as electrolysis

8/22/2019 As Electrolysis

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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) +-
http://www.google.com/url?sa=i&rct=j&q=&esrc=s&frm=1&source=images&cd=&cad=rja&docid=nQxXXhit_UUuuM&tbnid=RuETvUzFJsk-UM:&ved=0CAUQjRw&url=http://www.electronicspoint.com/audio-controlled-power-supply-t239361p2.html&ei=5fksUpSFBYHQrQfQhICgDw&bvm=bv.51773540,d.bmk&psig=AFQjCNFe28PGqopklHr4558jKzTBeZF3kw&ust=1378765547790882


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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)

-

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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as electrolysis

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