CONDITIONS FOR A CHEMICAL CHANGE OR REACTION
Channel: Ramancoeducation
Date: 01/12/2022

The conditions for a chemical reaction or chemical change are :

1. Close contact. A chemical reaction occurs only when the substances are brought in close contact with each other. The close contact can be brought about by grinding the reactants together or dissolving the reactants in a suitable solvent like water, etc.

2. The state of the reactants. The kinetic energy of molecules is least in solids and maximum in gases. So, frequency of collision will also be maximum for gases and hence reaction will be facilitated most when the reactants are in the gaseous state.

3. Heat. Some reactions occur only by heating, e.g. potassium chlorate decomposes only on heating.
Heat increases the temperature and it in turn increases kinetic energy. So with increase in temperature, number of collisions increase resulting in increase in rate of chemical reaction.
Generally, the rate of reaction doubles for a 10°C rise in temperature.
4. Light. Some reactions take place only when the reactants are exposed to light. Such reactions are called photochemical reactions.
5. Pressure. Some reactions take place only when the reactants are subjected to high pressure.
6. Electricity. Some reactions occur when an electric discharge is passed.
e.g. Acidulated water decomposes on passing electric current through it giving hydrogen and oxygen gases.
7. Catalyst. A catalyst can increase or decrease the rate of reaction, without itself undergoing any change.
A catalyst is a substance which increases or decreases the rate of reaction but remains unchanged at the end of reaction. Example : Potassium chlorate decomposes at 230°C instead of 400°C if heated in the presence of manganese dioxide (Catalyst).

Promoter : A substance which increases the efficiency of a catalyst is known as promoter.
Example : Molybdenum acts as a promoter during manufacture of ammonia.

8. Concentration of the reactants. With the increase in the concentration of the reactants, the rate of a reaction increases. One probable reason is increase in the number of collisions. More is the cone, of reactants more is the number of collisions and faster is the rate of reaction.