# Important Notes on Chemical Equilibrium for JEE Main 2020

Important Notes in Chemical Equilibrium

### 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**

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

**Equilibrium Constant (K):****Equilibrium constant in terms of concentration (K**_{C}) is given by

_{C}) is given by

**Equilibrium constant in the terms of partial pressure (K _{P}) is given by **

**The equilibrium constant is in terms of mole fraction (K _{x}) is given by **

**The Relation between K _{p} & K_{c} is given by **

**Relation between K _{P} and K_{X 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 K_{1} and K_{2} are the constant of two reactions then on combining them the resulting equation has equilibrium constant which will be equal to **K = K _{1} . K_{2}**

Equilibrium constants

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

On adding

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

_{}

On reversing,

(c) If a chemical reaction having equilibrium constant K_{1} is multiplied by a factor n then the resulting equation has equilibrium constant K = (K_{1})^{n}, n can be fraction

Eg.

Multiplying by

**How K**_{eq}depends on the temperature:-

**Equilibrium constant always depends on the temperature of the reaction**

By the above statement means k_{p} and k_{c} will always lremain constant at **constant temperature** without any effect of pressure, concentration, volume or catalyst.

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

**Enthalpy of reaction:-**

If T_{2} > T_{1} then

K_{2} > K_{1} provided ΔH = +ve : this is case if endothermic reaction

K_{2} < K_{1} 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 > 10^{3}) 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 > K**_{C} reaction goes in backward direction until equilibrium in reached

-if **Q < K _{C}** reaction goes in forward direction until equilibrium is maintained

-if **Q = K**_{C} 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

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

where M_{T} = Theoretical molecular wt. M_{0} = 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 *l*n k vs 1/Temp. when plotted then it is a straight line with the slope equal to and the intercept is

* 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.

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 **

**Thermodynamics of Equilibrium**

For a general reaction, is given by

If plot of *l*nk vs 1/T is plotted then it is a straight line with slope , and intercept

**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