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Types of Chemical Reactions

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.

Science > Chemistry > Chemical Equilibrium > Types of Chemical Reactions

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