EQUILIBRIUM Class 11
Equilibrium Class 11
Introduction
Equilibrium is the most important characteristic property of reversible reactions. These reactions for which the forward reaction occurs to a much greater extent are considered to be unidirectional in nature and whenever the rate of forward reaction is equal to the rate of backward reaction, equilibrium is attained, not to forget that equilibrium exists only in a closed system.
It is the state of the system at which temperature, pressure, volume, and composition have fixed values and do not vary with time. Chemical Reactions can be divided into two categories:
Irreversible Reactions: The reactions that proceed to completion and the products fail to recombine to give back reactants. For example:
Reversible Reactions: The reactions that never go to completion and products recombine to give back reactants. For example:
Physical Equilibrium:
We know that solid, liquid, and gas are the three states of substance. Therefore, three types of physical equilibrium are possible. These are
Solid(s) ⇌ liquid (l)
Liquid(l) ⇌ gas(g)
Solid(s) ⇌ gas(g)
Here the sign double half arrows (⇌) pointing in opposite directions are both for the reversible change as well as for the equilibrium state.
Equilibrium Class 11
1. Solid(s) – liquid(l) equilibrium: At equilibrium, two processes take place at the same rate i.e., Ice(s) ⇌ water(l)
H2O(s) ⇌ H2O(l)
At equilibrium,
Rate of melting of ice = Rate of freezing of water
The temperature at which the solid and liquid states of a pure substance are in equilibrium at the atmospheric.
pressure is called the normal freezing point or melting point of that substance.
2. Liquid(l) – gas(g) equilibrium:
H2O(l) ⇌ H2O(g)
In such type of equilibrium,
Rate of vaporization of water = Rate of condensation of water vapour
3. Solid(s) – gas(g) equilibrium: Such type of equilibrium is attained in case of volatile solids.
Example: If solid iodine is placed in a closed vessel, violet vapours start appearing in the vessel.
The intensity of violet vapour increases with time and ultimately it becomes constant.
I2(s) ⇌ I2(g)
In this equilibrium,
Rate of sublimation = Rate of condensation
4. Solids in liquids: Suppose sugar is added continuously into a fixed volume of water at room temperature and stirred thoroughly with a glass rod. First, the sugar will keep on dissolving but then a stage will come when no more sugar dissolves. Instead, it settles down at the bottom. The solution is now said to be saturated and in a state of equilibrium. In this state
Rate of dissolution = Rate of precipitation
Sugar(s) ⇌ Sugar (in solution)
Equilibrium Class 11
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