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Understanding Concept Of Chemical Equilibrium

Chemical Bonding Assignment / Homework Help
In our daily life we observe one substance changing into another substance. Such transformation of substances is termed as reaction. The substances undergoing the change are termed as reactants and the substances obtained after the reaction are termed as products.


Rate of reaction:

The rate at which a substance reacts i.e. undergoes transformation is termed as rate of reaction. As the reaction occurs the concentration of the reactants decreases with an increase in the concentration of the products. The concentration of the reactants in expressed in terms of moles per liter which is termed as active mass.

Reversible and Irreversible reactions:

A reaction is said to be reversible if the products can be converted back to reactants under the same conditions in which the reactants were converted to products. Reversible reactions proceed in both forward as well as backward direction.

N2 +3H2 ↔ 2NH 3

3Fe + 4H2 O ↔ Fe3 O4 + 4H2

2SO2 +O2 ↔ 2SO3

A reaction is said to be irreversible if the products cannot be converted back in to reactants under the same conditions in which the reactants were converted to products are termed as irreversible reactions. These reactions proceed only in one direction.

Examples of irreversible reactions include:

NaCl + AgNO3 → AgCl + NaNO3

NaCl + H2SO4 → NaHSO4+ HCl

The irreversible nature of the certain reactions is due to conversion of one or more products into insoluble or volatile species. In the above reactions AgCl and HCl are volatile substances. Thus, the volatile products make the reaction unidirectional.

Chemical Equilibrium

When a reversible reaction is carried out in a closed vessel, a stage is reached when the speed of forward and backward reactions is the same and the system is said to be in chemical equilibrium. At the state of equilibrium the concentrations of the reactants and the products remains the constant.

Identification of the state of chemical equilibrium

As said above, the concentration of the reactants and products remains the same at the point of chemical equilibrium as long as the conditions are maintained stable. Properties that help in the identification of the chemical equilibrium are pressure, concentration of the reactants and products, density and color. These properties remain constant as long as the chemical equilibrium is maintained.

A simple example for the state of equilibrium is the dissociation of calcium carbonate. In this reaction chemical equilibrium is said to occur when the pressure of carbon dioxide becomes constant.

CaCO3 ↔ CaO + CO2

Another example is the reaction between hydrogen and iodine forming hydrogen iodide.

H2 + I2 ↔ 2HI

In this reaction, the concentration of the reactants, hydrogen and iodine and the product hydrogen iodide remains constant at the point of chemical equilibrium.

Solved problems
  • When is the reaction said to be in the state of chemical equilibrium?
    • When the reaction proceeds continuously
    • When the concentration of the reactants and products remains the same
    • When all the reactants are converted into products
    • All the above
    Answer: b
  • What is an irreversible reaction?
    • Reaction in which products can be converted back to reactants by altering the reaction conditions
    • Reactions in which products can be converted back to reactants under same conditions as that of the forward reaction
    • Reactions in which products cannot be converted back to reactants under same conditions
    • A and C
    Answer: d
  • Which of the following is a reversible reaction?
    • NaCl + AgNO3 → AgCl + NaNO3
    • NaCl + H2SO4 → NaHSO4+ HCl
    • H2 + I2 ↔ 2HI
    • All
    Answer: c
  • When do reactions become irreversible?
    • When volatile reactants are used as reactants
    • When volatile products are formed
    • Both a and b
    • None
    Answer: b

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