The state of equilibrium is a condition where the forces applied on an object from two opposite ends is equal, which keeps them under a balanced condition.There are three types of equilibriums such as a) mechanical, c) thermal and b) chemical equilibrium.
a) Mechanical Equilibrium: If the external force applied on an object is zero then it does not undergo any acceleration. This means that it remains in a steady state of equilibrium.
b) Thermal Equilibrium: When to two bodies of different temperature are kept in close contact with each other, the heat flows from on body to other. This happens until the temperatures of both the objects have reached an equal value or obtain equilibrium.
c) Chemical Equilibrium: A state of chemical reaction in which the formation of and conversion of material to product attains a constant value or a chemical reaction in which there is no increase or decrease in formation of product or intermediate or reactant is called chemical equilibrium.
There are mainly two types of chemical reactions: a) Reversible and b) irreversible reaction. An example of reversible reaction where chemical equilibrium is obtained is shown in equation 1a. Here, the formation of HI and its dissociation to hydrogen and iodine is attains an equilibrium such that rate of forward and backward reaction becomes equal.
H2+I2 2HI ——— — (1a)
Laws of Mass Action
When the chemical reaction reaches equilibrium it obeys laws of mass action. It states that in the given generic reaction (2a)
Aa + Bb Cc + Dd ——— — (2a)
Kc= [C]c [D]d/ [A]a [B]b ——— — (2b)
(Upper case characters represent chemical species and lower case characters show stoichiometric coefficients). In equation 2b [A] represents molar concentration of chemical species a. Further, numerator of the equation of product raised to the power of its stoichiometric coefficient. The denominator shows product of reactants.
If the chemical species are gases then the equation 2b can be written as equation 2c
KP= [PC]c [PD]d/ [PA]a [PB]b ——— — (2c)
KP is equilibrium rate constant; PC and PD are partial pressure of product of reactions, PA and PB are partial pressure of products. The lower case represents the concentrations of products and reactants of reactions. Further, KC and K P can be correlated using ideal gas law as shown in equation 2d.
P=RTC ——— — (2d)
Chemical equilibrium can classified into a) Dynamic equilibrium and b) Static equilibrium
a) Dynamic Equilibrium
The equilibrium is attained when forward and backward reaction rates of reactant and product are equal is called dynamic equilibrium. It does not take into consideration formation of equal concentration of product or reactants. It can be applicable to a reversible reaction. A reaction is at a state of dynamic reaction when rate of forward reaction is equal to rate of reverse reaction.
Dynamic equilibrium is useful in predicting whether the forward and backward reaction is spontaneous or non spontaneous. To explain this three quantities are being considered equilibrium constant (K), reaction quotient (Q) and Gibbs free energy (G).
K= [aC]c [aD]d/ [aA]a [aB]b (at equilibrium)
Q=[aC]c [aD]d/ [aA]a [aB]b (at any point of reaction)
aC activity of C, aD corresponds to activity of D.
G=dGro + RTlnQ
(dGro gibbs free energy for reaction under standard condition)
* G is negative, the forward reaction is spontaneous and reverse reaction non spontaneous.
* Q> K, reverse reaction is spontaneous and if Q<K forward reaction is spontaneous.
* At equilibrium Q=K and G=0
b) Static Equilibrium
It is also a mechanical equilibrium, which occurs when all the particles in reaction are at rest. It is also called as stead state from physics point of view. For example conversion of graphite to diamond where the reaction can be considered static when there is no force acting upon graphite (reactant) and diamond (product). The static equilibrium is applicable to irreversible reaction. A reaction is at static equilibrium if the rate of forward and backward reaction is zero.
Relation between equilibrium and rate constants
For the given reaction where reactants A and B are in equilibrium
A B —— — (3a)
Equilibrium constant is given by
Ke= [B]e/[A]e ——— — (3b)
Ke= equilibrium constant. The rate of the reaction is given by
d[A]/dt= -kf[A]+kb[B] ——— — (3c)
kf : Rate of forward reaction
kb: Rate of backward reaction
\ Ke= kf/kb ——— — (3d)
Le Chatelier’s Principle
For a reaction under dynamic equilibrium if there is a change in reaction conditions, there will be a shift in equilibrium to counteract this change. This change in reaction conditions can be on the account of changes in concentration, pressure or temperature.