1st Lecture on Chemical Equilibrium Chemistry Part II11th Standard by Rizwana Mohammed

1. The Malegaon High School & Jr. College
Malegaon, (Nashik), 423203
1st Lecture on Chemical
Equilibrium
Chemistry Part II, 11th Science
By
Rizwana Mohammad

2. Chemical equilibrium
Reversible reaction:
• The reaction which takes place in both directions is called as reversible
reaction.
• e.g. H2(g) + I2(g) ⇌ 2HI(g)
CH3COOH(aq) ⇌ CH3COO(aq) + H(aq)
• The reversible reaction is represented by a double arrow ⇌.
• One pointing in the forward direction and other in the reverse direction.
• The direction from reactants to products is the forward reaction.
• The opposite reaction from products to reactants is the reverse or
backward reaction.
Irreversible reaction:
• The reaction which takes place in only direction is called irreversible
reaction.
• e.g. CaCO3(g)
heat
CaO(s) + CO2(g)
• Irreversible reaction is represented by one arrow pointing in forward
direction.
+-

3. Equilibrium in physical processes:
a. Liquid-Vapour equilibrium:
• Look at a reversible physical process of evaporation of liquid water into
water vapour in closed vessel.
• Liquid evaporates in closed vessel, and exert vapour pressure.
• The vapours condense back into liquid state because the container is
closed
• Initially rate of evaporation is high and rate condensation is low.
• At equilibrium state the two rates become equal.
• In this state, the number of molecules leaving the liquid surface equals
the number those return to liquid from the vapour state.

4. • The equilibrium state may be represented as:
H2O(l) ⇌ H2O(vapour)
• At equilibrium, the pressure exerted by the gaseous water
molecules at a given temperature remains constant, known as the
equilibrium vapour pressure of water (or saturated vapour pressure
of water or aqueous tension).
• The saturated vapour pressure increases with increase in
temperature.
b. Solid-liquid equilibrium:
• Consider a mixture of ice and water in a perfectly insulated thermos
flask at 273K.
• It is an example of solid-liquid equilibrium.
• Ice and water are at constant temperature.
• They remain in what is called equilibrium.

5. c. Solid-vapour equilibrium:
• Place some iodine crystals in a closed vessel.
• Observe the change in colour intensity in it.
• After some time the vessel gets filled up with violet coloured vapour.
• The intensity of violet colour becomes stable after certain time.
• We see both, that is, solid iodine and iodine vapour in the closed vessel.
• It means that solid iodine sublimes to give iodine vapour and the iodine
vapour condenses to form solid iodine.
• The stable intensity of the colour indicates a state of equilibrium between
solid and vapour iodine.
I2(s) I2(g)
• Camphor(s) ⇌ Camphor(g)
• Ammonium chloride(s) ⇌ Ammonium chloride(g)
sublimation
condensation
Fig : Solid iodine in equilibrium
with its vapour

6. Equilibrium in chemical process:
• “When there is no further change in concentration of reactant and
product, we say that the reaction has attained equilibrium, with the
rates of forward and reverse reactions being equal.“
• Consider the following dissociation reaction.
2HI(g) ⇌ H2(g) + I2(g)
• The reaction is carried out in a closed vessel starting with hydrogen
iodide.
• At first, there is an increase in the intensity of violet colour.
• After certain time the increase in the intensity of violet colour stops.
• At this stage, it is observed that reaction mixture contains the HI, H2
and I2 with their concentrations being constant over time.