Chemical Reactions and Their Types

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) + O_2(g) → CO_2(g)

Carbon C_(s) and are oxygen O_2(g) reactants, while carbon dioxide CO_2(g) is a product.

Types of Chemical Reactions:

Depending upon the phase of Reactants and Products:

Homogeneous Chemical Reactions:

A reaction in which all the substances involved exist in a single homogeneous phase is called homogeneous reaction.

Examples:

N_2(g)      +        3 H_2(g)       →           NH_3(g)

H_2(g)      +           I_2(g)         →          2HI_(g)

2SO_2(g) +          O_2(g)        →           2SO_3(g)

Characteristics of Homogeneous Reaction:

1. All the species involved (reactants and products) are in the same phase.
2. Thus the whole mixture has only a single phase.
3. There is no separation boundary between the species involved.

Heterogeneous Chemical Reactions:

A reaction in which the substance involved are present in different phases is called heterogeneous reaction.

Examples :

C_(s)         +   O_2(g)     →    CO_2(g)

CaCO_3(s)     →   CaO(s)    +   CO_2(g)

Characteristics of Heterogeneous Reaction:

1. All the species involved (reactants and products) are not in the same phase.
2. Thus the mixture contains more than two phases.
3. There is a clear separation boundary between the species involved.

Depending upon the evolution or absorption of heat:

Exothermic Chemical Reactions:

The chemical reactions in which heat is evolved are called exothermic reactions.

Example:
Nitrogen combines with hydrogen to give ammonia with evolution of 100 kJ of heat

N_2(g)      +        3 H_2(g)       →           NH_3(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.

Endothermic Chemical Reactions:

The chemical reactions in which heat is absorbed are called endothermic reactions.

Examples :

Nitrogen combines with oxygen to give nitric oxide with absorption of 100 kJ of heat

_N2(g)   +    O_2(g)         →   2NO_(g)   – 180 kJ

Hydrogen reacts with iodine to give hydrogen iodide with absorbtion of 51.88 kJ.

H_2(g)      +           I_2(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.

Depending upon the direction of the reaction:

Reversible Chemical Reactions:

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

A   +  B   ⇌   C   +   D

Characteristics of Reversible Reactions:

1. A reversible reaction is a reaction which can proceed in both the directions, forward and backward.
2. A reversible process is one whose direction can be reversed by an infinitesimal change in the conditions.
3. It is a hypothetical (imaginary) reaction.
4. It is a non-spontaneous process. It is to be arranged artificially.
5. This process is infinitesimally slow.
6. There is an equilibrium at every stage of the process.
7. The direction of the process can be reversed at any stage by an infinitesimal change in one of the state functions.
8. The driving and opposing forces differ by an infinitesimally small amount.
9. Maximum work can be obtained.

Irreversible Chemical Reactions:

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 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 irreversible reaction.

Examples:

C_(s)    +   O_2(g)   →   CO_2(g)

H_2(g) +   O_2(g)   →   2H₂O_(l)

Characteristics of Irreversible Reaction:

1. An irreversible reaction is a reaction which can proceed only in one direction. i.e. forward direction only.
2. The irreversible process is one whose direction cannot be reversed by changing the conditions slightly.
3. It is a natural and real process.
4. It is a spontaneous process. it takes place naturally.
5. This process is comparatively fast.
6. Equilibrium is reached at the end of the process.
7. The direction of the process cannot be reversed by small changes in state functions.
8. The driving forces are quite larger than opposing forces.
9. Maximum work cannot be obtained.