Science and Technology for UPSC Examinations [Chemistry] Part 4- Chemical Reactions Basics

Chemical Reactions

Chemical reactions are happening around us all the time and even in our body. Respiration and digestion in all living organisms, rusting of iron, cooking of food, formation of brown colour on a cut piece of apple etc., are some of the important and common chemical reactions taking place in our daily life.

The chemical reactions can be slow reactions or fast reactions.

Burning of Camphor and coal, explosive reaction of sodium with water and phosphorous with moist air are examples of fast reactions. setting of cement and rusting of iron take at least a few days or weeks time. These are very slow reactions.

A change is considered to be chemical reaction only if chemical bonds are broken in reactants and new bonds are formed in products. In other words, chemical reaction is a bond breaking and bond making process. In some cases, the products of a reaction recombine to form the reactants back. These are called reversible reactions. Others are irreversible reactions.

The rate of a chemical reaction is defined as the change in the concentration of the reactant or product per unit time. The rate of a reaction is positive, when expressed in terms of change in concentration of products but negative in terms of change in concentration of reactants.

A typical graph showing the change in the concentration of a reactant or product of a reaction is called Reaction Rate Graph.

Factors Affecting Chemical Reaction Rate

The Factors affecting the rate of reaction are summarized as below:

• Concentration of Reactants: The rate of reaction generally increases with increase in concentration of the reactants.
• Temperature: Generally increase in temperature increases the rate of reaction. When the temperature increases, the reactant molecules acquire higher energy and can easily form the products. Cooked food gets spoilt quickly during summer than winter. Souring of milk is faster in summer. These are all due to the fact that the chemical reactions responsible for these changes take place faster at higher temperatures.
• Presence of Light: There are certain reactions which take place or are accelerated by the absorption of light by the reactants. Such reactions are known as photochemical reactions. These reactions do not occur if the reactants are shielded from light. The combination of hydrogen and chlorine to produce hydrogen chloride does not take place at measurable rate in the dark. The plants prepare starch from carbon dioxide and water in the presence of sunlight by the process of photosynthesis. This reaction is slow in dim sunlight but it is much faster in bright sunlight.
• Presence of Catalyst: A catalyst is a substance which is added to a reaction mixture to alter the rate of chemical reaction where the mass and the chemical composition of the catalyst remain unchanged at the end of the reaction. Many industrially important reactions such as manufacture of ammonia, sulphuric acid, nitric acid and polythene are carried out using suitable catalysts.

Chemical Equilibrium:

In reversible chemical reactions, there is a point when forward and backward reactions proceed simultaneously at the same rate. This is called Chemical Equilibrium. For instance when hydrogen and iodine are taken in a closed vessel maintained at 717 K, hydrogen molecule combines with iodine molecule to form hydrogen iodide.

H₂(g) + I₂(g) ↔ 2HI(g)

Since the reaction is reversible in nature, the molecules of hydrogen iodide formed begin to dissociate to form hydrogen and iodine.

Exothermic and Endothermic Reactions

The chemical reactions which proceed with the evolution of heat energy are called exothermic reactions. Heat evolved is expressed in Joules (J) or kilo Joules (kJ).

All combustion reactions are exothermic. These reactions proceed with the evolution of heat energy. An important exothermic reaction occurs in our body cells which is nothing but respiration. During respiration, glucose in food burns in oxygen and gives out heat energy. Another example is formation of ammonia. When nitrogen combines with hydrogen in the presence of iron catalyst to form ammonia, a lot of heat is produced. Thus, the formation of ammonia is an exothermic reaction

N₂(g) + 3H₂(g) 2NH₃(g) + Heat

The chemical reactions which proceed with the absorption of heat energy are called endothermic reactions.

Why heat is evolved or absorbed in chemical reactions?

Chemical reaction involves the rearrangement of atoms. During the reaction, certain bonds are broken while certain new bonds are formed between the atoms. If the energy required to break the bonds is more than the energy released during the formation of bonds, then there is net absorption of energy and the reaction is endothermic. Both exothermic and endothermic reactions are due to the internal (intrinsic) energy that is stored within the structural units of every substance. When nitrogen and oxygen are heated to a very high temperature, they combine to form nitrogen monoxide and a lot of heat is absorbed in this reaction. Thus, the formation of nitrogen monoxide is an endothermic reaction.

N₂(g) + O₂(g) + Heat ↔2NO(g)

Impact of Temperature on Exothermic and Endothermic Reactions

Increase in temperature favours endothermic reactions and decrease in temperature favours the exothermic reactions.