All living beings born in this beautiful world adopt and follow their own lifestyles. Have you observed and analyzed your daily life from the viewpoint of a chemist? Chemical reactions take place around us, all the time, and even in our bodies. Any change can be classified as a physical change or a chemical change. Physical changes can be easily reversed but, it is not easy to reverse a chemical change. What is the reason? During chemical changes, new substances are formed, and it is difficult to regenerate the original substances. Chemical changes are more permanent than physical changes. All chemical changes are accompanied by chemical reactions.
The lustrous white color of the silver anklet slowly changes into a slightly black color. We say the silver anklet has got tarnished. Can you guess the reason behind it?
Do you feel the heat? Let us learn what happens. Calcium oxide reacts with water to produce slaked lime (calcium hydroxide). This reaction is exothermic and is accompanied by a hissing sound and the formation of bubbles, leading to the release of a considerable amount of heat.
Since there are numerous chemical reactions, the study of these reactions can be made easier by classifying them.
Let us see the different types of
classifications of chemical reactions.
1. Combination reaction
A +B =A B
A combines with B to form a new product AB. It is the simple representation of a combination reaction.
2Mg + O2→ 2MgO
The liberation of a reddish brown gas(NO2) is observed. This is because of the decomposition of lead nitrate into lead oxide, nitrogen dioxide, and oxygen. ∆ 2Pb(NO3)2 → 2PbO + 4NO2↑ + O2↑
A few more examples of decomposition reaction are:
Decomposition of ammonium dichromate
(NH4)2Cr2O7 → Cr2O3↑ + N2↑ + 4H2O↑
Double Decomposition Reaction (Double Displacement Reaction)
AB+CD=AD+CB
Oxidation:
A chemical reaction that involves the addition of oxygen or removal of hydrogen or loss of electron(s) is called Oxidation.
2Mg + O2 →2MgO (addition of oxygen)H2S + Br2→ 2HBr + S (removal of
hydrogen)
Fe2+→ Fe3+ + e- (loss of electron)
Redox Reaction:
Redox reaction is a chemical reaction in which oxidation and reduction take place simultaneously.
Zn + CuSO4 → Cu + ZnSO4 Write an equation of any other redox reaction.
Oxidation also has damaging effects on food and eatables. When food containing fat and oil is left as such for a long time, it becomes stale. The stale food develops a bad taste and foul smell. This is very common in curd or cheese, particularly in summer. Oils and fats are slowly oxidized to certain foul-smelling compounds.
A substance that alters the rate of reaction without undergoing any change in mass and composition is known as a catalyst.
For acids, we use the term basicity, which refers to the number of replaceable hydrogen atoms present in one molecule of acid. For example, acetic acid has four hydrogen atoms, but only one can be replaced.
Few metals do not liberate hydrogen gas on reaction with acids. e.g. Ag, Cu. Lime stone, chalk and marble are different physical forms of calcium carbonate. They react with acids giving the corresponding salt, carbon dioxide and water.
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