iron ill ? in in
Sn2+ Tin Therefore Tiny Lovely Town
Pb2+ Lead Let London Peace Lose
H+ Hydrogen Highly ? Honolulu His
Cu2+ Copper Clever City Causing Control
Ag+ Silver Slay Strange ?
Au+ Gold Good Pigs Gazes ?

13. Anions Your own
F- Fat Flower
SO42- ‘So So
NO3- Nui’ Nice
Cl- Clean Can
Br- Brother’s Buy
I- I-phone’s In
OH- ‘O’ ready Orchard

14. Copper(II) sulphate, CuSO4 solution
consists of
Ions that are present
Cu2+ ions, H+ ions, SO42- ions , OH- ions
that move freely.

15. CATHODE ANODE
electrodes Carbon
Competing
ions Cu2+ ions, H+ ions
Cations
selectively Cu2+ ions
discharged
Cu2+ ion is lower than H+ ions
Why?
in the electrochemistry series.
Half
equation Cu2+ + 2 e- → Cu
Observation
A brown solid is deposited
at cathode.
Products
Copper, Cu metal is formed
at the cathode.

16. CATHODE ANODE
Carbon electrodes
Competing
SO42- ions , OH- ions ions
Anions
OH- ions selectively
discharged
OH-ion is lower than SO42- ions
Why?
in the electrochemistry series.
Half
4OH- → O2 + 2H2O + 4 e- equation
Colourless gas bubbles
Observation
are released at anode
oxygen gas, O2 and water, H2O Products
are produced at the anode.

17. Copper(II) sulphate, CuSO4 solution
consists of
Ions that are present
Cu2+ ions, H+ ions, SO42- ions , OH- ions that move freely.
CATHODE ANODE
electrodes Carbon Carbon electrodes
Competing Competing
ions Cu2+ ions, H+ ions SO42- ions , OH- ions ions
Cations Anions
selectively Cu2+ ions OH- ions selectively
discharged discharged
Cu2+ ion is lower than H+ ions OH-ion is lower than SO42- ions
Why? Why?
in the electrochemistry series. in the electrochemistry series.
Half Half
equation Cu2+ + 2 e- → Cu 4OH- → O2 + 2 H2O + 4 e- equation
Observation
A brown solid is deposited at Colourless gas bubbles are Observation
cathode. released at anode
Products Copper, Cu metal is formed at oxygen gas, O2 and water, H2O Products
the cathode. are produced at the anode.

18. Describe Electrolysis of
an aqueous solution
Example:
Copper(II) sulphate, CuSO4 solution
0.1 mol dm-3
Carbon electrodes

19. • Copper(II) sulphate, CuSO4 solution consists of
Cu2+ ions, H+ ions, Cl2- ions and OH- ions
that move freely.
• During electrolysis,
the cations Cu2+ ions and H+ ions move to the
cathode;
the anions SO42- ions and OH- ions move to the
anode.

20. At CATHODE,
• Copper (II) ions , Cu2+ ions are selectively
discharged
by receiving electrons to form Cu atom.
This is because Cu2+ ions has a lower position
than H+ ions in the electrochemical series.
Cu2+ (aq) + 2 e- → Cu (s)
• A brown solid is deposited at cathode.
• Copper, Cu metal is formed at the cathode.

21. At ANODE,
• Hydroxide ions , OH- are selectively discharged
by donating electrons to form O2 and H2O
molecules.
This is because OH- ions has a lower position
than SO42- ions in the electrochemical series.
4OH- (aq) → O2 (g) + 2H2O(l) + 4 e-
• Colourless gas bubbles are released at anode.
(The gas relights a glowing wooden splinter.)
• Oxygen gas, O2 and water, H2O are produced at
the anode.

22. Overall equation:
2Cu2+ (aq) + 4 e- → 2Cu (s)
4OH- (aq) → O2 (g) + 2H2O(l) + 4 e-
2Cu2+ (aq) + 4OH- (aq) → 2Cu (s)+ O2 (g) + 2H2O(l)

23. Describe Electrolysis of
an aqueous solution
Copper(II) chloride, CuCl2 solution
0.1 mol dm-3
Carbon electrodes

24. Sodium chloride solution
Cations Anions
+ Cl-
Na
+ OH -
H

25. Sodium chloride Copper (II) sulphate
solution solution
Cations Anions Cations Anions
Na+ Cl- Cu2+ SO42-
H+ OH- H+
OH-
The selection of ions to be discharged is based on:
3 Factors :
• Position of ions in the electrochemical series
• Concentration of ions in the electrolytes
• Types of electrodes used in the electrolysis

