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6. EQUILIBRIUM
Reversible and irreversible reactions
A reaction that takes place in only one direction is called an irreversible reaction.
e.g. Reaction between NaOH and HCl
NaOH + HCl → NaCl + H2O
Reversible reactions are those which take place in both directions. i.e. here reactants
combined to form products and the products recombine to form reactants.
E.g. Haber process for the preparation of ammonia.
N2(g) + 3H2(g) 2NH3(g)
The process by which reactants are converted to products is called forward reaction and the
process by which products recombine to form reactants is called backward reaction. After sometimes,
the rate of forward reaction becomes equal to the rate of backward reaction and the reaction attains
equilibrium. Thus equilibrium is a state in which the rates of forward and backward reactions are equal.
Equilibrium is dynamic in nature. i.e. at equilibrium the reaction does not stop. The
reactant molecules collide to form products and the product molecules collide to form the
reactants and the rates of these reactions are equal.
Physical equilibrium
Equilibrium involving physical process is called physical equilibrium. E.g. melting of ice,
evaporation of water, sublimation, dissolution of solids or gases in liquids etc.
1. Solid-Liquid Equilibrium
When ice and water are kept in a perfectly insulated thermos flask at 273K and 1 atm pressure,
there exist an equilibrium between ice and water. i.e. ice is converted to water (melting or fusion) and
water is converted to ice (freezing) at the same rate. At this stage, there is no change in the mass of ice
and water.
H2O(s) ⇌ H2O(l)
Ice and water are in equilibrium only at particular temperature and pressure. For any pure
substance at atmospheric pressure, the temperature at which the solid and liquid phases are at
equilibrium is called the normal melting point or normal freezing point of the substance.
2. Liquid-Vapour Equilibrium
Water and water vapour are in equilibrium position at atmospheric pressure (1.013 bar) and at
100°C in a closed vessel. At this stage, the conversion of water to water-vapour [evaporation] and the
reverse process [condensation] takes place in equal rates.
H2O(l) ⇌ H2O(g)
For any pure liquid, the temperature at which the liquid and vapours are at equilibrium at one
atmospheric pressure (1.013 bar), is called normal boiling point of the liquid. The normal boiling point
of water is 100°C.
Boiling point of the liquid depends on the atmospheric pressure. It also depends on the altitude
of the place. At high altitude the boiling point decreases.
3. Solid – vapour Equilibrium
If we place solid iodine in a closed vessel, after sometimes the vessel gets filled up with violet
vapour and the intensity of colour increases with time. After certain time the intensity of colour
becomes constant and at this stage, equilibrium is attained. I2(solid) ⇌ I2 (vapour)
4. Dissolution of Solid in Liquids
Consider the dissolution of sugar in a fixed amount of water. When we add sugar into water, at
first, we can dissolve more amount of sugar, since the solution is unsaturated. But after sometime, the
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rate of dissolution of sugar decreases and rate of its crystallisation increases. i.e. the sugar molecules in
the solution separate out as solid sugar. At equilibrium state, the rate of dissolution becomes equal to
the rate of crystallisation. At this stage, the solution is called saturated solution. It is a solution in which
no more solute can be dissolved at a given temperature. The concentration of the solute in a saturated
solution depends upon the temperature.
Sugar (solution) ⇌ Sugar (solid)
5. Dissolution of Gases in Liquids
The solubility of a gas in a liquid depends on nature of the gas, pressure of the gas and
temperature. When pressure of the gas increases, its solubility also increases. A quantitative relation
between pressure and solubility was proposed by Henry and is known as Henry’s law. The law states
that the mass of a gas dissolved in a given mass of a solvent is proportional to the
pressure of the gas above the solvent, at a constant temperature.
In soda water, CO2 gas is dissolved in water at high pressure. There is an equilibrium between the
CO2(g) in the solution and that in the free state above the solution.
i.e., CO2 (gas) ⇌ CO2 (in solution)
When the bottle is opened, some of the dissolved carbon dioxide gas, escapes out with a fizzing
sound and a new equilibrium at low pressure is attained.
The solubility of a gas in a liquid decreases with increase in temperature.
General Characteristics of Physical Equilibrium
(i) Equilibrium is possible only in a closed system at a given temperature.
(ii) Both the opposing processes (forward and backward processes) occur at the same rate and
there is a dynamic but stable condition.
(iii) All measurable properties of the system remain constant at equilibrium.
(iv) When equilibrium is attained for a physical process, it is characterised by constant value of
one of its parameters at a given temperature.
(v) The magnitude of such quantities at any stage indicates the extent to which the physical
process has proceeded before reaching equilibrium.
Chemical Equilibrium
Equilibrium associated with chemical reactions is called chemical equilibrium. At equilibrium, the
concentrations of reactants and products are constant.
Consider a hypothetical reaction, A + B ⇌ C + D
As the reaction proceeds, the concentration of the reactants decreases and that of the products
increases. After sometimes, the two reactions occur at the same rates and an equilibrium state is
reached. This can be illustrated by the following graph.
After the equilibrium is attained, the concentration of the reactants and products become
constant. At equilibrium, the rate of forward reaction (rf) = the rate of backward reaction (rb).
