Electrolysis
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Electrolysis. What is the name of the scientist who investigated the theory of electrolysis in 1830?. Definition of Electrolyis Electrolysis is the conduction of electricity by an electrolyte (usually an ionic compound), when molten or dissolved in water , leading to the

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Electrolysis

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Electrolysis

Electrolysis

What is the name of the scientist who investigated the theory of electrolysis in 1830?

Definition of Electrolyis

Electrolysis is theconduction of electricity by an electrolyte (usually an ionic compound), when

molten or dissolved in water, leading to the

decomposition of the electrolyte.


Electrolysis

Electrolytes conduct electricity because they contain

mobileions.

During electrolysis,

electrical energy is converted into

chemical energy.


Electrolysis

Electrolytes

Electrolytes is a compound in solution or molten state that conducts electricity with the decomposition at the electrodes as it does so.

Electrolytes are usually molten ionic compounds or solutions of ionic salts.

Examples: hydrochloric acid, aqueous sodium hydroxide, aqueous sodium chloride


Electrolysis

Non-electrolytes

Electrolytes

Aqueous ethanoic acid

Alcohol

Dilute nitric acid

Sugar solution

Pure deionised water

Molten lead(II) bromide

Copper(II) sulfate

solution

Pure ethanoic acid

Lemon juice

Sodium carbonate solution


Electrolysis

Why do ionic compounds conduct electricity in molten and aqueous states but not in solid state?

In solid state, the

ions are held in fixed positions by

strong electrostatic forces.

In molten or aqueous states, the strong

electrostatic forces are weakened/overcome and the ions are now

mobile. These mobile ions are able to conduct electricity.


Electrolysis

Question for discussion

1. What particles are responsible for electrical conductivity?

  • Mobile electrons

  • present in conductors such as metals & graphite

  • Mobile ions

  • Present in electrolytes such as dilute HCl or molten NaCl


Electrolysis

Question for discussion

2.What substance will remain unchanged when an electric current pass through it?

Metals and graphite (elements)

3.What substance will decompose when an electric current pass through it?

Electrolytes (E.g molten NaCl)


Electrolysis

Cathode

Anode

Anode

Electrolysis of Molten lead (II) bromide

Electrons flow from anode to cathode

Positive

electrode

Negative

electrode

ADD

ADD


Electrolysis

Molten lead(II) bromide contains:

Br-

Pb2+

And

Positive ions, Pb2+, are attracted to negative electrode (cathode)

Always remember:

Cations are attracted to cathode

Negative ions, Br-, are attracted to positive electrode (anode)

Always remember:

Anions are attracted to anode.


Electrolysis

Reactions at the electrodes:

Reactions at the anode:

2

Br- (l)  Br2 (g)

+ 2e-

Recall, in balancing ionic equations,

charge must also be balanced.

Charge

-2

0

-2

Bromide ions lose electrons to form bromine molecules.

Thus in general oxidation occurs at the anode.

Remember: An Ox

Oxidation

Anode

Observation at anode:

Reddish-brown and pungent gas was observed.


Electrolysis

Reaction at cathode,

Pb2+(l) Pb(l)

+ 2e-

+2

-2

0

Charge

Lead(II) ions gain electrons to form lead atoms. Lead (II) ions are

said to be discharged.

In general reduction takes place at cathode.

Remember:

Red Cat

Cathode

Reduction

In the electrolysis of binary molten compounds,

a metal is discharged at the cathode;

a non-metal is produced at the anode.

A binary compound is a compound containing only two elements.

Eg NaCl , PbBr2


Electrolysis

Eqn

at anode

Eqn

at cathode

Molten

electrolyte

Ions

Product

Product

NaCl

Pb2+, O2-

PbO

2 O2- (l)  O2 (g) +4e-

oxygen gas

Pb2+ +2 e-  Pb

lead

metal

Calcium

metal

Cal2

Ca2+, l-

2 I- (l) I2(g) +2e-

iodine

Ca2+ +2 e-  Ca

Try practice 1 on page 3 of notes

Na+, Cl-

2 Cl- (l)  Cl2(g) +2e-

Chlorine gas

(yellowish-

Green gas)

Na+(l) + e-  Na(l)

Sodium

metal


Electrolysis

Question for discussion (NOT in note)

During the electrolysis of molten lead(II) oxide, do you expect the lead form to be solid or liquid? Why?

What information is needed?

Mp of PbO = 888 C

Mp of Pb = 327.5 C

Lead formed would be in molten/liquid form.


Electrolysis

Electrolysis of Solutions

Aqueous electrolytes or solutions are mixtures of two electrolytes : the compound and

water.

