Types of chemical reactions
Elements compound and mixture of Class 8
TYPES OF CHEMICAL REACTIONS
- The reactions in which a compound splits up into two or more simpler substances are known as decomposition reactions. The decomposition reactions are carried out by applying heat, light or electricity. Heat, light or electricity provide energy which breaks a compound into two or more simpler compounds. Please note that a decomposition reaction is just the opposite of a combination reaction. We will now study some examples of decomposition reactions.
- Example 1. When calcium carbonate is heated, it decomposes to give calcium oxide and carbon dioxide :
CaCO3(s) CaO(s) + CO2(g)
Calcium carbonate Calcium oxide Carbon dioxide
- In this reaction, one substance, calcium carbonate is breaking up into two simpler substances, calcium oxide and carbon dioxide, so this is a decomposition reaction. Please note that calcium carbonate is also called ‘limestone’ and the calcium oxide formed from it is called ‘lime’ (or quicklime). The decomposition of calcium carbonate (limestone) on heating is an important reaction used in various industries. This is because, calcium oxide (lime) obtained by the decomposition of calcium carbonate has many uses in industry. For example, calcium oxide (or lime) is used on a large scale in the manufacture of cement and glass.
- When a decomposition reaction is carried out by heating, it is called ‘thermal decomposition. ‘Thermal’ means ‘relating to heat’.
The decomposition of calcium carbonate into calcium oxide and carbon dioxide is an example of thermal decomposition (because it is carried out by heating).
- The reactions in which one element takes the place of another element in a compound, are known as displacement reactions. In general, a more reactive element displaces a less reactive element from its compound. The examples of some important displacement reactions are given below.
- Example . When a strip of zinc metal is placed in copper sulphate solution, then zinc sulphate solution and copper are obtained :
CuSO4 (aq) + Zn (s) ZnSO4 (aq) + Cu (s)
Copper sulphate Zinc Zinc sulphate Copper
(Blue solution) (Colourless solution)
Double decomposition (or) double displacement reactions:
- The reactions in which two compounds react by an exchange of ions to form two new compounds are called double displacement reactions. A double displacement reaction usually occurs in solution and one of the products, being insoluble, precipitates out (separates as a solid). Some of the examples of double displacement reactions are given below :
- Example : When silver nitrate solution is added to sodium chloride solution, then a white precipitate of silver chloride is formed along with sodium nitrate solution.
Oxidation and reduction reactions:
- The addition of oxygen to a substance is called oxidation.
- The removal of hydrogen from a substance is also called oxidation.
- The addition of hydrogen to a substance is called reduction.
- The removal of oxygen from a substance is also called reduction.
It is obvious from the above definitions that the process of reduction is just the opposite of oxidation. Moreover, oxidation and reduction occur together. We will now define the oxidising agents and reducing agents.
Oxidising agent :
- The substance which gives oxygen for oxidation is called an oxidising agent.
- The substance which removes hydrogen is also called an oxidising agent.
Reducing agent :
- The substance which gives hydrogen for reduction is called a reducing agent.
- The substance which removes oxygen is also called a reducing agent.
- The oxidation and reduction reactions are also called redox reactions (In the name ‘redox’, the term ‘red’ stands for ‘reduction’ and ‘ox’ stands for oxidation). We will now give some examples of oxidation and reduction reactions.
- Example :When copper oxide is heated with hydrogen, then copper metal and water are formed :
- The process of depositing a thin and compact layer of a superior metal over an inferior metal by the process of electrolysis is called electroplating.
- Reasons for Electroplating
- 1. To prevent metallic articles from rusting: For example, articles made from iron or steel are electroplated with nickel or chromium, so as to prevent them from rusting.
- 2. To improve appearance of articles: For example, decoration articles made from copper or brass (inferior metals) are electroplated with gold or silver (superior metals) to improve their appearance.
- How to obtain smooth and firm deposit of a superior metal during electroplating?
- By adopting following precautions, a smooth and firm deposit of a superior metal can be obtained.
- 1. The surface of article to be electroplated must be very clean, i.e., it should be free from rust and grease.
- 2. The electrolytic solution should be of appropriate concentration.
- 3. The electrolyte should be maintained at a proper temperature.
- 4. A small current should be passed over a longer period of time. This prevents electrolyte from heating.
- 5. A direct electric current should be applied.
- Electroplating an article with silver
- During silver plating, the article to be electroplated is made cathode, whereas pure silver is made anode.
- Electrolyte: Sodium silver cyanide