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

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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

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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.