Extraction
of metals
Define oxidation in terms of gain of
oxygen and reduction in terms of loss of oxygen (C4.01)
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The reaction of a substance with oxygen is obviously
oxidation:
2Mg(s) + O2(g)
à
2MgO(s)
CH4(g) + 2O2(g)
à
CO2(g) + 2H2O(l)
C6H12O6(s) + 6O2(g)
à
6CO2(g) + 6H2O(l)
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*
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Reduction can be seen as the inverse process where oxygen
is removed:
CuO(s) + H2(g)
à
Cu(s) + H2O(g) on heating
Fe2O3(s) + 3CO(g)
à
2Fe(l) + 3CO2(g) in blast furnace
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In the latter reaction the carbon monoxide has been
oxidised according to the first definition.
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Oxidation and reduction always occur together.
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Define oxidation in terms of loss of
electrons and reduction in terms of gain of electrons (C4.02)
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The reaction of magnesium with oxygen
2Mg(s) + O2(g)
à
2MgO(s)
gives a compound containing Mg2+
ions.
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The reaction of magnesium with
chlorine gives ionic magnesium chloride
Mg(s) + Cl2(g)
à
MgCl2(s)
which also contains Mg2+
ions. From the point of view of the magnesium the same process has
occurred:
Mg
à
Mg2+ + 2e—
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Therefore oxidation is loss
of electrons.
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The reaction of copper(II) oxide
with hydrogen is reduction according to the first definition; it also
results in copper(II) ions changing to copper metal, i.e.
Cu2+ + 2 e --
à
Cu
so that reduction is gain
of electrons.
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OILRIG: Oxidation
Is Loss
Reduction is
Gain (of
electrons).
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Oxidation and reduction occur
during electrolysis:
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At
the anode electrons are removed from the negative ion; at the
Anode
you get oxidAtion.
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At
the cathode electrons are added to the positive ion; at the
Cathode
you get reduCtion.
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Understand that the extraction of
metals involves reduction of their ores (C4.03)
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Apart from gold and silver which occur native (i.e. as
the metal) metals have to be extracted from their ores, in which the
metal exists as positive ions; Fe2+ in Fe2O3,
Al3+ in Al2O3, Na+ in NaCl.
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The extraction involves adding electrons to the metal ion
to make the metal, i.e. it is reduction.
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Recall how the way in which a
particular metal is extracted from its ores is related to its position
in the reactivity series (C4.04)
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If a metal is reactive it forms ions easily; therefore
conversion of its ions to the free metal is difficult.
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Reactive metals (alkali (group 1) and alkaline earth
(group 2) metals, and aluminium) have to be extracted using electrolysis
to reduce the ions.
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Less reactive metals (Zn, and other metals less reactive
than Zn) can be extracted using carbon or carbon monoxide to reduce the
ores, which are usually oxides or have been converted to the oxide.
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Understand that
processes involving the use of large amounts of electricity are
relatively expensive (C4.05)
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Electrolysis uses very large amounts of electricity, which is expensive
in terms of fuel costs.
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The
process is much more expensive than reductions using C or CO; thus
aluminium, the commonest metal in the Earth’s crust, is expensive,
whereas iron is rather cheap.
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Describe the
extraction of aluminium from purified bauxite including simple cell
diagram, nature of electrolyte and electrodes, and reactions (C4.06)
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Bauxite is a mixture of aluminium oxide, iron oxide, silicon dioxide and
variable amounts of water.
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Pure
aluminium oxide is extracted from bauxite and must be electrolysed.
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The
aluminium oxide cannot be melted on an industrial scale since its
melting temperature is over 2000oC. It is dissolved in molten
cryolite, Na3AlF6, at about 950oC.
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The
anode and the cathode of the cell are both graphite.
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At the
anode, oxide ions are oxidised to oxygen which immediately reacts with
the graphite anode, this having to be replaced from time to time:
2O2
– à
O2 + 4e –
C + O2
à
CO2
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Aluminium ions are reduced to molten aluminium at the cathode:
Al3+
+ 3 e – à
Al
The
molten metal is siphoned out of the cell from time to time.
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Understand why
aluminium is less reactive than expected and appreciate how anodising is
achieved and the change that takes place during the process (C8.01)
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Aluminium is much less reactive than might be expected from its position
in the Periodic Table and from the difficulty of its extraction from
aluminium oxide.
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The
lack of reactivity is due to a layer of aluminium oxide which sticks
strongly to the surface of the metal and protects it from attack.
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The
thickness of the oxide layer is increased by anodising; the aluminium
object is made the anode of an electrolytic cell using a dilute
sulphuric acid electrolyte:
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At the
anode oxygen is liberated from hydroxide ions in the electrolyte:
4OH
– (aq)
à
O2(g) + 2H2O(l)
The
oxygen is liberated as very fine bubbles and reacts with the aluminium
to form aluminium oxide – the layer can be made thicker by electrolysing
for a longer time.
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The
lead cathode is inert; hydrogen is liberated there from H+
ions in the electrolyte.
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The
aluminium oxide layer can be coloured in the electrolytic process by
incorporating dyestuffs into the electrolyte.
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Anodised objects can be made highly abrasion resistant by
giving them thick oxide coatings.