+1 Chemistry Notes for 2023-24 : Prepared by ANIL KUMAR K L, APHSS ADICHANALLOOR Page 64
6. EQUILIBRIUM
Reversible and irreversible reactions
A reaction that takes place in only one direction is called an irreversible reaction.
e.g. Reaction between NaOH and HCl
NaOH + HCl → NaCl + H2O
Reversible reactions are those which take place in both directions. i.e. here reactants
combined to form products and the products recombine to form reactants.
E.g. Haber process for the preparation of ammonia.
N2(g) + 3H2(g) 2NH3(g)
The process by which reactants are converted to products is called forward reaction and the
process by which products recombine to form reactants is called backward reaction. After sometimes,
the rate of forward reaction becomes equal to the rate of backward reaction and the reaction attains
equilibrium. Thus equilibrium is a state in which the rates of forward and backward reactions are equal.
Equilibrium is dynamic in nature. i.e. at equilibrium the reaction does not stop. The
reactant molecules collide to form products and the product molecules collide to form the
reactants and the rates of these reactions are equal.
Physical equilibrium
Equilibrium involving physical process is called physical equilibrium. E.g. melting of ice,
evaporation of water, sublimation, dissolution of solids or gases in liquids etc.
1. Solid-Liquid Equilibrium
When ice and water are kept in a perfectly insulated thermos flask at 273K and 1 atm pressure,
there exist an equilibrium between ice and water. i.e. ice is converted to water (melting or fusion) and
water is converted to ice (freezing) at the same rate. At this stage, there is no change in the mass of ice
and water.
H2O(s) ⇌ H2O(l)
Ice and water are in equilibrium only at particular temperature and pressure. For any pure
substance at atmospheric pressure, the temperature at which the solid and liquid phases are at
equilibrium is called the normal melting point or normal freezing point of the substance.
2. Liquid-Vapour Equilibrium
Water and water vapour are in equilibrium position at atmospheric pressure (1.013 bar) and at
100°C in a closed vessel. At this stage, the conversion of water to water-vapour [evaporation] and the
reverse process [condensation] takes place in equal rates.
H2O(l) ⇌ H2O(g)
For any pure liquid, the temperature at which the liquid and vapours are at equilibrium at one
atmospheric pressure (1.013 bar), is called normal boiling point of the liquid. The normal boiling point
of water is 100°C.
Boiling point of the liquid depends on the atmospheric pressure. It also depends on the altitude
of the place. At high altitude the boiling point decreases.
3. Solid – vapour Equilibrium
If we place solid iodine in a closed vessel, after sometimes the vessel gets filled up with violet
vapour and the intensity of colour increases with time. After certain time the intensity of colour
becomes constant and at this stage, equilibrium is attained. I2(solid) ⇌ I2 (vapour)
4. Dissolution of Solid in Liquids
Consider the dissolution of sugar in a fixed amount of water. When we add sugar into water, at
first, we can dissolve more amount of sugar, since the solution is unsaturated. But after sometime, the
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rate of dissolution of sugar decreases and rate of its crystallisation increases. i.e. the sugar molecules in
the solution separate out as solid sugar. At equilibrium state, the rate of dissolution becomes equal to
the rate of crystallisation. At this stage, the solution is called saturated solution. It is a solution in which
no more solute can be dissolved at a given temperature. The concentration of the solute in a saturated
solution depends upon the temperature.
Sugar (solution) ⇌ Sugar (solid)
5. Dissolution of Gases in Liquids
The solubility of a gas in a liquid depends on nature of the gas, pressure of the gas and
temperature. When pressure of the gas increases, its solubility also increases. A quantitative relation
between pressure and solubility was proposed by Henry and is known as Henry’s law. The law states
that the mass of a gas dissolved in a given mass of a solvent is proportional to the
pressure of the gas above the solvent, at a constant temperature.
In soda water, CO2 gas is dissolved in water at high pressure. There is an equilibrium between the
CO2(g) in the solution and that in the free state above the solution.
i.e., CO2 (gas) ⇌ CO2 (in solution)
When the bottle is opened, some of the dissolved carbon dioxide gas, escapes out with a fizzing
sound and a new equilibrium at low pressure is attained.
The solubility of a gas in a liquid decreases with increase in temperature.
General Characteristics of Physical Equilibrium
(i) Equilibrium is possible only in a closed system at a given temperature.
(ii) Both the opposing processes (forward and backward processes) occur at the same rate and
there is a dynamic but stable condition.
(iii) All measurable properties of the system remain constant at equilibrium.
(iv) When equilibrium is attained for a physical process, it is characterised by constant value of
one of its parameters at a given temperature.
(v) The magnitude of such quantities at any stage indicates the extent to which the physical
process has proceeded before reaching equilibrium.
Chemical Equilibrium
Equilibrium associated with chemical reactions is called chemical equilibrium. At equilibrium, the
concentrations of reactants and products are constant.
Consider a hypothetical reaction, A + B ⇌ C + D
As the reaction proceeds, the concentration of the reactants decreases and that of the products
increases. After sometimes, the two reactions occur at the same rates and an equilibrium state is
reached. This can be illustrated by the following graph.
After the equilibrium is attained, the concentration of the reactants and products become
constant. At equilibrium, the rate of forward reaction (rf) = the rate of backward reaction (rb).
+1 Chemistry Notes for 2023-24 : Prepared by ANIL KUMAR K L, APHSS ADICHANALLOOR Page 64
