Options(electrochemistry)
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History
• Galvani noted the muscular reaction in
dissected frog’s legs when they were in
contact with two different metals.
Allesandro Volta
• Volta produced cells and
batteries by separating
copper and zinc plates with
salt solution.
Humphry Davy
• Humphry Davy passed electric
current into molten salts and
electrolysed them. Discovered
Na, K, Ca, Mg, Ba and Sr.
Michael Faraday
• Michael Faraday worked
with Davy. He discovered
the electric motor.
• Furthered the study of
electrolysis (Faraday’s
laws).
Electrochemistry
• A simple cell consists of two different metals
immersed in a solution of ions. The voltage of
the cell depends on the difference in activity
between the two metals.
• The standard electrode potential is measured
against the hydrogen electrode.
A TYPICAL COMBINATION OF HALF CELLS
1.10V
ZINC
COPPER
ZINC
SULPHATE (1M)
COPPER
SULPHATE (1M)
E° = - 0.76V
E° = + 0.34V
zinc is more reactive - it dissolves to give ions
Zn(s) ——> Zn2+(aq) + 2e¯
the electrons produced go round the external circuit to the copper electrode
electrons are picked up by copper ions
As a result, copper is deposited
overall reaction
Cu2+(aq) + 2e¯ ——> Cu(s)
Zn(s) + Cu2+(aq) ——> Zn2+(aq) + Cu(s)
Corrosion
Corrosion is any undesired process whereby a
metal is converted to one of its compounds.
It can be prevented by:
–
–
–
–
Painting and greasing.
Galvanising-coating with zinc
Coating the surface with another metal-tin can
Using sacrificial anodes. Iron placed in contact with a
metal that is more easily oxidised
– Alloying with another element- stainless steel is a
mixture of Iron, chromium and carbon.
Rusting of Iron
Electrolysis of Molten Salts.
Electrolysis of molten
Lead
Bromide(PbBr2).
• Cathode reaction
Pb2+ + 2e– Pb
• Anode reaction
2Br– Br2 + 2e–
Electrolysis of Sodium from molten
Sodium Chloride(Downs cell)
The Downs cell consists of a graphite anode(doesn’t
react with chlorine) surmounted by a hood to
remove chlorine gas, and a circular steel cathode.
Added to the Sodium Chloride is Calcium Chloride
which helps to reduce the melting point.
Cathode reaction:
Na+ + e-  Na
Anode Reaction:
Cl- -e-  ½ Cl2
Downes cell
3. Extraction of Aluminium from
Bauxite
2 stages
Aluminium oxide extracted from Bauxite.
1. Bauxite is crushed and mixed with hot Sodium Hydroxide.
2. Digestion- The mixture is heated and the hydrated Aluminium oxide
reacts with the Sodium Hydroxide to form Sodium Aluminate.
AL2O3.3H2O +2NaOH  2NaAlO2 + 4H2O
3. Removal of impurities by filtration.(Red Mud)
4. Precipitation of Aluminium Oxide(Seeding)
4NaAlO2 + 4H2O  Al2O3.3H2O + 2NaOH
5.Removal of water of Crystallisation
Production of Aluminium from
Alumina.
• In the smelter the alumina is heated with
cryolite which lowers the mp.
• Anode and Cathode are made of Graphite
Production of Aluminium from
Alumina
• During the electrolysis, the following reactions
occur:
• Cathode: 2Al3+(l) + 6e- → 2Al(l)
• Anode:
3O2-(l) - 6e- → 11/2O2(g)
• The molten aluminium formed settles at the
bottom of the tank, and is siphoned from
there.
• The electrolysis is carried out at a high
temperature, at which the large carbon
anodes are steadily burnt away by the oxygen
liberated there, and have to be replaced at
regular intervals.
• The electrolysis of alumina consumes
enormous amounts of electricity, and is
therefore normally carried out where
electricity is relatively cheap.
Uses Of Aluminium
• Used to make strong, light, corrosion-resistant
alloys, which are used in the construction of
aeroplanes and ships.
• Aluminium is a good conductor of heat and so
is used to make saucepans and as a cooking
foil.
• It is an excellent electrical conductor, and so is
used in electricity wires.
Recycling of Aluminium
• The recycling of aluminium is an economically
viable process.
• It is also desirable from an environmental
point of view, saving energy and conserving
natural resources.
• When aluminium is being recycled, the
aluminium waste is melted.
• The impurities are either burned off or
separated from the aluminium
Anodising
• The purpose of anodising is to produce a very
hard oxide coating on the surface of
aluminium, or one that is very resistant to
corrosion.
• In anodising, the aluminium is used as an
anode in the electrolysis of dilute sulfuric acid.
• Graphite, platinum or another inert cathode
can be used.
Anodising
The Aluminium is coated with a thicker layer of
aluminium oxide to prevent corrosion.
Anode Reaction
2Al +3H2O  Al2O3 + 6H+ + 6eCathode Reaction
2H+ + 2e-  H2
Manufacture of Iron and Steel
Iron is extracted from its
ores in a blast furnace.
The raw materials are:
1.Iron ore, mainly
haematite(Fe2O3)
2.Coke, a form of Carbon
3.Limestone CaCO3
The PROCESS
Hot air is blasted through to burn the coke and
to form carbon monoxide
1.The coke burns in the furnace to melt the Iron
and it makes Carbon Monoxide
C + O2  CO2
C + ½ O2  CO
2. The coke reduces the iron oxide
3C + Fe2O3  2Fe + 3CO
3. The Carbon Monoxide also reduces the iron
oxide.
3CO + Fe2O3  2Fe + 3CO2
4. The limestone decomposes to lime which
reacts with impurities in the ore, mainly SiO2 to
form a slag.
CaCO3  CaO + CO2
CaO + SiO2  CaSiO3
The Manufacture of Steel
Pig iron is changed to steel by
• Removing the impurities
• Adjusting the amount of carbon
The process for making steel is called
the basic oxygen steel-making process
or BOS process.
Oxygen gas is blown through the oxygen lance at high pressure, this reacts with
impurities such as carbon and sulphur and oxidises them. These leave as gas (carbon
dioxide and sulphur dioxide) and pure iron remains.
Calculated amounts of carbon or other metals are added to make a range of different
alloys.
The Electric Arc method of
manufacturing steel.(Video)
• Pig iron, scrap iron and lime CaO are added to
the furnace. Oxygen is blown through to
oxidise the impurities. The lime removes other
impurities. Other metals are added depending
on the type of steel required.
Electric Arc Process
The main stages in the process are:
1. Charging the furnace-iron, scrap iron and lime are
added to the furnace
2. Melting- large current passed through carbon
electrodes to melt the iron.
3. Refining.
CaO + SiO2  CaSiO3 (Slag)
4. Tapping-removal of Slag
5. Casting- the steel is molded and cut into required
lengths.