Post on 02-Jan-2016
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
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)