CBSE Class 12 Chemistry 2016 Previous Year Question Paper

CHAPTER GENERAL PRINCIPLES AND PROCESSES OF ISOLATION OF ELEMENTS

Syllabus

Principles and methods of extraction - concentration, oxidation, reduction-electrolytic method and refining; occurrence and principles of extraction of aluminium, copper, zinc and iron.

Chapter Analysis

2018

List of Topics D OD OD D/OD

Methods and Principles 1Q 1 Q.

of Isolation of Elements (3 marks) (3 marks)* (3 marks)

Refining of Nickel, 1 Q. Extraction of Aluminium (3 marks) and Extraction of Iron Extraction of Gold and 1 Q.

Role of NaCN and Zn (3 marks)

• *One question of 3 marks on Principles of Vapour Phase Refining, Zone Refining and Chromatography was asked.

On the basis of above analysis, it can be said that from exam point of view, Methods and Principles of Isolation of Elements are the most important topics of the chapter.

TOPIC-1 Principles and Methods of Extraction

TOPIC - 1

Principles and Methods of

extraction. .... P. 95

• Minerals : The naturally occurring chemical substances in the earth's crust which are obtained by mining.

Revision Notes

TOPIC - 2

Principles of Extraction of

Aluminium, Copper, Zinc and

Iron .... P. 108

• Ore: The mineral from which a particular metal can be extracted conveniently and economically.

Gangue: The earthy materials associated with the ores.

Occurrence of metals :

In free state: Very few metals exist in the free or native state. Only metals like gold, platinum and mercury are occasionally found in the free state, i.e., in the pure form.

In the combined state: The rest of the metals occur in the combined form of compounds such as oxides, carbonates, sulphides, sulphates, silicates, chlorides, nitrates, phosphates etc.

Note: Copper and silver are two metals which occur in free as well as combined state as sulphides, oxides or halides ores.

➣ Metallurgy: Metallurgy is the entire scientific and technological process used to obtain the pure metal from its ore.

> Flux: The substance which is added in the ore to convert non-fusible gangue to fusible compound is called flux. There are three types of flux: acidic flux (Silica borax) and basic flux (Limestone) and neutral flux (Graphite).

> Slag: The fusible compound formed by combination of flux and gangue is called slag.

96 Oswaal CBSE Chapterwise & Topicwise Question Bank, CHEMISTRY, Class – XII

The processes involved in the metallurgy:

1. Concentration of the ore
2. Isolation of metal from its concentrated ore
3. Refining or purification of metals

Metallurgy: The whole process of obtaining a pure metal from one of it's ore is known as metallurgy. A general scheme of various metallurgical operations employed for the extraction of metals from ores is given below: Ore Crushing and grinding

Levigation

Concentration Magnetic separation of the ore or dressing Froth flotation

Leaching

Concentrated ore

Roasting

Calcination

Extraction of metal from the Reduction concentrated ore Chemical reduction

Electrolytic reduction

Crude metal

Poling

Purification or Chromatographic method dressing of • Electrolytic refining crude metal

Zone refining

Van Arkel method

Pure metal

➣ Concentration of ore: It is a process used for removing the gangue from the ore and increasing ore's grade on the basis of difference between the physical or chemical properties of the gangue and the ore. The concentration of the ore is carried out by the following methods:

1. Crushing and Grinding: The huge lumps are first broken into small pieces in the jaw crushers and then powdered with the help of a ball mill or stamp mill. This process is termed as pulverisation.

1. Levigation or Gravity separation: It is based on difference in densities (gravities) of ore and the gangue. In this process, ores is washed with stream of water under pressure, light impurities are washed away whereas heavy ore are left behind. Eg. Generally oxides and carbonates ores are concentrated by this method.

1. Magnetic separation method: Ore and gangue are separated, if only one of them is magnetic in nature. Magnetic separation method is used to remove tungsten FeWO₄-magnetic ore particles from cassiterite (non magnetic–SnO₂). It is also used to concentrate magnetite (Fe₃O₄), chromite (FeCr₂O₄) and pyrolusite (MnO₂)

from unwanted gangue. Finely ground ore

Magnetic roller

Magnetic particles

Non-magnetic particles

Fig. 1: Magnetic separation method

1. Froth flotation process: This process makes use of the principle of preferential wetting of solid surfaces by various liquids. This process is used for the concentration of sulphide ores e.g., ores of lead, zinc and copper,