as electrolysis
TRANSCRIPT
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Electrolysis
(i) Molten Ionic Compounds
e.g molten lead(II) bromide using carbon electrodes
AnodeCathode
Overall reaction:
(ii) Aqueous Ionic Compounds
Factors to be considered when predicting the reactions that occur during
electrolysis of an aqueous ionic compound.
The position of the cation and anion in the electrochemical series
The concentration of the electrolyte
The nature of the electrolyte
The nature of the electrodes
The position in the electrochemical series(ecs)
Cations AnionsK+ SO4
2-
Ca2+ Cl-
Na+ Br-
Mg2+ I-
Zn2+ OH-(H2O)Fe2+
Ease of
discharge
of cation
increases
Ease of
discharge
of anion
increases
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Sn2+
Pb2+
H+(H2O)Cu2+
Ag+
The least electropositive element of the cation of the electrochemical
series will be reduced at the cathode.
The least electronegative element of the anion at the bottom of the
electrochemical will be oxidized at the anode.
e.g electrolysis of aqueous potassium chloride using carbon electrodes
At the cathode:
K+ (aq) + e K(s) higher in ecs
2H2O(l) + 2e H2(g) +2OH-(aq) lower in ecs
Water will be reduced to hydrogen gas ( K+ will not be discharged)
At the anode:
2Cl-(aq) Cl2(g) + 2e higher in ecs
2H2O(l) O2(g) + 4H+(aq) + 4e lower in ecs
Water will be oxidized to oxygen gas( Cl- will not be discharged)
Overall electrolytic reaction:
Try completing the table (carbon electrodes are used):
Electrolyte Cathode Anode
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NaCl(l)Dilute NaCl(aq)CuSO4(aq)H2SO4(aq)Dilute NaOH(aq)
Concentration of the electrolyte
If the concentrations of the ions which could be discharged at an
electrode are very differet, then the ion present in high concentration
may be discharged in preference to the others, event though its ease
of discharged is unfavorable.
e.g electrolysis of concentrated aqueous potassium chloride
At the cathode, hydrogen gas will be discharged.
At the anode, chlorine gas will now be discharged instead of oxygen
gas regardless of its unfavorable ease of discharge.
Nature of the electrolyte
If concentrated aqueous copper (II) chloride is used instead of
concentrated aqueous potassium chloride, the products of electrolysis
will be different.
At the anode, chlorine gas will be formed.
At the cathode,
Cu2+(aq) + 2e Cu(s) lower in ecs than H+
2H2O(l) + 2e H2(g) + 2OH-(aq)
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Copper will be deposited in preference to the evolution of hydrogen
gas.
Nature of electrodes
If the anode is made of a metal such as silver or copper, the anodemay dissolve to form ions in the solution.
e.g electrolysis of aqueous copper(II) sulfate using copper electrodes
At the anode, copper will dissolve into the solution as copper ions.
At the cathode, copper ions will be deposited as copper metal.
(iii) Industrial Processes
(i) Electrolysis of Brine using a Diaphragm Cell
Electrolyte Purified concentrated aqueous sodiumchloride(brine) . (Purification needed to removeCa2+ and Mg2+ which would precipitateinsoluble hydroxides and block the pores of thediaphragm)
Anode Graphite or Titanium coated with Ru-Ti oxideThe electrolyte is added to the anodecompartment. The electrolyte soaks through
the diaphragm into the cathode compartment.Electrolysis takes place.Chlorine gas is discharged preferentially due tothe high concentration of Cl-.
The chlorine gas may be piped from the top ofthe anode compartment; stored in cylinders.
Cathode Steel or NickelAsbestos fibres are deposited in the steel
Carbon
chlorine
Spent
Steel mesh
Steam
NaOH as by-
product
Abestos
H2
NaCl solution
Anode:2Cl-(aq) Cl2(g) +2e
Cathode: 2H2O(l)+ 4e H2 (g) +-
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mesh Diaphragm is porous but separates electrolysisproducts.
Hydrogen gas may be piped out of the cathode
compartment
Na+ are attracted to the cathode.Na+ combine with the OH- produced to formNaOH.NaOH settles at the bottom of the cell.
Overall reaction:
Just a note
Some unreacted sodium chloride will settle at the bottom of the
cell, together with the sodium hydroxide. The solution is
evaporated. Most of the NaCl crystallizes on cooling, leaving a
solution rich in NaOH.
(ii) Extraction of Aluminium form Molten Al2O3 or Cryolite
Preliminary step
Bauxite, Al2O3xH2O, is purified and heated to give aluminium
oxide(alumina Al2O3)
Alumina has a high melting point, is not readily soluble in water and
cannot be an electrolyte.
Preparation of electrolyte:
Sodium hexafluoroaluminate(III) (cryolite, Na3AlF6) is added to
Al2O3.
Cryolite is used because it
- It acts as a solvent and an electrolyte
- Has a much lower melting point than alumina
- Has a lower vapour pressure than molten aluminium
- Has a lower density than molten aluminium
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- Can dissolve sufficient alumina to allow deposition of
aluminium at much lower temperature(energy saving)
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A little fluorspar is added to lower the melting point further.
Small quantity of aluminium fluoride is added to reduce the
solubility of the molten aluminium, when formed, in the molten
cryolite.
Electrolyte Alumina and cryolite (Al2O3 and Na3AlF6)
Anode
The carbon anode is gradually consumed and needs to be
replaced regularly.
Cathode
Aluminium settles at the bottom of the cell and is siphoned
out
Overall reaction:
Carbon
rods(anode)Molten cryolite and
aluminium oxide
Aluminium tap
Carbon lined box(cathode)
Molten
aluminium
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Disadvantages of such extraction method:
o
o
(iii) Electrolytic Purification of Copper
Electrolyte Aqueous copper(II) sulfate
Anode Impure copper
Copper anode dissolves into the electrolyte as Cu2+ which
migrate to the cathode to be reduced to copper.
Cathode Pure copper
The anodic sludge may contain traces of precious metals such as
gold, silver, platinium which are below copper in the ecs.
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