ELECTROCHEMISTRY AS

Redox reactions Oxidation : loses electrons/oxidation

number increases /loses hydrogen/accepts oxygen

Reduction : accepts electrons/oxidation number decreases /accepts hydrogen/loses oxygen

Calculation of oxidation numbers

APPLICATIONS OF ELECTROLYSIS 1.Electrolysis is the chemical

decomposition of a substance (electrolyte) by an electric current

2.Electrodes : Anode ( + ) Cathode ( -)

3.Electrolyte : ionic compound ( molten or aqueous solution )

Consists of cations ( positive ions ) And negative ions ( anions ) 4.During electrolysis : a.Cations → cathode ( reduction ) b.Anions → anode ( oxidation )

5.Example : Electrolyte : molten PbBr2

Cathode (reduction) : Pb2+ (l) + 2e- → Pb (s) Anode ( oxidation ) : 2 Br- (l) - 2e- → Br2 (g)

More than one cation and / or anion selective discharge

EXTRACTION OF ALUMINIUM

a.Extracted by electrolysis of molten salts ( not aqueous solutions )

b.Importance : Al widely used due to its properties

Light , strong , good electrical conductor ,does not corrode

c.Extracted from ore : bauxite d.Electrolyte : pure alumina ( Al2O3 ) and

cryolite ( Na3AlF6 )

Function of cryolite : to lower m.p of Al2O3 ( from 2050o C to about 950o C )

e.Electrodes : graphite anode and cathode

f.Equations : i)Cathode : Al3+ (l) + 3e- → Al (s) ii)Anode : 2 O2- (l) → O2 (g) + 4e-

Iii)Overall equation : 2 Al2O3 (l) → 4 Al(s) + 3 O2 (g)

g.Al which is more dense sinks to the bottom and is syphoned off

h.At the high operating temp ,carbon anode blocks replaced often due to oxidation to CO2 by O2 evolved

C (s) + O2 (g) → CO2 (g) , H highly exothermic ,heat evolved helps to

partly maintain electrolyte in molten state i.Pollutants : i) CO(g) : from incomplete combustion of anode ii) fluorine : from cryolite : 2F- → F2 + 2e-

Fluorine is corrosive and toxic

PURIFICATION OF COPPER

a.Impure Cu obtained by roasting its ore in air , then purified by electrolysis.

b.Electrodes : Anode ( impure copper ) Cathode ( pure copper )

c.Electrolyte : aqueous Cu2+ (eg aqueous CuSO4 )

d.Equations : i)Anode : Cu (s) → Cu2+ (aq) + 2e-

ii)Cathode : Cu2+ (aq) + 2e- → Cu (s) Copper transferred from anode to

cathode Observations : Anode dissolves Pure copper deposited at cathode

e.Impurities : example i) metals more reactive/electropositive than Cu :

Fe and Zn ii) metals less reactive/electropositive than Cu :

Ag and Au f.Fe and Zn also ionises , enters solution as

Fe2+ (aq) and Zn2+ (aq) However at cathode : only Cu2+ discharged. Fe2+ and Zn2+ remains in solution

g.Ag and Au remain undissolved and fall to the cell bottom as anode

sludge ( from which they can be recovered )

ELECTROLYSIS OF BRINE(concentrated NaCl)

a.Using the diaphragm cell Consists of 2 chambers ( anode and

cathode chamber ) b.Electrolyte : purified brine Purification removes Mg2+ and Ca2+

which may form insoluble hydroxides that then clogs the diaphragm

c.Electrodes : i)Anode : titanium or inert electrode

(graphite) it resists corrosion by the very reactive

chlorine formed

ii)Cathode : steel or graphite

Diagram

Steel cathode (1)

Titanium anode (1)

Container + compartment + electrodes + diaphragm (1)

d. Equations : i) Anode : 2Cl- -> Cl2(g) + 2e-

ii) Cathode : 2H+ (aq) + 2e- → H2 (g)

Another possible way of writing equations : From H2O : H+ (discharged) and OH- (unchanged) Cathode : 2H2O + 2e- → H2 + 2OH-

From NaCl : Cl- (discharged) and Na+(unchanged) Anode : 2Na+ + 2Cl- - 2e- → Cl2 + 2Na+ Overall equation : 2H2O + 2NaCl → H2 + 2NaOH + Cl2

Molar ratio of products : H2 : NaOH : Cl2 = 1 : 2 : 1

e. Na+ goes through diaphragm to cathode chamber

f. NaOH forms through the following reaction :

Na+ + OH- → NaOH (aq) and flows out of cell NaOH used as detergent , soap , paper

industries

g.Level of brine on left side (anode) is g.Level of brine on left side (anode) is deliberately higher than the right side deliberately higher than the right side ( cathode) ………. ( cathode) ………. WHYWHY??

SO THAT THE BRINE WILL SLOWLY FLOW THROUGH THE ASBESTOS DIAPHRAGM TOWARDS THE CATHODE, CARRYING THE SODIUM IONS WITH IT AND PREVENTING THE REVERSE FLOW OF SODIUM HYDROXIDE TOWARDS THE ANODE WHERE IT WILL REACT WITH THE CHLORINE.

h. Products obtained : i) chlorine Uses: purify water supply disinfectant , bleach used in plastics , polymers (eg PVC)

ii) hydrogen Uses : manufacture of ammonia ,

margarine and HCl , as fuel iii) aqueous NaOH Uses : manufacture of soap, paper and

detergent

Note : If question specifies manufacture of

chlorine from electrolysis of brine. Chlorine ( main product ) Hydrogen and NaOH ( by products )

i. Other products : i)H2 and Cl2 can be combined to make HCl:

H2 + Cl2 -> 2HCl

ii) Cl2 and NaOH(aq) :

(1)Cl2 and cold NaOH(aq) ( 15o C) produces

sodium chlorate(I), NaClO NaClO used as : bleach and disinfectant

Cl2 + 2NaOH -> NaCl + NaClO + H2O

Or Cl2 + 2OH- -> Cl- + ClO- + H2O

0 -1 +1Type of reaction : Chlorine undergoes

disproportionation

Oxidation no of Cl increases from 0 to +1(oxd)

and decreases from 0 to -1(red)

(2)Cl2 and hot NaOH(aq) ( 70o C) produces sodium chlorate(V), NaClO3

NaClO3 used as : weedkiller

3Cl2 + 6 NaOH -> 5NaCl + NaClO3 + 3H2O

Or 3Cl2 + 6OH- -> 5Cl- + ClO3- + 3H2O

0 -1 +5

Type of reaction : Chlorine undergoes disproportionation