DPS BOKARO, SEC-IV

METALLURGY-2 STUDY MATERIAL FOR CLASS-12

CHEMISTRY DEPARTMENT

09/04/2020

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.

That is, an element will reduce the oxide of other metals which lie

above it in Ellingham diagram.

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