Water ionises to a very small extent to give a very small amount of H+ and OH-

H2O  H+ + OH-


Electrolysis

From water

From HCl and water

2H+ (aq) -  H2(g)

+2 e-

Electrolysis of Solutions

Eg 1 : Electrolysis of concentrated hydrochloric acid

Ions present : H+, Cl-, OH-

Ions at cathode:

H+

Eqn at cathode:

Ions at anode:

Cl-, OH-

There are two anions at anode.

Which ion will discharge at anode ?


Electrolysis

Factors in deciding which ion is discharged are:

  • Its position in electrochemical series

  • Concentration of the ion

  • Type of electrode


Electrolysis

Least readily

discharged

Hydrogen (from water)

(Ions formed by metals

above hydrogen in the

electrochemical series are not

discharged.)

Zn2+

Fe2+

Reference

point

Hydrogen

Pb2+ (Depends on concentration)

copper

silver

Most readily

discharged

Electrochemical Series

In an electrochemical series, metals or non-metals are arranged

in order of their reactivity.

Metallic ions

Product at cathode

K+ (Group I)

Na+ (Group I)

Ca2+ (Group II)

Mg2+ (Group II)

Al3+ (Group III)

H+

Cu2+

Ag+


Electrolysis

Oxygen (from water)

(Sulfate and nitrate ion are not

discharged.)

SO42-

NO3-

Least readily

discharged

O2 if electrolyte is

Dilute; respective

Halogens if

Electrolyte is

concentrated

Cl-

Br-

I-

OH-

oxygen

Most readily

discharged

Non-metallic Ions

Product at anode

Note:

When the non-metallic ions are close in their position in the

electrochemical series, order of discharged is influenced by

the concentration of anion.


Electrolysis

E.g In the electrolysis of concentrated HCl, both OH- & Cl-migrate to the anode. But concentration of Cl- is larger than that of OH- .

Hence Cl- is preferentially discharged instead.

2 Cl- (l)  Cl2(g) +2e-


Electrolysis

Eg 2Electrolysis of dilute sulfuric acid

Ions present in dilute sulfuric acid:

H+, OH-, SO42-

Ions migrating to cathode:

H+

Equation representing reaction at cathode:

H+ (aq)  H2(g)

2

+2 e-

Hydrogen gas

Products at cathode:

Observation:

Effervescence of colourless and odourless gas.


Electrolysis

+4e-

OH- (aq)  H2O(l)+ O2(g)

4

2

Eg 2Electrolysis of dilute sulfuric acid

Ions migrating to anode:

OH-

SO42-

Equation representing reaction at anode:

Reason:

sulfate ion which is high up in the electrochemical series is very

stable. A large amount of energy is required for sulfate to give up its electrons, hence it is not discharged. Thus, hydroxide ions, OH- , is preferentially discharged to give oxygen gas.

Product at anode:

oxygen gas (and water)

Observation:

Effervescence of colourless and odourless gas.


Electrolysis

The volume of H2 collected during electrolysis of dilute sulfuric acid is twice that of oxygen. Why?

Anode: 4OH- 2H2O + O2 + 4e-

Cathode: 2H+ + 2e-  H2

For 1 mole of oxygen produced at the anode, 4 moles of electrons are released.

These 4 moles of electrons will reduce hydrogen ions at the cathode to produce 2 moles of hydrogen gas.

Thus volume of hydrogen is twice that of oxygen.

Overall equation can be represented as:

2H2O  2H2 + O2

As water molecules are removed/decomposed, concentration of sulfuric acid increases.


Electrolysis

Electrolysis of Dilute sulfuric acid


Electrolysis

Eg 2Electrolysis of aqueous copper(II) sulfate using

graphite electrodes

Ions present :

H+

Cu2+

SO42-

OH-

Ions migrating to cathode:

H+

Cu2+

Equation representing reaction at cathode:

Cu2+ (aq) + 2e- Cu(s)

Products at cathode:

Copper (brown/pink solid deposited on cathode)

[link]


Electrolysis

4

2

Eg 2Electrolysis of aqueous copper (II) sulfate using

graphite electrodes

Ions migrating to anode:

OH-

SO42-

Equation representing reaction at anode:

OH- (aq)  H2O(l)+ O2(g)

+4e-

Products at anode: oxygen and water


Electrolysis

Eg 2Electrolysis of aqueous copper (II) sulfate using

graphite electrodes

What happens to the electrolyte after some time?

The blue electrolyte fades away and eventually becomes colourless. This is due to the reduction of copper (II) ion to copper atoms, leading to a decrease in concentration of copper (II) ions in solution over time.

What happens to the pH of the electrolyte? Why?

The pH of electrolyte decreases. As hydroxide ions are discharged, concentration of hydroxide ions decreases and is less than that of hydrogen ions, hence making the solution acidic.


