Important Notes on Chemical Equilibrium for JEE Main 2020

Important Notes in Chemical Equilibrium

By Poojashree AgarwalUpdated : Sep 10, 2020 , 19:32 IST
Important Notes on Chemical Equilibrium for JEE Main 2020

Chemical Equilibrium

It is the state in which both reactants and products are present in concentrations which have no further tendency to change with time. 

At the  Equilibrium

  1. The Rate of the forward reaction is always equal to the rate of backward reaction
  2.  The Concentration (mole/litre) of reactant and product is constant.
  3. The net GiBBs energy is constant ΔG = O
  4. The equilibrium constant is equal to Q.

Equilibrium Constant (K):image001Equilibrium constant in terms of concentration (KC) is given by 


Equilibrium constant in the terms of partial pressure (KP) is given by 


The equilibrium constant is in terms of mole fraction (Kx) is given by 


The Relation between Kp & Kc is given by 


Relation between KP and KX is given by 


here  P = Total pressure at equilibrium.

Δn = (c+d)-(a+b), calculation of Δn involves only gaseous components.

So in the above equation n is the difference of the suml of moles between gaseous products and gaseous reactants which can be Positive o negative both 


So there are many reasons that how K(eq) depends on the stoichiometry of the reaction. See below points

(a) If  K1 and K2  are the constant of two reactions then on combining them  the resulting equation has equilibrium constant  which will be equal to K = K1 . K2

Equilibrium constants

Let the reaction is A to B and B to C and both the reactions are reversible


On adding image008

(b) If the reaction having eq. constant K1 is reversed then resulting equation has eq. constant 1/K1


On reversing, image010

(c) If a chemical reaction having equilibrium constant K1 is multiplied by a factor n then the resulting equation has equilibrium constant K = (K1)n, n can be fraction

Eg. image011

Multiplying by image012

  •  How  Keq depends on the temperature:-

Equilibrium constant  always depends on the temperature of the reaction

By the above statement means kp and kc will always  lremain constant at constant temperature without any effect of  pressure, concentration, volume or catalyst.

But kx depends on pressure provided Δn≠0, i.e. even at constant temperature, kx will undergo a change by the change in total pressure at equilibrium.

image013 Enthalpy of reaction:-


If T2 > T1 then

K2 > K1 provided ΔH = +ve : this is case if endothermic reaction

K2 < K1 if ΔH = -ve : this is case of exothermic reaction

Noe: In the above equation, the unit of R and ΔH should be same.


Relation between equilibrium constant and standard free energy change.

ΔG° = -2.303 RT log K

Where ΔG° = standard free energy change

T = Absolute temperature

R = universal gas constant.

Significance of value of Equilibrium constant (k):

How to use neglection in the problem:


Case. I

If K is large (k > 103) then the concentration  of prodeuct is  larger than the reactant. Therefore,  concentration of the reactant can be neglected with respect to the product. In this case, the reaction is product loving  and equilibrium will be more in forward direction as compared to the  backward direction.

Case. II

If K is very small (K < 10-3)

[Product] << [Reactant]


Reaction Quotient (Q):


The reaction quotient is 

-if Q > KC reaction goes in backward direction until equilibrium in reached

-if Q < KC reaction goes in forward direction until equilibrium is maintained

-if Q = KC Reaction is at equilibrium

Degree of dissociation (α)

α = no of moles dissociated / initial no. of moles taken

= fraction of moles dissociated out of 1 mole.

Note: % dissociation = α x 100

  • Observed molecular weight and Observed Vapour Density of the mixture

Observed molecular weight of


D = vapour density without dissociation image017

d = vapour density of mixture = observed v.d. image018


where MT = Theoretical molecular wt. M0 = observed molecular wt. or molecular wt. or molecular wt. of the mixture at eq.

Note: It is not applicable for n = 1 [eg. Dissociation of HI & NO].

Significance of n:


Change in  Equilibrium due to external factor:

Le Chatelier’s Principle:

A principle stating that if a constraint (such as a change in pressure, temperature, or concentration of a reactant) is applied to a system in equilibrium, the equilibrium will shift so as to tend to counteract the effect of the constraint

Change due to concentration:

The equilibrium proceeds in forward direction with increase in concentration of Reactant where it happens vice versa in case of products.


Effect of volume:

 If volume is inversely proportional to the  pressure. if volume is increased then the pressure automatically  decreases. Eventually the reaction will proceed accordingly with the change in equilibrium

* If volume is increased then, for

Δn>0 reaction will move in the forward direction

Δn<0 reaction will move in the backward direction

Δn=0 reaction will remain unchanged.

Effect of Pressure:

* If pressure is increased at equilibrium then reaction will try to decreases the pressure, hence it will shift in the direction in which less no. of moles of gases are formed.

Effect of inert gas addition:

(i) Constant pressure: If inert gas is added then to maintain the pressure constant, volume is increased.

Hence, equilibrium will shift in the direction in which larger no. of moles of gas is formed.

Δn>0 reaction will shift in the forward direction

Δn<0 reaction will shift in the backward direction

Δn=0 reaction will not shift.

(ii) Constant volume: Inert gas addition has no effect at constant volume.

Effect of Temperature:

Equilibrium constant depends on temperature only.

Int the  plot of ln k vs 1/Temp.  when plotted then it is a straight line with the slope equal to  image021 and the  intercept is image022


* For endothermic (ΔH>0) reaction, value of the equilibrium increases with increase in temperature.

* For exothermic (ΔH<0) reaction, value of the equilibrium constant decreases with increase in temperature.


Vapour Pressure of Liquid:

Vapour pressure i s the pressure exerted by the molecule of gasses on the liquid. Aqueous tension is the vapour pressure of liquid.

image023 constant at fixed temperature

Hence, V.P. of any liquid never depends on the  pressure, volume and  the change in concentration.

e.g. at 25°C, vapour pressure of water ≈ 24 mm of Hg

Relative Humidity image024

Thermodynamics of Equilibrium

For a general reaction, image025 is given by


If plot of lnk vs 1/T is plotted then it is a straight line with slope image027, and intercept image022





Vant Hoff equation


Note: * ΔH° should be substituted with sign.

* For endothermic (ΔH>0) reaction value of the equilibrium constant increases with the rise in temperature

* For exothermic (ΔH<0) reaction, value of the equilibrium constant decreases with increase in temperature

Simultaneous Equilibrium:

The kind of Equilibrium in which more than two equilibria exists one or more than one species is known as simultaneous equilibrium.IF there's a system like this then equilibrium of common species in all the equilibrium will always be same.


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Poojashree Agarwal
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Poojashree Agarwal