electrolysis part 3 aqueous solution
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Electrolysis of an aqueous solution
6th & 8th July 20104S4
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
Learning outcomes
Electrolysis of an aqueous solution
solutionDilute
NaCl
solution
Carbon
electrodes
Electrolyte: Molten NaCl Electrolyte: NaCl solution
Cations Anions
H+ OH-Na+ Cl-
Cations Anions
Na+ Cl-
Sodium chloride solution
Cations Anions
H+ OH-
Na+ Cl-
Exercise:Solution Cations Anions
Dilute sulphuric acidH2SO4
H+ SO42- , OH-
Silver bromide solutionAgBr
Ag+, H+ Br- , OH-
Sodium nitrate solutionNaNO3
Na+, H+ NO3-, OH-
Electrochemical series
a series to show the tendency of the ions to donate/ receive electrons.
Cations
K+
Na+
Ca2+
Mg2+
Al3+
Zn2+
Fe 2+
Sn2+
Pb2+
H+
Cu2+
Ag+
Au+
Anions
F-
SO42-
NO3-
Cl-
Br-
I-
OH-
Easi
er t
o d
isch
arge
Electrochemical
series
Homework for this week
Copy & Memorise
the ELECTROCHEMICAL SERIES
Quiz on next week
Electrochemical series
K+ Potassium
Na+ Sodium
Ca2+ Calcium
Mg2+ Magnesium
Al3+ Aluminium
Zn2+ Zinc
Fe 2+ Iron
Sn2+ Tin
Pb2+ Lead
H+ Hydrogen
Cu2+ Copper
Ag+ Silver
Au+ Gold
Yourown
Pork Potential Pretty Pop
Sometimes Scientists Sally Stars
Can Can Could Can
Make Make Marry Make
A A A Any
Zulu Zoo Zulu Zombie
ill ? in in
Therefore Tiny Lovely Town
Let London Peace Lose
Highly ? Honolulu His
Clever City Causing Control
Slay Strange ?
Good Pigs Gazes ?
Anions
F-
SO42-
NO3-
Cl-
Br-
I-
OH-
Your own
Fat Flower
‘So So
Nui’ Nice
Clean Can
Brother’s Buy
I-phone’s In
‘O’ ready Orchard
Ions that are present
Copper(II) sulphate, CuSO4 solution
consists of
Cu2+ ions, H+ ions, SO42- ions , OH- ions
that move freely.
electrodes Carbon
Competing ions Cu2+ ions, H+ ions
Cationsselectively discharged
Cu2+ ions
Why?Cu2+ ion is lower than H+ ions
in the electrochemistry series.
Half equation Cu2+ + 2 e- → Cu
ObservationA brown solid is deposited
at cathode.
ProductsCopper, Cu metal is formed
at the cathode.
CATHODE ANODE
electrodesCarbon
Competing ionsSO4
2- ions , OH- ions
Anions selectivelydischarged
OH- ions
Why?OH-ion is lower than SO4
2- ions
in the electrochemistry series.
Half equation4OH- → O2 + 2H2O + 4 e-
ObservationColourless gas bubbles
are released at anode
Productsoxygen gas, O2 and water, H2O
are produced at the anode.
CATHODE ANODE
electrodes Carbon
Competing ions Cu2+ ions, H+ ions
Cationsselectively discharged
Cu2+ ions
Why?Cu2+ ion is lower than H+ ions
in the electrochemistry series.
Half equation Cu2+ + 2 e- → Cu
ObservationA brown solid is deposited at
cathode.
Products Copper, Cu metal is formed at the cathode.
Ions that are presentCu2+ ions, H+ ions, SO4
2- ions , OH- ions that move freely.
Copper(II) sulphate, CuSO4 solution consists of
CATHODE ANODE
electrodesCarbon
Competing ionsSO4
2- ions , OH- ions
Anions selectivelydischarged
OH- ions
Why?OH-ion is lower than SO4
2- ions
in the electrochemistry series.
Half equation4OH- → O2 + 2 H2O + 4 e-
ObservationColourless gas bubbles are
released at anode
Productsoxygen gas, O2 and water, H2O
are produced at the anode.
Describe Electrolysis of an aqueous solution
Example: Copper(II) sulphate, CuSO4 solution0.1 mol dm-3
Carbon electrodes
• 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.
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.
• A brown solid is deposited at cathode.
• Copper, Cu metal is formed at the cathode.
Cu2+ (aq) + 2 e- → Cu (s)
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 SO4
2- ions in the electrochemical series.
• 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.
4OH- (aq) → O2 (g) + 2H2O(l) + 4 e-
2Cu2+ (aq) + 4 e- → 2Cu (s)
4OH- (aq) → O2 (g) + 2H2O(l) + 4 e-
Overall equation:
2Cu2+ (aq) + 4OH- (aq) → 2Cu (s)+ O2 (g) + 2H2O(l)
Describe Electrolysis of an aqueous solution
Copper(II) chloride, CuCl2 solution0.1 mol dm-3
Carbon electrodes
Sodium chloride solution
Cations Anions
H+ OH-
Na+ Cl-
• Position of ions in the electrochemical series
• Concentration of ions in the electrolytes
• Types of electrodes used in the electrolysis
The selection of ions to be discharged is based on:
Sodium chloride solution
Cations Anions
H+ OH-
Na+ Cl-
Copper (II) sulphatesolution
Cations Anions
Cu2+
H+
SO42-
OH-
3 Factors :
Factors Ions will be discharged
Position of ions in the electrochemical series
The ions that are LOWER in the ELECTROCHEMICAL SERIES will be selectively discharged.
