Metals and Non-Metals - Part XII

7. Study of some metals
We will now study a little more about three important metals : Aluminium (Al), iron (Fe) and copper (Cu). These three metals are indispensable in modern life. Aluminium and its alloys are used as electrical wires, utensils, pressure cookers, car engines, etc. Iron in the form of steel is used as utensils, pipes, equipments, etc. Copper and its alloys are used as electrical wires, pipes, vessels, etc.

Aluminium
Aluminium has chemical symbol Al. It has 13 protons and 14 neutrons. An atom of Al is represented as ²⁷₁₃Al. Al is largely separated by electrolysis method. Al is characterized as having very useful properties. It is light in weight, is a good electrical conductor, is highly malleable and highly ductile. Al is also resistant to corrosion. Alloys of Al find wide applications in various industries.

Since Al has 13 protons, it also has 13 electrons. The electronic configuration of Al is K-shell – 2 electrons, L-shell – 8 electrons and M-shell has 3 electrons. Al is placed in Group III A of the periodic table, just after magnesium. Al shows metallic properties and gives off its 3 electrons in the outermost shell easily to achieve a stable electronic configuration. It is a very reactive metal.  

Extraction of Al from bauxite : The process of concentration of ore is done by chemical separation. This process is known as Bayer's process. Bauxite ore is reacted with hot NaOH. A water soluble sodium aluminate is formed. The gangue particles are usually Fe and Si that do not dissolve in NaOH. The sodium aluminate formed is removed by adding water and filtering the solution.  

The process of conversion of the ore into metal-oxide is done in two steps. The sodium aluminate solution is diluted with HCl to form aluminium hydroxide. This separates from the solution as it precipitates out.   

The aluminium hydroxide is filtered and watered several times to remove NaCl. Then the precipitate is dried and heated strongly. The heat removes water and converts the hydroxide into pure oxide. The water formed is evaporated out.

The next step of reduction of Al-oxide into pure Al is done by electrolysis. The Al₂O₃ is mixed with cryolite (Na₃AlF₆) and melted. This forms electrolyte of the cell. This particular process is called Hall’s process.  

Al₂O₃ is not a conductor of electricity. Cryolite is therefore added so that the electrolyte is conductor of electricity. Also cryolite has Al atoms, which can also get separated out. But in the overall reaction, the concentration of the electrolyte remains unchanged. The cell is lined with carbon that acts as a cathode. A series of carbon rods are placed which act as anodes. When electrical current is passed, Al metal is released at the cathode. Oxygen is released at the anode. Oxygen reacts with carbon anode and sometimes carbon dioxide is also released. Pure Al is heavier than the electrolyte and collects at the bottom of the tank. This is then drained off and purified.

The step by step reactions are shown below.  

The Al obtained by electrolysis is more than 99% pure. Hence further purification of Al is not performed.

Al is not obtained from reduction of its oxide Al₂O₃ because this is a very stable compound. Carbon is unable to reduce this oxide into pure Al.

Physical properties of aluminium : Al is a whitish metal with a blue tinge. Its density is low (2.6 gm/cc). Al is a very ductile and malleable metal. It is a good conductor of heat and electricity.

Chemical properties of aluminium :
1. Valence : Since Al has 3 electrons in its M-shell, it gives them off easily. Al therefore has valence = 3+ .

2. Action of air : Al is a reactive metal and it reacts with air. When Al is exposed to air, it reacts with oxygen to form a thin layer of Al₂O₃. This acts as a protective layer and stops further corrosion of Al. Al₂O₃ is unreactive and rest of the Al remains unattacked by air. The reaction is exothermic.

3. Action of water : Al reacts with water to form a hydroxide. The surface layer of pure Al forms the aluminium hydroxide, which acts like a protective layer for the rest of the Al. Hydrogen gas is emitted in this reaction.

4. Reaction with acids : Al reacts with HCl to form Al-chloride and releases hydrogen.  

Al reacts with dilute sulphuric acid to become aluminium sulphate, releasing hydrogen. With concentrated sulphuric acid, Al reacts by releasing sulphur dioxide and water.  

Al does not react with nitric acid to give a nitrate; since nitric acid is oxidizing, Al forms an oxide layer Al₂O₃ on the surface. This oxide layer prevents more reaction. Thus Al is unreactive to nitric acid.

5. Action of alkalis : Al reacts with NaOH and KOH to form sodium and potassium aluminate, releasing hydrogen.

6. Reaction with chlorine : Aluminium chloride is formed when Al is heated in chlorine atmosphere. The reaction is shown below.  

7. Reducing action : Al strongly attracts oxygen. Hence it is a good reducing agent. The example given below shows how Al reduces Fe (III) oxide. This is an exothermic reaction and the temperature of the reaction goes up to as much as 3500°C.  

8. Test for aluminium : If Al is present in ionic form or salt form in a solution, it can be tested by pouring ammonium hydroxide. A white precipitate of aluminium hydroxide separates out and ammonium chloride solution remains behind. This is a practical test for the presence of Al.  

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