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Relate the uses of
aluminium to its properties (C4.07)
Appreciate the need
for alloying aluminium to increase its strength (C8.02)
Understand the
important uses of aluminium and its alloys (C8.03)
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Aluminium is
-
soft
and of low density – it needs to be alloyed to be useful for
engineering applications
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malleable and ductile
-
a
very good conductor of electricity
-
resistant to corrosion (see below)
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It is
used as an alloy (Duralumin) with magnesium and some copper for making
aircraft (because of low density; not ‘light’) and with magnesium
for car bodies (Birmabrite).
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It is
used with a steel core (for strength) for overhead power lines, and
without the steel for underground cables.
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It is
used to make a wide variety of items for construction, e.g. window
frames, because of its ease of extrusion into complex shapes and its
corrosion resistance.
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Recall that carbon and
carbon monoxide can reduce the oxides of less reactive metals (C4.08)
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A wide
variety of oxides of less reactive metals can be reduced by carbon or
carbon monoxide:
Fe2O3
+ 3C à
2Fe + 3CO
Fe2O3
+ 3CO à
2Fe + 3CO2
CuO +
C à
Cu + CO
CuO +
CO à
Cu + CO2
Heating the oxides in a blast furnace with coke will give both reactions
at the same time, depending on the temperature.
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Carbon
or carbon monoxide reduction of oxides can be used to obtain zinc and
metals less reactive than zinc.
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The
extraction of iron is the largest use of the process.
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Describe the
extraction of iron in the blast furnace, including outline diagram, raw
materials, reactions and the formation and uses of slag (C4.09)
Explain the chemical
reactions occurring in different parts of the blast furnace and the
energy changes associated with them (C8.04)
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The
structure of the blast furnace is essentially:
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The
top of the furnace is charged with a mixture of:
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coke: this produces heat and the reducing agent, CO;
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iron
ore, either Fe2O3 or Fe3O4
-
limestone, used to remove silica impurity from the ore.
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Coke
is added to produce heat and the reducing agent, CO; these reactions
occur near the air inlet.
C + O2
à
CO2 +
heat;
an exothermic reaction.
CO2
+ C à
2CO an endothermic reaction
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CO
reduces the iron ore; there is also some reduction with unburnt carbon
higher up the furnace. The reaction with CO is endothermic and uses
about half the energy in the furnace:
Fe2O3
+ 3CO
à
2Fe + 3CO2
Fe2O3
+ 3C à
2Fe + 3CO
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The
main impurity in the ore is silica, SiO2. This is acidic and
reacts with CaO (a base) to give molten slag CaSiO3. CaO is
produced from limestone by heating in an endothermic reaction:
CaCO3
à
CaO + CO2
CaO +
SiO2
à
CaSiO3
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The
molten slag floats on top of the molten iron.
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It
matters how impure the iron ore is – removal of SiO2 as slag
is an endothermic process and therefore costs fuel.
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Slag
is used to make building blocks and for road-fill.
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Understand that impure
iron from the blast furnace and pure iron have very limited uses (C8.05)
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Pig-iron from the blast furnace contains about 4% carbon.
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It
is strong but very brittle so will not withstand sharp blows.
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It
is used for street furniture – bollards, drain and manhole covers, for
example.
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Describe the
production of mild steel by lowering the carbon content in the impure
iron using high pressure oxygen (C8.06)
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The
carbon content of pig-iron is lowered by blowing oxygen through the
molten iron – this oxidises the carbon to gaseous CO2.
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Mild
steel contains about 0.15% carbon and is the most widely-used steel.
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Understand the uses of
mild steel (C8.07)
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Mild
steel is used for almost all non-specialist steel products – cars,
domestic goods, constructional steel.
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Mild
steel rusts easily in presence of air and water, so alloy steels are
used where this is such a serious problem that the much greater coast of
alloys is acceptable.
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Understand the uses of
alloy steels such as stainless steel, titanium steel and manganese steel
(C8.08)
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Stainless steel consists of an alloy of iron, chromium and nickel:
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It
is expensive so is not used for large-scale construction.
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Used
for cutlery, kitchen fittings, surgical instruments, exhaust systems
for cars (at a price!).
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Stainless steel is attacked by high concentrations of acids.
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High-quality stainless steel is non-magnetic. It will contain 18% Cr
and 8% Ni and is called ‘18:8 stainless’.
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Alloy
steels are used for various purposes:
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Titanium steels are used for cutting tools;
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Vanadium steels are used for spanners and springs;
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Manganese steel is especially hard and is used for safes;
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Alnico contains Al, Ni and Co in addition to iron and is used for
permanent magnets.
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Describe the
purification of copper by electrolysis, including a simple diagram of
the cell (C4.10)
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Copper
from the blast furnace contain numerous impurities; it is purified by
electrolysis using the cell shown:
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The
copper anode is not an inert electrode – it does not remove electrons
from an ion in solution, but loses electrons itself.
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At the
anode copper is oxidised to copper(II) ions:
Cu
à
Cu2+ + 2 e –
These
dissolve into solution.
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The
impurities:
§
Iron,
this forms Fe2+ ions in solution and means that the
electrolyte has to be replaced periodically;
§
Precious metals such as platinum, ruthenium, rhodium and palladium which
are in the insoluble anode mud. This is processed to extract
these elements. |
Thanks to Kent White of
TM Technologies for
helpful comment on this page. |
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