negative electrode (anode)

positive electrode (cathode)

anodecathode

electrolyte

electron flow

+

Electrolysis

What is electrolysis?

• By using electricity to decompose chemical substances in which a redox reaction is forced to take place

What is cathode and anode?

• Red cat, An ox

What is positive and negative pole?

• According to the battery

A. Electrolysis of molten substance by using inert electrode

• No preference of discharge of ions• + pole attracts negative ions, vice versa

Pb2+(l) + 2e- Pb(l)2Br-(l) Br2(g) +2e-

rubber bands

graphite electrodes

wooden piece

electrolysis cell

very dilute sodium chloride solution

partition

e– e–

– +

6 V

Na+

H+

Cl-

OH-

2H+(aq) + 2e- H2(g)

4OH-(aq) O2(g) + 2H2O(l)+ 4e-

B. Electrolysis of dilute solution by using inert electrode

B. Electrolysis of dilute solution by using inert electrode

• Preference of discharge of ions (according to the E.C.S)• negative ions go to anode and discharge in the order: OH-

>I->Br->Cl- >>>>SO42-, NO3

-

• e.g. OH- ions discharge , as it is more readily to give electrons than other negative ions

• positive ions go to anode and discharge in the order : Ag+>Cu2+>H+ >>>>other metal ions

• e.g. H+ ions discharge, as it is more readily to accept electrons.( Cu2+/Ag+ discharges if they are present)

C. Electrolysis of conc solution by using inert electrode

2Cl(aq) Cl2(g) + 2e2H+(aq) + 2e H2(g)

C. Electrolysis of conc solution by using inert electrode

• By concentration effect, the ions to be discharge may be different from the dilution solution.

• e.g. negative ions discharge according to the order of ease to lose e-(s): OH->I->Br->Cl-

• although OH- is more readily to give electrons, however due to concentration effect (I-/Br-/Cl- are present in larger amount), I-/Br-/Cl- are discharged instead.

• e.g. positive ions discharge according to the order of ease to gain e-(s): : Ag+>Cu2+>H+ >>>>other metal ions

• The ion discharge is same as in dilute solution unless Mercury is used as anode.

D. Electrolysis of dilute solution by using non-inert electrode

OH-

SO4-

Cu2+

H+

Cu2+(aq) + 2e Cu(s) Cu(s) Cu2+(aq) + 2e

The solution finally becomes……

D. Electrolysis of dilute solution by using non-inert electrode

• Metals ALWAYS give electrons more readily than OH- ion.

• + pole: metal electrode discharges, and dissolves, but not OH- ion.

• Would the preference of discharge of ions be affected at the – pole by using non-inert electrode?

• NO (metal solids always give electrons)

e.g.1 Electrolysis of dil Zn(NO3)2

dil Zn(NO3)2

Zn Pt

OH-

NO3-

H+

Zn2+

Zn Zn2++2e-

2H++2e- H2

e.g.2 Electrolysis of conc sodium chloride using mercury as cathode

graphite (as anode)

concentrated sodium chloride solution (electrolyte)

bare platinum wire

water

plastic-covered wire

mercury (as cathode)

+

Na+

H+

OH-

Cl-

Reason: 1) Mercury prefers to form alloy with the metal formed. 2) conc effect

?

Na+(aq) + e Na(s)

Na(s) + Hg(l) Na/Hg(l)

sodium amalgam (alloy)

Due to concentration effect,

At the cathode (mercury)At the cathode (mercury)

Reduction

At the anode (graphite)At the anode (graphite)

2Cl(aq) Cl2(g) + 2e

Oxidation

The sodium chloride solution becomes more and more dilute.

Overall reaction: 2Hg(l) + 2Na+(aq) + 2Cl(aq) 2Na/Hg(l) + Cl2(g)

Overall reaction at cathode: 2Na/Hg(l) + 2H2O(l) 2NaOH(aq) +H2(g) + 2Hg(l)

+ (oxidation) (oxidation)anode

(reduction)+ (reduction)cathodePolarity of electrode (and

change)

A redox reaction is forced to take place

A spontaneous redox reaction takes place

Type of reaction involved

A device for bringing about chemical changes by electricity

A device for generating electricity by chemical changes

Function

Electrolytic cellChemical cell

FACTORS AFFECTING THE RATE OF ELECTROLYSIS

Increase

the currentSpeed up

electrolysis

Increase the voltage

Decrease the

resistance

Uses of electrolysis

A. Extraction of some reactive metals

Metals high in the

E.C.S

Metals high in the

E.C.S

Aluminiu

m

Aluminiu

m

Molten chlorides of

metals

Molten chlorides of

metals

Molten oxide of aluminium

Molten oxide of aluminium

electrolysis

electrolysis

e.g. Extraction of Aluminium

O2-

O2-

O2-

O2-

O2-

O2-

O2-

O2-

Al3+ Al3+ Al3+ Al3+ Al3+ Al3+

Al3+ +3e- Al

2O2- O2 + 4e-

B. Purification of metals

e.g. Purify copper metal

+-

OH-

SO42-

Cu2+

H+

What material is – electrode made up of?

