Science > Chemistry > Chemical Equilibrium > Types of Chemical Reactions
In this article, we shall discuss only those types of chemical reactions which are linked to the topic of chemical equilibrium.
Chemical Reaction:
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.
Reactants:
One or more substances which react and undergo chemical change are called reactants of the chemical reaction.
Products:
The new substances formed during the chemical reaction between the reactants are called products of the chemical reaction.
Example:
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 Chemical Reactions:
On the Basis of the phase of Reactants and Products:
Homogeneous Reaction:
A reaction in which all the substances involved exist in a single homogeneous phase is called homogeneous reaction.
Examples:
N2(g) + 3 H2(g) → NH3(g)
H2(g) + I2(g) → 2HI(g)
2SO2(g) + O2(g) → 2SO3(g)
Characteristics of Homogeneous Reaction:
- All the species involved (reactants and products) are in the same phase.
- Thus the whole mixture has only a single phase.
- There is no separation boundary between the species involved.
Heterogeneous Reaction:
A reaction in which the substance involved are present in different phases is called heterogeneous reaction.
Examples :
C(s) + O2(g)
→ CO2(g)
CaCO3(s) → CaO(s) + CO2(g)
Characteristics of Heterogeneous Reaction:
- All the species involved (reactants and products) are not in the same phase.
- The mixture contains more than two phases.
- There is a clear separation boundary between the species involved.
On the Basis of the evolution or absorption of heat:
Exothermic Reaction:
The chemical reactions in which heat is evolved are called exothermic reactions.
Example:
Nitrogen combines with hydrogen to give ammonia with the evolution of 100 kJ of
heat
N2(g) + 3 H2(g) → NH3(g) + 100 kJ
Characteristic of Exothermic Reaction:
- In an exothermic reaction, heat is evolved.
- For an exothermic reaction, the change in enthalpy is negative.
- In an exothermic reaction, the enthalpy of reactants is more compared to that of products.
- Products are more stable than the reactants.
Endothermic Reaction:
The chemical
reactions in which heat is absorbed are called endothermic reactions.
Examples :
a) Nitrogen combines with oxygen to give nitric oxide with the absorption of
100 kJ of heat
N2(g) + O2(g) → 2NO(g) – 180 kJ
b) Hydrogen reacts with iodine to give hydrogen iodide with the absorption of 51.88 kJ.
H2(g) + I2(g) → 2HI(g) – 51.88 kJ
Characteristic of Endothermic Reaction:
- In an endothermic reaction, heat is absorbed.
- For an endothermic reaction, the change in enthalpy is positive.
- In an endothermic reaction, the enthalpy of products is more compared to that of reactants.
- Reactants are more stable than the products.
On the Basis of the direction of the reaction:
Reversible Reaction:
A chemical reaction, in which the products formed react with each other to give back the original reactants, is called a reversible reaction. conventionally the reaction proceeding from left to right is called as a forward reaction while that proceeding from right to left is called as a forward reaction.
Explanation:
A + B → C + D (Forward Reactions)
C + D → A + B (Backward Reaction)
Here A & B are reactants, reacting with each other to form the products C & D. As the reaction proceeds the amounts of C & D will go on increasing and those of A and B will go on decreasing. But under the same conditions, C and D will react with each other to form A and B. Thus it is a reversible reaction. Both the reactions can be combined as
Characteristics of Reversible Reactions:
- A reversible reaction is a reaction which can proceed in both the directions, forward and backward directions.
- A reversible process is one whose direction can be reversed by an infinitesimal change in the conditions.
- It is a hypothetical (imaginary) reaction.
- It is a non-spontaneous process. It is to be arranged artificially.
- This process is infinitesimally slow.
- There is an equilibrium at every stage of the process.
- The direction of the process can be reversed at any stage by an infinitesimal change in one of the state functions.
- The driving and opposing forces differ by infinitesimally small amount.
- Maximum work can be obtained.
Irreversible Reaction:
A chemical reaction in which the products formed do not react with each other to produce the original reactants is called an irreversible reaction.
Explanation:
A + B → C + D
In the above example, A & B are reactants, reacting with each other to form C and D products. But under the same conditions products, C and D will not react with each other to form original products. Hence only forward reaction is possible. Hence it is an irreversible reaction.
Examples:
C(s) + O2(g)
→ CO2(g)
H2(g) + O2(g) → 2H2O(l)
Characteristics of Irreversible Reaction:
- An irreversible reaction is a reaction which can proceed only in one direction. I.e. forward direction only.
- The irreversible process is one whose direction cannot be reversed by changing the conditions slightly.
- It is a natural and real process.
- It is a spontaneous process. it takes place naturally.
- This process is comparatively fast.
- Equilibrium is reached at the end of the process.
- The direction of the process cannot be reversed by small changes in state functions.
- The driving forces are quite larger than opposing forces.
- Maximum work cannot be obtained.