Equilibrium Constant for Heterogeneous Equilibria - Chemical Equilibrium | Physical Chemistry PDF Download

Equilibrium Constant for Heterogeneous Equilibria 
The equilibrium which involves reactants and products in different physical states. The law of mass action can also be applied on heterogenous equilibria as it was applied for homogenous equilibria (involving reactants and products in same physical states).
(i) Thermal Dissociation of Solid Ammonium Chloride: 
The thermal dissociation of NH₄Cl_(s) takes place in a closed container according to the equation

NH₄Cl_(s) ⇌ NH_3(g) + HCl_(g)

Let us consider 1 mole of NH4Cl(s) is kept in a closed container of vo lume ‘V’ litre at temperature TK and if x mole of NH₄Cl dissociates at equilibrium then

Applying law of mass action,

As NH₄Cl is a pure solid, so there is no appreciable change in its concentration. Thus, 

And

(ii) Thermal Dissociation of Ag₂CO₃:

Ag₂CO_3(s) dissociates thermally according to the equation.

Ag₂CO_3(s) ⇌ Ag₂O_(s) + CO_2(g)

Applying law of mass action, at constant temperature, we get,

Now, let us consider that 1 mole of Ag₂CO_3(s) is heated in a closed container of volume V and x mol of Ag₂CO_3(s) dissociates at equilibrium, then

Now, as Ag₂CO₃ and Ag₂O are solids, so their concentration can be assumed to be constants
Thus

7. Significance of magnitude of equilibrium constant: 
· A very large value of K_c or K_p signifies that the forward reactions has gone to completion or very nearly so.
· A very small value of K_c or K_p signifies that the forward reaction has not occur to any significant extent.
· If the numerical value of K_c or K_p is neit her very large or very small, a reaction is mo st likely to reach a state of equillibrium in which both reactants and products are present.

8. Predictions of direction of the reaction: 

Q_C is called React ion Quotient: 
Q_C is the ratio of concentration of products and reactants each raised to their schoitiometric coefficient as in balanced chemical equation. Hence, three cases arises:

Q_C < K_C  Forward direction
Q_C > K_C  Backward direction
Q_C = K_C  Equilibrium  

Note: While dealing with K_P and α, take init ial moles of reactant as 1 because ult imately they will be cancelled out.

9. Extent of Reaction:
Magnitude of equilibrium constant tells us the extent of reaction. It resembles the extent by which the reactant has been reacted or product has been formed at equilibrium.

Case I: If equilibrium constant (say K_C) is very-very small.

(Concentration of product is very low and concentration of reactant is very high) 

It clearly explains that numerator is very small and denominator is very-very high.

Hence, by this we can infer that reaction has a very low approach in forward direction and reactions has just started.

Case II: If equilibrium constant (i.e. KC) is very-very high. 

(Concentration of product is very high and concentration of reactant is very low).

Hence, it explains that reaction has almost completed.

Illustration: A(g) ⇌B(g) + C(g) 

Initial moles of reactant is 1 mole and volume of the container is 1L. Calculate the moles of B(g) at Equilibrium Kc = 10^–50 

Solution: Since the value of K_C is very low.

Since,  x <<<<1
Hence, K_C = x² = 10^–50

x = 10^–25

Illustration: If initially 1 mole of all the species present one taken in 10 L closed vessel, find equilibrium concentration of each species. 

A(g) ⇌ B(g) +C(g) k_c = 10^–2

Solution: A(g) ⇌ B(g) + C(g)

At any time concentration

Since Q_C > K_C, so reaction will move in backward direct ion to attain equilibrium.

Equilibrium concentration

Equilibrium concentration of A(g) = 0.16
Equilibrium concentration of B(g) = 0.04
Equilibrium concentration of C(g) = 0.04

Illustration: Calculate K_P for the reaction

N₂O₄(g) ⇌ 2 NO₂(g)

When NO₂ is 30% dissociated and total pressure at equilibrium is 2 bar. 

Solution:

Given, α = 0.3

We are dealing with K_P and α. Hence init ial mole of N₂O₄ = 1

Partial Pressure

Put α = 0.3
K_P = 0.79

10. Dealing with degree of dissociation (α)

In terms of moles

In terms of α