Heterogeneous equilibria

Heterogeneous equilibria

The chemical equilibrium in which the reactants and products are not in the same phases are called heterogeneous equilibrium. An example of heterogeneous equilibrium can be the decomposition of calcium carbonate which upon heating forms calcium oxide and carbondioxide under equilibrium conditions. When the reaction is carried out in a closed vessel, the following heterogeneous equilibrium is established.

CaCO3    -- > < --   CaOs) + CO2(g)

The equilibrium constant expression for CaCO₃  dissociation can be written as

K = [CO₂] [CaO] / [CaCO₃]

[CaCO₃]

But CaO and CaCO₃ are pure solids. The activity or concentration of pure solids is unity.

Thus Kc     =    [CO2]

in terms of partial pressures,

Kp = pco₂, where Pco2 is the pressure of CO2 alone in equilibrium. There are many examples of heterogeneous chemical equilibria with K_p and K_c values different depending on the number of product and reactant molecules.

(i) The equilibrium constant expression for decomposition of liquid water would be

2H2O(l)  -- > < --- 2H2(g) + O2(g)

K  = [H₂]² [O₂] / [H₂O]² but activity of liquid H₂O = 1.0

K_c  = [H₂]² [O₂]

and K_p  = (p_H2)² (p_O2)     p = partial pressure.

(ii) Consider the equilibrium reaction of decomposition of NH₄ Cl.

NH₄Cl_(s) -- > < -- NH3(g) + HCl(g)

K_c = [NH₃] [HCl] because [NH₄Cl₍₅₎] = 1.0

K_p = P_NH3   P_HCl.P=Partial Pressure

(iii) Consider the hydrogen gas evolution equilibrium such as

3Fe(s) + 4H₂O_(g) -- > < -- Fe₃O₄   +   4H₂(g)

K_c = [H₂]⁴ / [H₂O]⁴

k_p  = (p_H2)² / (p_H2O)⁴

since activities of Fe(G) and Fe3O4(s) are 1.0.