metallurgy -2
TRANSCRIPT
Reduction of oxide to the metal
Reduction of the metal oxide usually involves heating it with some
Other substance acting as a reducing agent (C or CO or even another
metal).
The method used to extract a metal from its ore depends upon the
stability of its compound in the ore, which in turn depends upon
the reactivity of the metal:
• The oxides of very reactive metals, such as aluminium,
form stable oxides and other compounds. A lot of energy is needed
to reduce them to extract the metal.
• The oxides of less reactive metals, such as iron, form less
stable oxides and other compounds. Relatively little energy is
needed to reduce them to extract the metal.
So, the method of extraction of a metal from its ore depends on the
metal's position in the reactivity series.
Reactivity and extraction method
The table displays some metals in decreasing order of reactivity and
the methods used to extract them.
Metal Method
Potassium Electrolysis
Sodium Electrolysis
Calcium Electrolysis
Magnesium Electrolysis
Aluminium Electrolysis
(Carbon) (Non-metal)
Metal Method
Zinc Reduction by carbon or carbon monoxide
Iron Reduction by carbon or carbon monoxide
Tin Reduction by carbon or carbon monoxide
Lead Reduction by carbon or carbon monoxide
(Hydrogen) (Non-metal)
Copper Various chemical reactions
Silver Various chemical reactions
Gold Various chemical reactions
Platinum Various chemical reactions
We can see from the table that reactive metals, such as aluminium,
are extracted by electrolysis, while a less reactive metal, such as iron,
may be extracted by reduction with carbon.
Because gold is so unreactive, it is found as the native metal and not
as a compound. It does not need to be chemically separated. However,
chemical reactions may be needed to remove other elements that
might contaminate the metal.
� Some metals can be extracted by heat alone such as mercury and silver, from their
corresponding oxides. Most metals are too reactive to be extracted by heat alone.
� Metal Oxide →HEAT Metal + Oxygen
2HgO →HEAT 2Hg + O2
� Some metals can be extracted by heating with carbon such as zinc, tin, lead, copper and
iron from their corresponding oxides. Metals above zinc in the reactivity series are too
reactive to be extracted by heating with carbon or carbon monoxide gas.
� Very reactive metals are strongly bonded in their ores and cannot be extracted using
carbon/carbon monoxide. More energy is needed and this is provided by electrolysis.
Electrolysis is a process which uses electricity to break down a substance. Metals above
zinc in the reactivity series are extracted using electrolysis such as aluminum. Aluminum
is extracted from its molten ore, bauxite (Al2O3).
� Titanium is produced by reducing titanium (IV) chloride using a more reactive metal
such as sodium or magnesium. This is the only way of producing high purity metal.
TiCl4 + 4Na Ti + 4NaCl
The more reactive metal sodium releases electron easily.
4Na 4Na+ + 4e
-
These electrons are used to reduce titanium chloride.
TiCl4 + 4e-
Ti + 4Cl-
Thermodynamic aspect of metallurgy
∆Hr& ∆Sr cannot decide the feasibility of a reaction separately at
constant Temperature (T) & Pressure (P)
� ∆Gr decides the spontaneity of a reaction.
∆G = ∆H - T∆S ---------------- (i)
� ∆Gr< 0 or negative for a spontaneous or feasible process.
� For ∆S +ve at high temperature, T∆S value increases,
So, -T∆S in eqn. (1) becomes more –ve, ∆G value becomes more –
ve, the reaction becomes spontaneous & vice versa
� If equilibrium constant value, K is large for a reaction, Reactants
⇋ Products, & T increases
∆G0r = - RT ln K = -2.303RT log K
∆G0rvalues become more –ve with increase in temperature &
reaction becomes spontaneous.
� For a coupled reaction: A → B, ∆G1 > 0 or +ve means non
spontaneous reaction and for C → D , ∆G2 < 0 or -ve means
spontaneous reaction.
Reactions (1) & (2) are coupled i.e. A + C → B + D
If ∆G1 + ∆G2 < 0 or –ve, both the reaction becomes spontaneous.
� Example: (1) 2FeO → 2Fe + O2, ∆G1 > 0 ≈ Non-spontaneous.
(2) C + O2 →CO2, ∆G2 < 0 ≈ Highly Spontaneous
(1) + (2): 2FeO + C + O2→ 2Fe + CO2 + O2 Overall reaction:
2FeO + C → Fe + CO2
Here if ∆G1 + ∆G2 < 0 or –ve, so the reaction is spontaneous.
� This is the basis of metallurgy.
Ellingham Diagrams:
� In Ellingham diagram the ∆G0 values are taken for per mole of O2.
� These diagrams were first constructed by Harold Ellingham in
1944.
� Ellingham diagram help us in predicting the feasibility of a
thermal reduction of an ore.