Concentration of ions in the electrolytes
The particular ions with HIGHER CONCENTRATIONwill be selectively discharged
Types of electrodes used in the electrolysis
ACTIVE ELECTRODES ionises (form ions)
Factors that affect the electrolysis of an aqueous solution
Cations
K+
Na+
Ca2+
Mg2+
Al3+
Zn2+
Fe 2+
Sn2+
Pb2+
H+
Cu2+
Ag+
Anions
F-
SO42-
NO3-
Cl-
Br-
I-
OH-
Easi
er t
o d
isch
arge
Position of ions
in the electrochemical series
which ions will be SELELCTED to be
discharged ?
Sodium
sulphate,
Na2SO4
solution,
Cations:
Anions:
Carb
on
0.1
mol
dm-3
Silver
nitrate,
AgNO3
solution
Cations:
Anions:
Carb
on
0.1
mol
dm-3
Observation & Product/s
CATHODE ANODE
H2 gas O2 gas, water
Silver, Ag metal
O2 gas, water
Na+
H+
SO42-
OH-
Ag+
H+
NO3-
OH-
Colourless gas bubbles are released.
Colourless gas bubbles are released.
Silvery grey solid is deposited.
Colourless gas bubbles are released.
Concentration of ions
in the electrolytes
A A
0.001 moldm-31.0 moldm-3
Dilute HCl Cations:
Anions:
Carb
on
0.001
mol
dm-3
Products
CATHODE ANODE
Na+
H+
Cl-
OH- H2 gas O2 gas, water
Colourless gas bubbles are released.
Colourless gas bubbles are released.
Concentrated
HClCations:
Anions:
Carb
on
1.0
mol
dm-3
Products
CATHODE ANODE
H+
Cl-
OH-H2 gas Cl2 gas
Colourless gas bubbles are released.
Greenish-yellow vapouris released.
Because: Cl- ions have higher concentration in the electrolyte. (even though Cl- ion has higher position than OH- ion in the electrochemical series.)
Types of electrodes
A A
Inert electrode Active electrode
CuSO4
solution,
Cations:
Anions:
Carb
on
0.1
mol d
m-3
Products
CATHODE ANODE
Changein solution
Cu2+
H+
SO42-
OH-
The intensity of the blue colour of the electrolyte decreases.
INERT electrode Because: concentration of the blue Cu2+ ions decreases as more copper is deposited at the cathode
Copper, Cu metal
O2 gas, water
A brown solid is deposited.
Colourlessgas bubbles are released.
CuSO4
solution,
Cations:
Anions:
Co
pp
er
0.1
mol d
m-3
Observation
CATHODE ANODE
Changein solution
SO42-
OH-
Cu2+
H+
ACTIVE electrode take part in chemical reaction during electrolysis
At anode,
ions present in the electrolyte will not be discharged.
Observation
CATHODE ANODE
Changein solution
Because: concentration of the blue Cu2+ ions REMAIN UNCHANGED.
The copperelectrode becomes THICKER.
The copperelectrodedissolves into the solution.The anode becomes THINNER.
Cu2+ + 2e- → Cu Cu → Cu2+ + 2e-
The intensity of the blue colour of the electrolyte remain unchanged.
Rate of formation of Cu2+ ions at anode= Rate of discharged of Cu2+ ions at cathode.
Discharge ionise
Glossary:
Form ions
Receive/ donate electrons to form atoms and molecules
A solution produced when a compound is dissolved in WATER.
Half equation
X n+ + n e- → X
K+
Na+
Ca2+
Mg2+
Al3+
Zn2+
Fe 2+
Sn2+
Pb2+
H+
Cu2+
Ag+
Au+
Eg.: Cu2+ + 2e- → Cu
Observations
Silvery grey solid is deposited at cathode.
K
Na
Ca
Mg
Al
Zn
Fe
Sn
Pb
CuBrown solid is deposited at cathode.
Half equations
F-
Cl-
Br-
I-
2 Y- → Y2 + 2 e-
4OH- → O2 + H2O+ 4 e-OH-
2 Br- → Br2 + 2 e-Eg.:
ObservationsA greenish yellow vapour is released at anode.
The gas turns the blue litmus paper to red and then white.
A brown vapour is released at anode.
The gas turns the blue litmus paper to red and then white.A brown liquid is produced at anode.
Br2
I2
Cl2
O2
Colourless gas bubbles are released at cathode.
The gas gives a ‘pop’ sound when tested with a lightedwooden splinter when bring near to the mouth of thetest tube.
H2
Colourless gas bubbles are released at anode.
The gas relights a glowing wooden splinter.
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