Homogeneous vs Heterogeneous Equilibrium
Chemical Equilibrium
5
JEE Qs
8%
Hard
50
min
Always identify the phase of each reactant and product to correctly write the equilibrium constant expression, especially for heterogeneous systems where pure solids/liquids are excluded.
🧮 Key Formulas
✅ Key Points for JEE
- 1Homogeneous equilibrium involves reactants and products all in the same phase (e.g., all gaseous, all liquid, or all aqueous solution). The equilibrium constant (Kc or Kp) includes all species.
- 2Heterogeneous equilibrium involves reactants and products in different phases (e.g., solid-gas, liquid-gas, solid-aqueous).
- 3The concentration (active mass) of pure solids and pure liquids is considered constant and is incorporated into the equilibrium constant. Therefore, they are omitted from the final expression for Kc or Kp.
- 4For heterogeneous equilibrium, only concentrations of gaseous species (as partial pressures or molar concentrations) and aqueous species (as molar concentrations) are included in the equilibrium constant expression.
- 5Adding or removing a pure solid or liquid reactant/product does not shift the equilibrium, as their effective concentration remains constant and they are not part of the K expression.
⚠️ Common Mistakes
- ✕Including concentrations/partial pressures of pure solids or liquids in the K expression for heterogeneous equilibria.
- ✕Incorrectly applying Le Chatelier's principle to changes in amount of pure solids/liquids in a heterogeneous system.
- ✕Failing to identify the phases of all species in a reaction before writing the equilibrium constant expression.
📝 Practice Questions
See allQ58.For the reaction, H2( g) + I2( g) ⇌2HI( g) Attainment of equillibrium is predicted correctly by : 2025 (24 Jan Shift 2) JEE Main Previous Year Paper (1) (2) (3) (4) −−
Q51.Consider the equilibrium CO( g) + 3H2( g) ⇌CH4( g) + H2O( g) If the pressure applied over the system increases by two fold at constant temperature then (A) Concentration of reactants and products increases. (B) Equilibrium will shift in forward direction. (C) Equilibrium constant increases since concentration of products increases. (D) Equilibrium constant remains unchanged as concentration of reactants and products remain same. Choose the correct answer from the options given below : (1) (A), (B) and (C) only (2) (A) and (B) only (3) (A), (B) and (D) only (4) (B) and (C) only
Q65.A vessel at 1000 K contains CO2 with a pressure of 0.5 atm . Some of CO2 is converted into CO on addition of graphite. If total pressure at equilibrium is 0.8 atm , then Kp is : (1) 1.8 atm (2) 0.3 atm (3) 3 atm (4) 0.18 atm
Q55.Consider the reaction X2Y( g) = X2( g) + 12 Y2( g) The equation representing correct relationship between the degree of dissociation (x) of X2Y(g) with its equilibrium constant Kp is ______ . Assume x to be very very small. (1) (2) x = 3√2Kpp x = 3√2Kp2p (3) (4) x = 3√Kpp x = 3√Kp2p 2025 (23 Jan Shift 2) JEE Main Previous Year Paper
Q53.At temperature T, compound AB2( g) dissociates as AB2( g) ⇌AB(g) + 12 B2( g) having degree of dissociation x (small compared to unity). The correct expression for x in terms of Kp and p is 2025 (29 Jan Shift 1) JEE Main Previous Year Paper (1) 4√2Kpp (2) 3√2Kpp (3) 3√2 pK2p (4) √Kp
Q37.At −20∘C and 1 atm pressure, a cylinder is filled with equal number of H2, I2 and HI molecules for the reaction H2( g) + I2( g) ⇌2HI(g), the Kp for the process is x × 10−1 . x = _____ [Given : R = 0.082 L atm K−1 mol−1 ] (1) 0.01 (2) 10 (3) 2 (4) 1
NCERT Chapters
- Class 11 Chemistry Ch 7: Equilibrium