Types of Chemical Reactions

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Terminology:

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:

  • Note: Only those types of chemical reactions are discussed which are linked to the topic of chemical equilibrium.

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

Types of Chemical Reactions 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:
    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 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:
    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.



Types of Chemical Reactions 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

Types of Chemical Reactions (Contd.)

  • Characteristics of Reversible Reactions:
    1. A reversible reaction is a reaction which can proceed in both the directions, forward and backward directions.
    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 infinitesimally small amount.
    9. 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 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:
    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.

Rate of Chemical Reaction:

  • The change in the concentration of the reactants (or products) per unit time is called the rate of reaction
  • Mathematically it can be expressed as,

The rate of Reaction = Change in Concentration of Products or Reactants/time in which change is taking place



  • Hence the rate of chemical reaction is the change in concentration of the reactants in unit time. It’s S.I. unit is mol dm-3 s-1.

Active Mass:

  • Active mass is molar concentration per unit volume of that substance.
  • It is denoted by enclosing the symbol or formula of that substance in square bracket. For solutions, it is expressed in “ moles dm-3”. For gases, it is expressed in “ mol dm-3” or pressure in the atmosphere (atm) or bar or pascal (Pa).

Factors Affecting the Rate of Chemical Reaction:

Concentration:

  • If the concentration of reactants increases then the rate of reaction increases.

Pressure:

  • Change in pressure plays an important role in gaseous reactions. There can be three types of gaseous reactions:

    Reaction with the increase  in volume:

    The reduction in pressure for a gaseous reaction accompanied by an increase in volume increases the rate of reaction.

    e.g.  PCI5(g)    ⇌    PCl3(g)   +     Cl2(g)

  • Reaction with the decrease in volume:

    The increase in pressure for a gaseous reaction accompanied by a decrease in volume increases the rate of reaction.

    e.g. N2(g) +  3 H2(g)   ⇌    2NH3(g)

  • Reaction with no change in volume:

    The gaseous reaction of equilibrium, involving no change in volume is independent of pressure.

     e.g.  H2(g) + Cl2(g)  ⇌  2HCl(g)

Temperature:

  • In general rate of reaction increases with increase in temperature.

Catalyst:

  • In a chemical reaction, catalyst increases the rate of reaction.

Light:

  • The reactions which take place in presence of light are called photochemical reactions. Such reactions are influenced by light.

Size of particles:

  • If solid reactants are used in powdered form instead of granular form, the rate of reaction increases due to increase in the surface area available for reaction


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