Types of Chemical Reactions

Some of the important types of chemical reactions are:

Combination Reactions: The reactions in which two or more substances combine to form a single substance are called combination reactions.

Decomposition Reactions: The reactions in which a compound breaks down into two or more simpler substances are known as decomposition reactions. Decomposition can be effected by application of heat, light, or electricity.

This is called electrolysis of water. Decomposition of silver chloride or bromide is used in black and white photography. Decomposition by means of  electricity is used to extract metals from their compounds. When molten metal chloride or oxide is decomposed by passing electricity, then metal is produced at the cathode. Decomposition of food into simpler substances takes place during the process of digestion in the body. For example, starch present in wheat and rice decomposes to give glucose whereas proteins give amino acids.

Displacement Reaction: The reactions in which one element replaces another element in a compound are known as displacement reactions. A more reactive element displaces a less reactive element from its compound. For example,

CuSO₄(aq) + Fe(s) → FeSO₄(aq) + Cu(s)

Double Displacement Reactions: The reactions in which two compounds exchange ions to form two new compounds are called double displacement reactions. For example,

BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)

Oxidation and Reduction Reactions

The addition of oxygen to a substance or removal of hydrogen from a substance is called oxidation. The removal of oxygen from a substance or addition of hydrogen to a substance is called reduction. Oxidation and reduction reactions takes place side by side and are called redox reactions.

The substance which gives oxygen for oxidation, or removes hydrogen (i.e., brings about oxidation), is called an oxidising agent. The substance which gives hydrogen for reduction, or removes oxygen (i.e., brings about reduction), is called a reducing agent.

In this reaction, zinc oxide is reduced to zinc and carbon is oxidised to carbon monoxide. Here, zinc oxide is the oxidising agent and carbon is the reducing agent. This reaction is used in the extraction of zinc metal. Carbon is used in the form of coke.

A common example of reduction is hydrogenation of vegetable oil (liquid) which leads to the production of vanaspati ghee (solid fat).

When food materials prepared in oils and fats are kept for a long time they get an unpleasant smell and taste. Such food materials are said to have become stale or rancid. This happens because oils and fats present in food materials undergo oxidation by atmospheric oxygen, and their oxidation products have unpleasant smell and taste. This condition produced by aerial oxidation of oils and fats is called Rancidity. Rancid food materials are unfit for eating.

Another common effect of oxidation reactions observed in daily life is the Corrosion of metals. Corrosion is the process in which metals are destroyed gradually by the action of air, moisture, or a chemical on their surface. It is mainly caused by aerial oxidation. Rusting of iron metal is a common example. Iron is oxidised by oxygen present in the air in the presence of moisture to form hydrated ferric oxide (rust) which is reddish brown in colour. Corrosion weakens iron and steel objects, like bodies of vehicles, bridges, ships, etc. Rust flakes off and exposes a fresh surface of iron, thus continuing the process.

The green copper hydroxide-copper carbonate coating associated with copper corrosion is a tough film that adheres to the copper surface (statues, buildings, utensils, etc.). Copper corrosion requires the presence of oxygen, water and carbon dioxide. All these substances are normally present in air. The corroded copper objects can be cleaned with dilute acid solution.

Silver tarnishes quickly in the presence of sulphur-containing air pollutants, such as hydrogen sulphide, and sulphur containing foods, such as eggs and mustard. This silverware tarnish is  a thin layer of black silver sulphide.

The bright surface of aluminium objects changes to a dull silver-white as a thin film of aluminium oxide forms through atmospheric oxidation. This layer of aluminium oxide protects aluminium objects from further corrosion. This layer can be made thicker, for greater protection, by electrolyses. This process is called anodizing. Aluminium objects, like pressure cookers, cooking utensils, saucepans, etc., are anodized.