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SS3: CHEMISTRY - 2ND TERM

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  1. Quality of Petrol (Octane Number)| Week 1
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  2. Natural Gas | Week 2
    3 Topics
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    1 Quiz
  3. Introduction to Metals | Week 3
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    1 Quiz
  4. The Alkali Metals | Week 4
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  5. Alkaline Earth Metals | Week 5
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  6. Aluminium & Tin | Week 6
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  7. Transition Metals of the First Series | Week 7
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    1 Quiz
  8. Ethical, Legal & Social Issues | Week 8
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  9. Fats & Oils - Soaps & Detergents | Week 9
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  10. Giant Molecules | Week 10
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As discussed, most of the elements especially metals are found in combined forms with other elements and these are called minerals. Often, these minerals are found mixed with other earthly materials called ores.

Ores can be divided into four groups:

i. Native ore: Native ores are are non-reactive, and contains metal in a free state. e.g. silver, gold, platinum, mercury, copper, etc.

ii. Oxidized ore: These contain oxides or oxysalts. oxysalts arecarbonates, phosphate and silicate of metal. Examples:
Oxide ores: (Haematite – Fe2O3, Bauxite – Al2O3. 2H2O. etc.)
Carbonate ores: (CaCO3, Calamine – ZnCO3 etc.).
Nitrate ores: (NaNO3, KNO3, etc.)
Sulphate ores: (Barite – BaSO4, Gypsum – CaSO4.2H2O)

iii. Sulphurized ore: These contain sulphides of metal like iron, and lead. Examples: FeS2 (Iron pyrites), Chalcopyrite, (CuFeS2 ), PbS (Galena), HgS (Cinnabar) etc.

iv. Halide ore: These contain halide of metals. Examples: Chloride ore: AgCl (Horn silver), CaF2 (Fluorspar) etc.

Metal can be extracted from ores by a series of methods ike the concentration of ore, isolation of metal and purification of the metal.

1. Concentration of the Ore:

This simply means getting rid of as much of the unwanted rocky material, like rocks, sand, silt, and many other impurities called gangue, before the ore is converted into the metal.

The unwanted material can be separated by employing physical or chemical processses.

Various physical and chemical processes are employed to concentrate or separate ore from the gangue matrix.

Physical Processes:

The physical methods include

Washing: Washing away the early material in a stream of water to leave behind heavier ores, as in the case of tin ores.

Froth Flotation: In this process, the ore is first crushed and treated with oil and water to form a mineral froth. The oil which is generally used in the froth floatation process is pine oil. The pine oil binds to the metal compounds, but not to the unwanted rocky material. The froth is then skimmed off by blowing air. This frees the ore from the unwanted material. Sulphide ores are examples of ores that can be concentrated by froth flotation.

Magnetic separation: This method involves passing magnetic ores, e.g copper ore, through a magnetic separator.

magnetic separation

The ore is finely crushed and passed over the magnetic roller, where one is magnetic, and the other is nonmagnetic. Ores with a high metal content will attract and attach to the magnetic roller and get deflected into a pile, and the non-magnetic gangue particles will repel and fall into the pile from the conveyer belt.

Hand-picking: This involves using a hammer to separate the ore from the unwanted materials.

Chemical Processes:

Roasting in air: If the ore is not an oxide, for example, sulphide ores, the ore concentrate is usually roasted in air to chang it to an oxide. This is done because oxides are easier to deal with during metal extraction, compared to sulphide ores. Calcination is the chemical method of spearating carbonate or hydrated ores (hydroxides).

In roasting or calcination, the ore is heated at the temperature below the melting point.

For example;

Sulphide ore is heated at the temperature below the melting point.

2Pbs + 3O2 → 2PbO + 2SO4
ZnS + 2O2 → ZnSO4

A carbonate ore produces carbon dioxide under heat exposure.

ZnCO3 → ZnO + CO2

The hydrated oxide ore releases water under heat exposure.

Al2O3.2H2O → Al2O3 + 2H2O

2. Isolation of Metal (Extraction of Metal from Ore):

Metals found in combined forms exist as positive ions. To convert them to the metal, you need to add electrons – reduction. The metallic ions must be reduced to their corresponding metal atoms.

Example of redox reactions.

Fe3+ + 3e- → Fe (addition of electrons = reduction)

Al3+ + 3e- → Al (addition of electrons = reduction)

This can be done by chemical and thermal methods or by electrolysis.

Chemical Method:

The less electropositive (reactive) metals (Pb, Sn, Fe, Zn) are usually obtained by reducing their oxides with coke or carbon(II)oxide. Carbon (as coke or charcoal) is cheap. It not only acts as a reducing agent, but it also acts as the fuel to provide heat for the process.

For Example,

\(\scriptsize \underset{copper\:oxide}{2CuO} \: + \: \underset{carbon – reducing\: agent}{C} \rightarrow \: \underset{copper\:metal}{2Cu} \: + \: \underset{carbon\:monoxide}{CO_2} \)

Other more reactive metals like magnesium and sodium can be used to reduce the ore.

Electrolytic Reduction (Electrolysis):

The method applied in the extraction of a particular metal from its ore depends on the stability of its ore which in turn depends on the position of the element in the activity series.

It is difficult to use this method for more electropositive (reactive) metals (K, Na, Ca, Mg) at the top of the activity series and above carbon in the activity series.

Reactivity Series 2

Instead a process called electrolysis has to used. During electrolysis, electrons are being added directly to the metal ions at the cathode (the negative electrode. The cathode acts as a reducing agent by supplying electrons to the metallic ions in the electrolyte, resulting in the deposition of the free metal. A disadvantage is that electrolysis requires a lot of energy and are usually expensive. They are only used when the chemical method is not possible.

The table below summarises the reactivity, compound of the element, and method of metal extraction.

activity series of metals and methods of metal extraction
Activity series of metals and methods of metal extraction.

3. Purification of Metal:

Metals that are extracted by reductive processes usually need to be further processed to make them industrially useful.

Evaluation Questions:

1. State any three physical properties of metals.
2. Name two metals that are extracted from their ores by electrolysis.
3. Briefly describe the various methods of extraction of metals from their ores.

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Evaluation Questions

1. State any three physical properties of metals.

Answer:

i. Except mercury (which is liquid) all metals are solids at room temperature.

ii. Metals are lustrous. They have the quality of reflecting light from their surface and can be polished.

iii. Metals are malleable i.e they can be hammered into thin sheets

 

2. Name two metals that are extracted from their ores by electrolysis.

Answer:

i. Aluminium (Al)
ii. Potassium (K)

 

3. Briefly describe the various methods of extraction of metals from their ores.

Answer:

– Electrolysis of fused hydroxides and chlorides
– Electrolysis of chlorides
– Electrolysis of oxides
– Roasting of trioxocarbonate (IV) and sulphide to form oxides
– Reduction of oxides by carbon or carbon (II) oxide
– Roasting in air
– Heating in air
– Mined as free element