Equilibrium is a state in which there are no observable changes as time goes by. When a chemical reaction has
reached the equilibrium state, the concentrations of reactants and products remain constant over time and there
are no visible changes in the system. However, there is much activity at the molecular level because reactant
molecules continue to from product molecules while product molecules react to yield reactant molecules
State of equilibrium means the balance of driving forces i.e. the factors taking the reaction in forward direction
and the backword direction are balancing each other
Types of equilibria on the basis of process
The nature and the properties of the equilibrium state are the same regardless of the
direction from which it is achieved. It can be achieved in both directions.
Note Active masses are dimensionless quantities but for our purposes we generally take them with dimensions
of molarity, partial pressure, etc
Predicting the direction of the reaction
Predicting the extent of the reaction
Characteristics of equilibrium constant & factors affecting it
The reaction between alcohol and acid to form ester is an example of homogeneous equilibrium in
liquid system
For pure solid and pure liquid, active mass is taken to be unity i.e. 1 as they remain constant throughout
the reaction
Observed molecular weight and Observed Vapour Density of the mixture
If a change is applied to the system at equilibrium, then equilibrium will be shifted in that direction in which
it can minimise the effect of change applied and the equilibrium is established again under new conditions
On increasing pressure, equilibrium will shift in the direction in which total pressure decreases i.e. total no. of moles in
the reaction decreases and vice versa
It is the pressure exerted by the vapours over it’s liquid when it is in equilibrium with the liquid. Vapour pressure
of water is also called aqueous tension
Thermodynamics of Equilibrium
Condition for Spontaneity
If in any container there are two or more equilibria existing simultaneously involving one or
more than one common species. Then in both/all equilibrium the concentration of common
species is the total concentration of that species due to all the equilibria under
consideration
If in any container there are two or more equilibria existing simultaneously involving one or
more than one common species. Then in both/all equilibrium the concentration of common
species is the total concentration of that species due to all the equilibria under
consideration