Watch Class 11 Chemistry Chemical Equilibrium ( Complete Chapter ) video Lessons !!
What is Chemical Equilibrium?
“A stage during a reversible reaction at which rate of forward reaction becomes equal to the rate of reverse reaction is called Chemical Equilibrium.”
H2(g) + I2(g) ⇌ 2HI(g)
This is a reversible reaction.
At beginning only forward reaction occurs and no reverse reaction as no products are there. But then backward reaction also starts. With the passage of time rate of forward reaction becomes slow and slow as concentrations of reactants drop first rapidly and then slowly, while rate of reverse reaction becomes fast and fast as concentration of products increases first rapidly and then slowly, and ultimately two rates become equal. This stage of the reaction is called equilibrium.
In above mentioned case, reaction started with formation of Hydrogen Iodide but with passage of time rate of formation of Hydrogen Iodide becomes slow and slow while rate of its dissociation becomes fast and fast and ultimately the two rates becomes equal and equilibrium establishes.
(i) Rate of forward and reverse reaction becomes equal.
(ii) Concentrations of reactants and products become constant i.e. no further change in concentration of reactants or products occurs.
(iii) For equilibrium to exist, a close system is usually necessary so that neither reactants nor products escape.
- Equilibriums are of two types
- (i) Static equilibrium
- (ii) Dynamic equilibrium.
- Static equilibrium is the one in which system is stationary e.g. a book lying on table is in static equilibrium.
- Dynamic equilibrium is the one in which system is moving with constant speed e.g. Earth is moving around Sun with constant speed so its equilibrium is dynamic equilibrium, similarly chemical equilibrium is also dynamic equilibrium.