Sunday, March 1, 2009

igcse chemisyrty notes -extraction of metals

Oxidation and reduction
Oxidation is a reaction where oxygen is gained.
eg iron + oxygen ---> iron oxide
Reduction is a reaction where oxygen is lost. Eg iron oxide is reduced to form iron.
Classify the following reactions as oxidation or reduction:
burning magnesium,
making aluminium from aluminium oxide,
burning carbon in air,
copper oxide + carbon --> copper + carbon dioxide
carbon monoxide forming carbon dioxide.
Electron loss and gain
Oxidation is also a loss of electrons
eg Fe --> Fe2+ + 2e-
Reduction also the gain of electrons eg (when Fe ions becomes Fe atoms
Identify all of the elements below as being oxidised or reduced in the following reactions:
Cu2+ + 2e- ---> Cu
lead ions forming lead atoms
Na ---> Na+ + e-
Ca + Cl2 --> CaCl2
Fe2+ ---> Fe3+ + e-
Reduction of metal ores
An ore is a material found in the ground which contains a metal. An ore is often a metal oxide mixed with rock . When a metal is extracted its ore is reduced. Metal loses oxygen from its oxide. Examples include: haematite which is mostly iron III oxide
bauxite which is mostly aluminium oxide Al2O3
malachite which is mostly copper carbonate CuCO3
Extraction and position in reactivity series
Uses and properties of aluminium
extraction method that uses a lot of electrical energy. Electrical energy is more expensive than energy from burning carbon
The extraction of aluminiumAluminium is found in the ground in an ore called bauxite. Bauxite is aluminium oxide (Al2O3) with iron oxide impurities.

After purification aluminium oxide is mixed with cryolite to lower the melting point from 2000º to 1000º, which saves money.
This mixture is heated and the molten liquid used as the electrolyte. Both electrodes are made of graphite (carbon). The anode (+ve) is graphite and the cathode (-ve) is a graphite lining to a steel case.
The carbon anodes react with oxygen so have to be replaced.
C + O2 --> CO2

At cathode - positive aluminium ions attracted, gain electrons and become atoms.

Al3+ + 3e- ---> Al

At anode - negative oxide ions attracted, lose electrons and become atoms.2O2- ----> O2 + 4e-
uses and properties of aluminium
overhead power cables-good electrical conductor, low density
drinks cans-Does not react with water
aircraft parts -high strength and low density
Carbon and carbon monoxide for reducing oxides
Carbon and carbon monoxide can both remove oxygen from other compounds so are good for reduction.
They are used to reduce the ores of metals below carbon in the reactivity series.
E.g. zinc, iron, tin and lead.
Iron extraction using the blast furnace Drag and drop labels test on blast furnace
A blast furnace is used in the process of extracting iron. The raw materials iron ore, coke and limestone are put in at the top. Hot air is blasted into this furnace at the bottom making the coke (carbon) burn faster and the temperature rises to about 1500º.
When the coke burns, carbon dioxide is produced. C + O2 ---> CO2 CO2 reacts with the unburnt coke to form carbon monoxide CO CO2 + C ---> 2CO Iron oxide Fe2O3 in the ore is reduced to iron by the reaction with the carbon monoxide. 3CO + Fe2O3 ---> 3CO2 + 2Fe Molten iron is a dense liquid, so runs to the bottom of the furnace and is tapped off.
Limestone CaCO3 helps remove impurities during the extraction by forming calcium oxide

CaO. CaCO3 ---> CaO + CO2

The rock impurities silicon dioxide SiO2 are then removed by the following reaction.

CaO + SiO2 ---> CaSiO3 CaSiO3
is known as slag and can be used in making cement and road building.

The purification of copperVery pure copper is needed for copper wires. Electrolysis is needed to purify copper. The anode is a mass of impure copper and the cathode is pure copper. The electrolyte is sulphuric acid. The impurities drop at the anode as sludge during electrolysis. At anode Cu ---> Cu2+ + 2e-

At cathode Cu2+ + 2e- ---> Cu

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