Electrolysis

Eqn

at cathode

Eqn

at anode

Electrolyte

Ions

Product

Product

Na+, Cl-,

H+, OH-,

Na+, Cl-,

H+, OH-,

Conc

NaCl

solution

Hydrogen

gas

Hydrogen

gas

2 H+(aq) +2e-H2(g)

Recall Na+ not dishcarged in aq. form.

2 H+(aq) +2e-H2(g)

Recall Na+ not dishcarged in aq. form.

4OH- (aq)2 H2O(l)

+ O2(g)

+4e-

Oxygen

and

water

Chlorine

gas

2Cl- (aq)Cl2(g) +2e-

Dilute

aqueous

potassium

sulfate

K+,

SO42-

H+, OH-

Hydrogen

gas

4OH- (aq)2 H2O(l)

+ O2(g)

+4e-

Oxygen

and

water

2 H+(aq) +2e-H2(g)

Dilute

NaCl

solution


Electrolysis

Reactive electrodes

Electrodes

Inert

electrodes

(Electrode has no reaction

with the electrolyte.

Eg graphite and platinum)

Reactive

electrodes

(Electrode can

dissolve in the

electrolyte.

Eg copper, silver

nickel)


Electrolysis

Copper

electrodes

aq. CuSO4

Go to cathode

Go to anode

Electrolysis of aqueous copper(II) sulfate using copper electrodes

Ions present in aqueous copper(II) sulfate:

Cu2+, H+ , OH-, SO42-

Substances at anode:

OH-, SO42- and copper anode.

Which substance is most easily oxidised?

Copper, a metal, has the greatest tendency to lose electrons and

is most easily oxidised to copper (II) ions.

Hence at the anode, Cu electrode dissolves.

Eqn at anode: Cu (s)  Cu2+(aq)+ 2e-

Hence anode loses weight and becomes smaller/thinner.


Electrolysis

Electrolysis of aqueous copper(II) sulfate using copper electrodes

What happens at the cathode?

Substances at cathode: H+, Cu2+, Copper cathode

Which substance is most easily reduced?

Copper(II) ions which are below hydrogen in the electrochemical series has the greatest tendency to gain electrons.

At the cathode, brown/pink copper is deposited:

Eqn at cathode: Cu2+(aq)+ 2e-  Cu (s)

Hence cathode gains weight and becomes thicker.

The concentration of the electrolyte, copper(II) sulfate,

remains unchanged.


Electrolysis

What happens to the concentration of copper(II) sulfate?

Why?

Cu2+(aq)+ 2e- Cu (s)Cathode

Cu (s)  Cu2+(aq)+ 2e-Anode

The concentration of the electrolyte, copper(II) sulfate,

remains unchanged.

For every mole of copper(II) ion reduced at the cathode,

there is equal no. of mole of copper(II) ions being formed in

solution due to oxidation of copper at anode.


Electrolysis

pure Cu

(cathode)

-

-

+

+

Impure

Cu

(anode)

pure Cu

(cathode)

Impure

Cu (anode)

Impurities

aq. CuSO4

Application of electrolysis using reactive electrodes - Refining copper

The electrolysis of copper(II) sulfate using copper electrodes is used to refine (purify) copper.

  • The impure Cu is theanode.

  • A piece of pure Cu is used at the cathode.

  • Impurities on the anode will fall off. They contain

  • silver and gold.


Electrolysis

Application of electrolysis using reactive electrodes - Refining copper

Reaction at anode:

Cu (s)  Cu2+ (aq)+ 2e-

Each copper atom

loses 2 electrons to form a copper(II) ion. Hence anode slowly dissolves. It loses weight/reduces in size and becomes thinner.

Reaction at cathode:

Cu2+ (aq)+ 2e-  Cu (s)

At the cathode, each copper(II) ion

gains 2 electrons to form a copper atom. Hence cathode gains weight and becomes thicker.

Concentration of electrolyte

remains unchanged.


Electrolysis

-

+

copper anode

object

at

cathode

copper (II) sulfate

solution

Electroplating

Electroplating is the process of coating a metallic

object with another metal by electrolysis.

Electroplating is used for protection against

corrosion and for decoration.

At the anode, each copper atom loses 2 electrons to form

copper(II) ions, hence copper electrode dissolves.

Cu(s)  Cu2+(aq) + 2e-

At the cathode, each copper(II)ion gains 2 electrons and is

discharged to form solid copper.

Copper metal is plated on the object.

Cu2+(aq)+ 2e-  Cu (s)

Video clip

The overall change is for copper to be transferred from the anode to the object.


Electrolysis

Electroplating

  • In electroplating,

  • the object to plated is the cathode.

  • the metal used for electroplating is

  • made the anode.

  • the electrolyte is a solution of ions of

  • the metal.

Different types of electroplating

1. Tin plating

2. Solder plating

3. Chromium plating

4. Nickel plating

5. Silver / gold plating


Electrolysis

After

Before

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