In a chemical reaction one or more substances, called reactants undergo a chemical change to produce new substances called products of the reaction. Thus a chemical reaction is a process in which reactants undergo a change to produce products.
One or more substances which react and undergo chemical change are called reactants of the chemical reaction.
The new substances formed during the chemical reaction between the reactants are called products of the chemical reaction.
In the reaction, C(s) + O2(g) → CO2(g)
Carbon C(s) and are oxygen O2(g) reactants, while carbon dioxide CO2(g) is a product.
Types of Reactions:
Depending upon the phase of Reactants and Products:
A reaction in which all the substances involved exist in a single homogeneous phase is called homogeneous reaction.
Characteristics of Homogeneous Reaction:
1. All the species involved (reactants and products) are in the same phase.
2. Thus the whole mixture has only single phase.
3. There is no separation boundary between the species involved.
A reaction in which the substance involved are present in different phases is called heterogeneous reaction.
Characteristics of Heterogeneous Reaction:
1. All the species involved (reactants and products) are not in the same phase.
2. Thus mixture contains more than two phases.
3. There is clear separation boundary between the species involved.
Depending upon the evolution or absorption of heat
The chemical reactions in which heat is evolved are called exothermic reactions.
Nitrogen combines with hydrogen to give ammonia with evolution of 100 kJ of heat
N2(g) + 3 H2(g) → NH3(g) + 100 kJ
Characteristic of Exothermic Reaction:
1. In an exothermic reaction, heat is evolved.
2. For an exothermic reaction, the change in enthalpy is negative.
3. In an exothermic reaction, the enthalpy of reactants is more compared to that of products.
4. Products are more stable than the reactants.
The chemical reactions in which heat is absorbed are called endothermic reactions. Examples :
a) Nitrogen combines with oxygen to give nitric oxide with absorption of 100 kJ of heat N2(g) + O2(g) → 2NO(g) – 180 kJ
b) Hydrogen reacts with iodine to give hydrogen iodide with absorbtion of 51.88 kJ.
H2(g) + I2(g) → 2HI(g) – 51.88 kJ Characteristic of Endothermic Reaction:
1. In an endothermic reaction, heat is absorbed.
2. For an endothermic reaction, the change in enthalpy is positive.
3. In an endothermic reaction, the enthalpy of products is more compared to that of reactants.
4. Reactants are more stable than the products.