Equilibrium

What Does Equilibrium Mean in Chemistry?

Equilibrium represents the state of a process in which the properties like temperature, pressure, concentration of the system does not show any change with the passage of time.

If a reaction is at chemical equilibrium, then the rate of forward reaction and backward reaction will be equal. 

The mixture of reactants and products in the equilibrium state is called an equilibrium mixture. 


Types of Equilibrium 


Physical Equilibrium 

If in equilibrium opposing processes involve only physical changes, then the equilibrium is called physical equilibrium. For example, equilibrium between water and vapor in evaporation of water. 

Water ↔ Vapor 


Characteristics of Physical Equilibrium – 

  • It is possible only in a closed system at given conditions such as temperature, pressure etc. 

  • Forward and backward reactions occur at same rate. 

  • All properties of the system which are measurable remain constant. 

  • When equilibrium is attained for physical process, it is characterized by constant value of one of its parameters at a given temperature. 

  • The magnitude of such quantities at any stage indicates the extent to which the physical process has proceeded before reaching equilibrium. 


Chemical Equilibrium 

If in equilibrium opposing processes involve chemical changes, then the equilibrium is called chemical equilibrium. For example, equilibrium between acetic acid, water and acetate and hydronium ions. Equation is given below –

CH3CO2H + H2O ⇌ CH3CO−2 + H3O+


Characteristics of Chemical Equilibrium – 

  • It can be achieved from either side or direction.  

  • It is dynamic in nature. 

  • The concentrations of reactants and products remain constant at equilibrium.

  • This type of equilibrium can be attained by the presence of a catalyst. 

The chemical equilibrium which involves equilibrium between ions in aqueous solution is called ionic equilibrium. 

Equilibrium can be established for both physical processes and chemical reactions. When the condition arrives in which there is no change in the concentrations of reactants and products and rate of forward reaction becomes equal to the rate of backward reaction then this type of equilibrium is called dynamic equilibrium. 

Homogeneous equilibria – In these types of equilibrium reactants and products are found in same phase.

Heterogeneous equilibria – In these types of equilibrium reactants and products are found in different phases. 

Reversible reaction – A chemical reaction which takes place in forward direction as well as in backward direction under the same conditions is called a reversible reaction. 

Irreversible reaction – A chemical reaction which takes place in forward direction only is called an irreversible reaction. 

Overall rate of a reaction – Overall rate of a reaction is calculated by subtraction of rate of backward reaction from the rate of forward reaction. 

Overall rate of a reaction = Rate of forward reaction – Rate of backward reaction

Here you need to note that at equilibrium the overall rate of a reversible reaction becomes zero. 


Law of Mass Action 

Law of mass action was given by Norwegian scientists Peter wage and Cato gulberg in 1864. The law explains the relationship between the velocity of a chemical reaction and molar concentrations of the reactants at a given temperature. 

The law of mass action states that the rate of a chemical reaction at a given temperature is directly proportional to the product of active masses of the reactants. Here, the meaning of active mass is molar concentration of reactant in per unit volume of it. Its unit is mol dm-3

Suppose for a general reaction – 

aA + bB ↔ cC + dD--------------(1)

rate of reaction ∝ [A]a [B]b

rate of reaction = K [A]a [B]b

where K = rate constant for the reaction at a given temperature

Unit of rate constant – K = \[\frac{\text{Rate of Reaction}}{[A]^{a}[B]^{b}}\] = \[\frac{molL^{-1}s^{-1}}{(molL^{1})^{a+b}}\] = (molL-1)1 to ns-1

For the reaction (1), rate of forward reaction (Rf) will be –

Rf   [A]a [B]b

Rf = Kf [A]a [B]b

where Kf = rate constant for the forward reaction at a given temperature

For the reaction (1), rate of backward reaction (Rb) will be –

Rb   [C]c [D]d

Rb = Kb [C]c [D]d

where Kb = rate constant for the backward reaction at a given temperature

At equilibrium, rate of forward reaction is equal to rate of backward reaction. So, we can write –

Kf [A]a [B]b = Kb [C]c [D]d

One rearranging the above equation -

\[\frac{K_{f}}{K_{b}}\] = \[\frac{[C]^{c}[D]^{d}}{[A]^{a}[B]^{b}}\]

At constant temperature, Kf and Kb are constant, so, \[\frac{K_{f}}{K_{b}}\] will also be a constant. Thus, we can write –

\[\frac{K_{f}}{K_{b}}\] = K\[_{c}\]

Kc is called equilibrium constant or rate constant for chemical reactions at equilibrium at a given temperature. 

At a given temperature, the product of concentrations of the reaction products raised to the respective stoichiometric coefficient in the balanced chemical equation divided by the product of concentration of the reactants raised to their individual stoichiometric coefficients has a constant value. This is known as the equilibrium law or law of chemical equilibrium. 


Equilibrium Constant – Few important points regarding equilibrium constant are listed below -

  • The value of the equilibrium constant for a particular reaction is always constant depending only upon the temperature of the reaction and is independent of the concentrations of the reactants with which we start or the direction from which the equilibrium is approached. 

  • If the reaction is reversed, the value of the equilibrium constant is inverse. 

  • If an equation having equilibrium constant K is divided by 2, the equilibrium constant for the new equation is the square root of K (that is, √K). 

