Explain any two properties of ionic compounds.

Ionic compounds are neutral compounds made up of positively charged ions called cations and negatively charged ions called anions. Two properties of ionic compounds are as follows: 1. Ionic compounds have high melting and boiling points because the force of attraction between ions of ionic compounds is very strong. 2. Ionic compounds dissociate into ions when dissolved in water. So, the solutions of ionic compounds and molten ionic compounds conduct electricity, but solid materials do not.

Explain the steps involved in the extraction of metals from their ores.

The common steps involved in the extraction of metals from their ores are: (1) Crushing and pulverisation (2) Concentration or dressing of the ore (3) Conversion of ore into metal oxide(calcination or roasting) (4) Reduction of metal oxides to free metal (5) Purification and refining of metal. (1) Crushing and pulverisation: Ores are broken into small pieces with the help of crushers. These pieces are then reduced to a fine powder with the help of a ball mill or a stamp mill. (2) Concentration or dressing of the ore: The process of removal of unwanted impurities (gangue) from the ore is called ore concentration. It can be done by hydraulic washing, magnetic separation, froth floatation, leaching, etc. (3) Conversion of ore into metal oxide: It can be done by two processes calcination (for carbonate ores) and roasting (sulphide ores). The calcination is carried out in a limited supply of air or absence of air. Roasting involves the heating of ore lower than its melting point in the presence of air or oxygen. (4) Reduction of metal oxide into free metal: This process depends upon the nature and chemical reactivity of the metal. For low reactive metals, the self-reduction process is used. For middle reactive metals, carbon reduction, reduction with CO or reduction with aluminium is involved. For high reactive metals, an electrolytic reduction is used. (5) Purification and refining of metal: Distillation refining is used for refining volatile metals e.g. Zn , Hg . Liquation is used for refining low melting point metals e.g. lead, tin. Electrolytic refining is used for metals like Cu , Al , etc.

What are amphoteric oxides?

The metal oxides which react with both acids as well as bases to produce salts and water are known as amphoteric oxides. These oxides have the properties of both acidic and basic oxides, so they can neutralise both acids and bases. Examples of amphoteric oxides are zinc oxide( ZnO ) and aluminium oxide( Al 2 O 3 ).

Explain the electrolytic reduction of Alumina by Hall’s process with the help of a labelled diagram.

Hall's process of electrolytic reduction of Alumina to obtain metallic aluminium: Alumina has a very high melting point ( > 2000 ° C ). The electrolysis of alumina is carried out in an electrolytic cell at a much lower temperature by dissolving it in molten cryolite ( Na 3 AlF 6 ) and fluorspar( CaF 2 ). The addition of cryolite and fluorspar reduces the melting point of alumina to about 1000 ° C . The electrolytic cell is a large steel tank having a lining of carbon (graphite) on the inner side. The carbon lining acts as a cathode. A set of carbon(graphite) rods dipped in the molten electrolyte acts as an anode. On passing electric current, electrolysis of alumina takes place. Aluminium is formed at the cathode and oxygen gas is liberated at the anode. The electrode reactions are as follows: At cathode: Al 3 + + 3 e - → Al (Reduction) At anode: 2 O 2 - → O 2 + 4 e - (Oxidation) The molten aluminium is heavier than the electrolyte used and hence, it sinks to the bottom of the tank from where it is removed periodically. The oxygen gas liberated at the anode reacts with the carbon anode and forms carbon dioxide gas. C ( s ) + O 2 ( g ) → CO 2 ( g ) As the anode gets oxidised during the electrolysis of alumina, it has to be replaced from time to time. The labelled diagram of Hall’s process is given below:

Explain the Frasch process for the extraction of sulphur with the help of a labelled diagram.

Frasch process is a method to extract sulphur from underground deposits. Elemental sulphur is found in the deposits in the ground. There are two properties of sulphur that are used in its extraction process. These are (i) its low melting point and (ii) low density. In the Frasch process, superheated water is pumped into the sulphur deposit, the sulphur then melts, which is then extracted by lifting it to the surface with compressed air. The extraction process involves the following steps: First, a well is drilled into the soil for mineral deposits. Then after this, the superheated water is pumped into the deposit (this water was approximately heated to 170 degrees Celsius). As a result of this, the sulphur in the deposit melts due to its low melting point. Now, besides the water pumped into the well, compressed air is also pumped into it. We know that sulphur has a low density and due to this reason, the molten sulphur comes to the surface. The low density of sulphur allows the compressed air to push it to the surface. Now, the sulphur can cool down and solidify since it is on the surface. From here, sulphur forms a rhombic crystal. Thus, in this way sulphur is extracted by Frasch process.

Explain the froth floatation process with the help of a labelled diagram.

The froth floatation method is a method used for the concentration of metal ores. It is used for the concentration of sulphide ores. In this technique, particles of interest are physically separated from a liquid phase as a result of differences in the ability of air bubbles to selectively adhere to the surface of the particles, based upon their hydrophobicity. The principle of froth floatation is that sulphide ores are preferentially wetted by pine oil, whereas the gangue particles are wetted by water.

Write the chemical formula of cryolite? Why is it added to bauxite in the extraction of aluminium?

The chemical formula of cryolite is Na 3 AlF 6 . It is sodium aluminium hexafluoride, an inorganic compound. In the extraction of aluminium, cryolite is added to bauxite to lower its melting point for easier electrolysis. The use of molten cryolite as a solvent reduces some of the energy costs involved in extracting aluminium. It also increases the conductivity of the solution.