19. Metals
Giant Metallic Structure
General Properties
Alloys
Displacement Reaction of Metals
Thermal Decomposition of Metal Carbonates
Methods of Extraction
Extraction of Iron
Raw Materials added into the blast furnace: haematite (iron ore), limestone (CaCO3), coke (C)
Rusting
Reactivity of Aluminium
Recycling Metals
- Strong electrostatic forces between positive metal ions and mobile delocalised valence electrons.
General Properties
- High melting and boiling points
- Malleable & ductile (easily bent into shape): This is because layers of atoms can slide easily over each other when a force is applied.
- Good conductors of heat and electricity: Metals consist of a lattice of positive ions surrounded by a sea of delocalised electrons which are free to move to conduct electricity.
Alloys
- An alloy is a mixture of a metal and another metal or non-metal.
- Steel = iron + carbon
- Brass = copper + zinc
- Stainless steel = iron + carbon + chromium + nickel (Steel which has higher percentage of carbon, is more brittle but stronger.)
- Alloys are stronger and harder than pure metals:
- Atoms of the added element are of a different size from that of the pure metal.
- This disrupts the orderly arrangement of the layers of metal atoms.
- When a force is applied, the layers of metal atoms cannot slide over each other.
Displacement Reaction of Metals
- A more reactive metal can displace a less reactive metal from its aqueous solution
Thermal Decomposition of Metal Carbonates
- The more reactive the metal, the more stable is the carbonate, the less easily it decomposes when heated. i.e. a higher temperature needed to decompose the carbonate.
Methods of Extraction
- K, Na, Ca, Mg, Al: Extracted by electrolysis of the molten compound
- Zn, Fe, Pb, Cu, Ag: Extracted by reduction of the metal ores using reducing agents like carbon, carbon monoxide and hydrogen
- Au: Exists in the uncombined state
Extraction of Iron
Raw Materials added into the blast furnace: haematite (iron ore), limestone (CaCO3), coke (C)
- Haematite, limestone and coke are fed in from the top of the blast furnace. Hot air is blown into the blast furnace from the sides.
- The coke burns in hot air to form carbon dioxide. The carbon dioxide is then reduced by the hot coke to form carbon monoxide.
- The carbon monoxide then reduces iron(III) oxide to hot molten iron, which flows to the bottom of the furnace to be tapped off. The carbon dioxide will escape from the top with the unreacted part of air, nitrogen, and some carbon monoxide, as waste gases.
- Limestone is added to remove acidic impurities like silica. It decomposes to form basic CaO which is used to neutralise acidic silicon dioxide to form slag. The slag flows to the bottom and settles above the molten iron (less dense than iron) to be tapped off to make roads.
Rusting
- Conditions for rusting : oxygen and water
- Methods of prevention of rusting :
- Barrier protection: Coating iron with a layer to prevent it from coming into contact with oxygen and water. e.g. painting, greasing, coating with a less reactive metal like tin or chromium, and plastic
- Sacrificial protection: The more reactive metal can act as a sacrificial metal and corrodes preferentially in place of the less reactive metal. e.g. zinc blocks protect hulls of ships & galvanisation : coating iron with a layer of zinc
Reactivity of Aluminium
- Aluminium appears to be unreactive as it is surrounded by a protective layer of inert and non–porous aluminium oxide, preventing aluminium from further reacting with the substance it is in contact with.
Recycling Metals
- Metal are extracted from metal ores in the Earth’s crust. As it is a finite and non-renewable resource, there is a need to recycle metals.
- Advantages
- Cheaper to recycle than to extract new metals from ores such as aluminium which requires high amount of energy from electrolysis.
- Conserve metal ores on earth.
- Reduce usage of land for storing dumped metals such as vehicles.
- Prevents pollution of the soil and water by metals ions caused by the corrosion of the abandoned metals by air and rain.
- Disadvantages
- Requires a lot of man-power to collect the metal.
- High cost of fuel involved in transportation of the metal.
- Toxic gases produced during the cleaning process , pollutes the air.