26. Factors that affect
the electrolysis of an aqueous solution
Factors Ions will be discharged
Position of ions The ions that are LOWER in the ELECTROCHEMICAL SERIES
in the electrochemical will be selectively discharged.
series
Concentration of ions The particular ions with HIGHER CONCENTRATION
in the electrolytes will be selectively discharged
Types of electrodes used ACTIVE ELECTRODES ionises (form ions)
in the electrolysis

27. Position of ions
in the electrochemical series
Cations Anions
K+
F-
Na+
Ca2+ SO42-
Easier to discharge
Mg2+
Al3+
NO3-
which ions will be Zn2+ Cl-
SELELCTED to be Fe 2+
discharged ?
Sn2+ Br-
Pb2+ I-
H+
Cu2+ OH-
Ag+

28. Observation & Product/s
CATHODE ANODE
Sodium Cations:
Carbon
0.1
Colourless gas Colourless gas
sulphate, Na+ mol
bubbles are bubbles are
Na2SO4 H+ dm-3
released. released.
solution,
Anions:
SO42-
OH- H2 gas O2 gas, water
Silver Cations: Carbon 0.1 Silvery grey Colourless gas
nitrate, Ag+ mol solid is bubbles are
AgNO3 H+ dm-3 deposited. released.
solution
Anions:
NO3- Silver, Ag
O2 gas, water
OH- metal

29. Concentration of ions
in the electrolytes
A A
0.001 moldm-3 1.0 moldm-3

30. Products
CATHODE ANODE
Dilute HCl Cations: 0.001
Carbon
Colourless gas Colourless gas
Na+ mol
bubbles are bubbles are
H+ dm-3
released. released.
Anions:
Cl-
OH- H2 gas O2 gas, water

31. Products
CATHODE ANODE
Concentrated Cations: 1.0
Carbon
HCl mol
Colourless gas Greenish-
H+ bubbles are yellow vapour
dm-3
released. is released.
Anions:
Cl-
OH- H2 gas Cl2 gas
Because:
Cl- ions have higher concentration in the electrolyte.
(even though Cl- ion has higher position than OH- ion in the
electrochemical series.)

32. Types of electrodes
A A
Inert electrode Active electrode

33. Products Change
CATHODE ANODE in solution
CuSO4 Cations: A brown
Carbon
0.1 mol dm-3
solution, Colourless The intensity
Cu2+ solid is
gas bubbles
H+
deposited.
are released. of the blue
colour of the
Anions: electrolyte
SO42- decreases.
Copper, O2 gas,
OH- Cu metal water
INERT electrode Because:
concentration of the
blue Cu2+ ions
decreases as more
copper is deposited
at the cathode

34. Observation Change
CATHODE ANODE in solution
CuSO4 Cations:
solution,
0.1 mol dm-3
Copper
Cu2+
H+
Anions:
SO42-
OH-
ACTIVE electrode
 take part in chemical reaction during electrolysis
At anode,
ions present in the electrolyte will not be discharged.

35. Observation Change
CATHODE ANODE in solution
The copper The copper The intensity of
electrode electrode the blue colour of
becomes dissolves into the
the electrolyte
THICKER. solution.
The anode becomes remain unchanged.
THINNER.
Cu2+ + 2e- → Cu Cu → Cu2+ + 2e-
Discharge ionise
Because:
concentration of the blue Cu2+ ions
REMAIN UNCHANGED.
Rate of formation of Cu2+ ions at anode
= Rate of discharged of Cu2+ ions at cathode.

36. Glossary:
A solution produced when a
compound is dissolved in WATER.
Receive/ donate electrons
to form atoms and molecules
Form ions

37. K+
Half equation
Na+
Ca2+
Mg2+
X n+ + n e- → X
Al3+
Zn2+ Eg.: Cu2+ + 2e- → Cu
Fe 2+
Sn2+
Pb2+
H+
Cu2+
Ag+
Au+

38. Observations
K
Na
Ca
Mg
Al Silvery grey solid is deposited at cathode.
Zn
Fe
Sn
Pb
Cu
Brown solid is deposited at cathode.

39. Half equations
F-
2 Y- → Y2 + 2 e-
Cl-
Br- Eg.: 2 Br- → Br2 + 2 e-
I-
OH- 4OH- → O2 + H2O+ 4 e-

40. Observations
A greenish yellow vapour is released at anode.
Cl2 The gas turns the blue litmus paper to red and then
white.
Br2 A brown vapour is released at anode.
The gas turns the blue litmus paper to red and then
white.
I2
A brown liquid is produced at anode.
Colourless gas bubbles are released at anode.
O2 The gas relights a glowing wooden splinter.
Colourless gas bubbles are released at cathode.
H2 The gas gives a ‘pop’ sound when tested with a lighted
wooden splinter when bring near to the mouth of the
test tube.