What is the assumption?

Cu2+(aq) + 2e Cu(s)

Cu(s) Cu2+(aq) + 2e

C. Electroplating

e.g Electroplating of nickel

- +

Ni2+(aq) + 2e Ni(s)

Ni(s) Ni2+(aq) + 2e

Ni2+

H+

OH-

SO42-

Can a plastic object be electroplated?

Figure 27.18

(a) A set-up for electroplating silver on a spoon.

(b) Electroplating silver on metal cups.

A+ –

silver anode

iron spoon (as cathode)

plating bath

silver nitrate solution

(a)

(b)

Ag+(aq) + eAg(s)

Ag(s) Ag+(aq) + e

Can we use this method to plate zinc on a coin?

This setting only suits for silver, copper, nickel plating.

Criterions:

1) Objects to be plated always place at - pole

2) The electrolyte should contain the plated metal ions.

D. Production of Cl2(g), H2(g), NaOH(aq)

by electrolysis of conc. brine

graphite (as anode)

concentrated sodium chloride solution (electrolyte)

bare platinum wire

water

plastic-covered wire

mercury (as cathode)

+

Cl-OH-

Na+ H+

Na+(aq) + e Na(s)

Na(s) + Hg(l) Na/Hg(l)

sodium amalgam (alloy)

Due to concentration effect,

At the cathode (mercury)At the cathode (mercury)

At the anode (graphite)At the anode (graphite)

2Cl(aq) Cl2(g) + 2e

Overall reaction at cathode: 2Na/Hg(l) + 2H2O(l) 2NaOH(aq) +H2(g) + 2Hg(l)

USEFUL CHEMICALS

E. Anodizing aluminium

The resistance to corrosion of aluminium can be enhanced

by anodizing it using electrolysis. A thicker protective layer of

aluminum oxide is formed on the surface.-+

4OH- O2 + 2H2O + 4e-

2H+(aq) + 2e H2

?

What are the advantages of anodization of aluminium??

WATER POLLUTION PROBLEMS IN HONG KONG

WATER POLLUTION PROBLEMS ASSOCIATED WITH

INDUSTRIAL EFFLUENTS

Liquid wastes of electroplating

Liquid wastes of electroplating

AcidsAcids

AlkalisAlkalis Metal saltsMetal salts

Toxic chemicals

Toxic chemicals

Organic solventsOrganic solvents

Plating sludgePlating sludge

27.10 Water pollution problems in Hong Kong27.10 Water pollution problems in Hong Kong

Figure 27.24 Water pollution due to industrial effluents.

(a) Industrial effluents

(b) Polluted water stream becomes colo

ured.

(c) Polluted water stream flows into the

sea

Figure 27.24 Water pollution due to industrial effluents.

(d) Dark blue water stain in harbour.

1)acids and alkalis e.g. H2SO4 [remove oxides]; NaOH [removes oil]

kill water plants and animals increase rusting rate of metal pipes and ships

2)heavy metal ions e.g. Cu2+ , Ni2+, Cr(VI), Pb2+

toxic to water plants and animals

3)cyanide (CN -) very toxic

1) Reducing the volume of waste solution (e.g. use less water for rinsing the object)

2) Recycling of the waste electrolyte.

3) Removal of toxic substances before disposingCommon methods are :

a) Adding Na2CO3 to remove the acids. e.g.Na2CO3 + H2SO4 Na2SO4 + CO2 + H2O

b) Adding NaOH to precipitate the metal ions. e.g.2NaOH(aq) + NiSO4(aq) Na2SO4(aq) + Ni(OH)2(s)

4) Reduce very toxic Cr(VI) compounds into less toxic Cr(III) ions.

CONTROL OF CHEMICAL WASTE DISPOSAL IN

HONG KONG

Chemical waste producers such as electroplating factories are

required to register with the Environmental Protection

Department (EPD) for proper treatment and disposal of chemical

wastes.

Figure 27.26 The Chemical W

aste Treatment Centre (CWT

C) at Tsing Yi Island of Hong

Kong. The Centre, having ope

rated since May 1993, is capa

ble of treating a great variety

of chemical wastes.