Metals

Properties

Metals are generally:

  • solids with high melting and boiling points
  • malleable
  • good conductors of electricity and heat

Alloys

Alloys are mixtures of a metal with other elements.

Alloys are used instead of pure metals because they may be:

  • more resistant to corrosion
  • stronger
  • harder
  • less dense

Alloys contain atoms of different sizes which distorts the regular arrangement of atoms in metals. This makes it harder for layers to slide over each other, therefore making an alloy less malleable.

Reactivity Series

The reactivity series is a list of metallic elements ranked their reactivity. Elements that are more reactive can displace less reactive elements from their compounds, forming a new compound.

The reactivity series (starting with the most reactive element) is:

  • Group 1
    • Potassium
    • Sodium
  • Group 2
    • Calcium
    • Magnesium
  • Group 3
    • Aluminium
  • Carbon
  • Transition Metals
    • Zinc
    • Iron
  • Hydrogen
  • Precious Metals
    • Copper
    • Silver
    • Gold
    • Platinum

Group 1 and 2 react violently with cold water to produce hydrogen gas.
Transition metals will not react with water under normal conditions but will react with steam.
Precious metals are less reactive than hydrogen and will not react with water or steam.

Only elements less reactive than carbon can be extracted from their ores through reduction with carbon.

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Typically, less reactive metals are more expensive. These metals are often found as pure elements in their ores and not as compounds.

The reactivity of a metal can be described as its tendency to form a positive ion.

Metal Extraction

The extraction of metals from their ores is dependent on the reactivity series.

Reduction with Carbon

Some metals are extracted using carbon. One such metal is iron.

  • Hematite is loaded into a blast furnace
  • Iron is extracted from hematite using carbon monoxide
  • Calcium carbonate is used to remove acidic impurities from the ore
  • Calcium carbonate thermally decomposes into calcium oxide which forms slag
  • Slag is separately removed from the furnace

Important reactions take place inside the blast furnace. Coke is used to make carbon monoxide.

C + O2 → CO2
C + CO2 → 2CO

Hematite is reduced by carbon monoxide to extract pure iron.

Fe2O3 + 3CO → 2Fe + 3CO2

Calcium carbonate used to remove acidic impurities decomposes due to high temperatures (thermal decomposition).

CaCO3 → CaO + CO2

Calcium oxide reacts with silica impurities from sand to form slag containing calcium silicate.

CaO + SiO2 → CaSiO3

Electrolysis

Metals more reactive than carbon are extracted using electrolysis. For example, aluminium is extracted from bauxite by electrolysis. Aluminium oxide (Al2O3) has a high melting point that needs to be lowered by mixing it with molten cryolite.

The extraction of aluminium by electrolysis, known as the Hall-Héroult process, uses a large container whose base is made of graphite. This base acts as the cathode (negative electrode) in the electrolysis. The anode (positive electrode), also made of graphite, is inserted into the electrolyte from above.

This ionic half-equation describes the reaction occurring at the cathode:

Al3+ + 3e- → Al

This ionic half-equation describes the reaction occurring at the anode:

2O2- → O2 + 4e-

The aluminium that forms at the cathode (the base of the container) pools at the bottom and can be tapped out as it is molten. The oxygen that forms at the anode, under the high temperature of the process, reacts with the carbon (graphite) anode to form carbon dioxide. The anode therefore gradually burns away and must be replaced.

Recycling

It is important to recycle metals because metal ores are a finite resource.

Uses of Metals

Aluminium

Aluminium is used in:

  • aircraft parts because of its strength and low density
  • food containers because of its resistance to corrosion

Aluminium has an apparent unreactivity and resistance to corrision because of a protective oxide layer that adheres to the surface of aluminium. This layer prevents further oxidation.

Steel

Steel is an alloy made from iron and carbon. Mild steel is used in car bodies and machinery while stainless steel (an alloy of steel with chromium and nickel) is used in cutlery. Steel is an alloy that is stronger and has a higher melting point than its constituents.

Zinc

Zinc is used for:

  • galvanising iron and steel to prevent corrosion (rusting)
  • making brass by mixing it with copper
Another alloy of copper is bronze, which is made from copper and tin.