  • If an equation having equilibrium constant K is multiplied by 2, the equilibrium constant for the new equation is the square of K (that is, K2

  • If an equation having equilibrium constant K is written in two steps (having equilibrium constant K1 and K2) then K1 × K2 = K. 

  • The value of the equilibrium constant is not affected by the addition of a catalyst to the reaction. This is because the catalyst increases the speed of the forward reaction and the backward reaction to the same extent.


Applications of Equilibrium Constants

Important features and applications of the equilibrium constant are listed below -

  • Expression for equilibrium constant is applicable only when concentrations of the reactants and products have attained constant value at equilibrium state. 

  • Its value is independent of initial concentrations of the reactants and products. 

  • It is temperature dependent. It shows an unique value for a balanced equation at a given temperature. 

  • For the reverse reaction, it is equal to the inverse of the equilibrium constant for the forward reaction. 

  • The equilibrium constant K for a reaction is related to the equilibrium constant of the corresponding reaction, whose equation is obtained by multiplying or dividing the equation for the original reaction by a small integer. 

  • It is used to predict the extent of a reaction on the basis of its magnitude. 

  • It is used to predict the direction of the reaction. 

  • It is used in calculations of the equilibrium concentrations. 


Predicting the Extent of a Reaction by Equilibrium Constant 

At equilibrium, if value of Kc is high –

(Kc > 103) Reaction will proceed nearly to completion or reaction will favour forward reaction

At equilibrium, if value of Kc is low –

(Kc > 10-3) Reaction will proceed rarely to completion or reaction will favour backward reaction

At equilibrium, if value of Kc is moderate –

(value of Kc is between 103 – 10-3) Reaction proceeds at equilibrium 


Calculating Equilibrium Concentrations 

Suppose we have the following information –

 1. The balanced reaction and value of Kc 

2. The initial concentration of the reactants, or the initial moles

3. Volume of the container 

We are asked to find the final equilibrium concentration of the reactants and products.

Then to calculate the equilibrium concentration of the reactants and products, we should follow following steps –

Step-1 Write down the balanced chemical equation for the reaction. 

Step-2 Under every reactant and product, write down the initial moles/concentration. 

Step-3 Subtract the amount reacting and add the amount produced in terms of a variable x and note down the equilibrium concentration in terms of x. If we are dealing in moles, then we will need to divide the moles by volume to obtain concentrations. 

Step-4 Substitute the equilibrium concentration in the expression of Kc and equate it to the value of Kc. 

Step-5 Solve the above equation and calculate the value of x and in case of multiple solutions select the value which is sensible from a reaction point of view. Then back substitute the value of x in the equilibrium concentration expression and obtain the actual value of the same.


Solubility Product (ksp

When a salt is dissolved in water then it starts breaking into ions and after sometime the solubility process attains equilibrium. 

AgCl(s) ↔️ Ag+ (aq) + Cl (aq) 

Ksp = [Ag+] [Cl] = Q = I.P. 

Where Ksp = solubility product 

Q = reaction quotient (It is defined as Kc but the concentrations in Qc are not necessarily equilibrium values)

I.P. = Ionic product

If I.P. < Ksp, then forward reaction will dominate, and more salt can be dissolved 

If I.P. = Ksp, then saturation will take place and no more salt can be dissolved

If I.P. > Ksp, then backward reaction will dominate and precipitation of solid salt will take place. 


Predicting the Direction of a Reaction 

• If Qc < Kc , net reaction goes from left to right.  It means the reaction will proceed in the direction of reactants.

• If Qc > Kc , net reaction goes from right to left. It means the reaction will proceed in the direction of products. 

• If Q = K , no net reaction occurs. It means the reaction mixture is already at equilibrium. 


Relationship Between Equilibrium Constant and Gibbs Free Energy (Keq and ΔG)

K = e\[^{-ΔG/RT}\]

If G is negative and K < 1 then, the reaction is spontaneous and proceeds in forward reaction. 

If G is positive for a reaction and K > 1 then, the reaction is nonspontaneous and proceeds in reserve or backward direction. 

If G is 0, reaction is at equilibrium. 


Le Chatelier’s Principle 

Le Chatelier’s principle states that - A change in any of the factors that determine the equilibrium conditions of a system will cause the system to change in such a manner so as to counteract the effect of the change. 

In other words, according to Le Chatelier’s principle, If a disturbance is introduced in an equilibrium mixture it will behave so as to undo the disturbance and re-establish the equilibrium.

Effect of change in concentration – If a reaction is at equilibrium and concentration of any reactant is increased then the equilibrium will shift in forward direction. While if concentration of any product is increased then the reaction will shift in the backward direction. If the concentrations are decreased, then the reverse happens. 

Effect of change in temperature - Exothermic reactions are favoured by low temperature whereas endothermic reactions are favoured by high temperature.

Effect of change in pressure – Change in pressure will not have any effect if moles of reactants and products are the same. When pressure is increased then reaction will shift in the direction having smaller number of moles and vice – versa. 

This ends our coverage on the topic “Equilibrium”. We hope you enjoyed learning and were able to grasp the concepts. You can get separate articles as well on various subtopics of this article such as reversible and irreversible reactions, rate of a reaction, law of mass action, pH scale etc. on Vedantu website. We hope after reading this article you will be able to solve problems based on the topic. If you are looking for solutions of NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.