Le Chatelier's Principle + Kp/Kc/Kx
Chemical Equilibrium
36
JEE Qs
8%
Hard
90
min
Master the systemic application of Le Chatelier's Principle to all types of stresses (concentration, pressure/volume, temperature, inert gas addition), particularly understanding how `Δn_g` governs pressure effects and that K is temperature-dependent only.
🧮 Key Formulas
✅ Key Points for JEE
- 1Le Chatelier's Principle predicts the *direction* of equilibrium shift to counteract an applied stress; the numerical value of K (equilibrium constant) *only* changes with temperature.
- 2For Kp-Kc and Kp-Kx relations, `Δn_g` is the difference in stoichiometric coefficients of *gaseous* products minus *gaseous* reactants. Pure solids and liquids are excluded from K expressions and `Δn_g` calculations.
- 3Adding an inert gas at constant volume has no effect on equilibrium position or K. Adding an inert gas at constant pressure (allowing volume to increase) shifts the equilibrium towards the side with a greater number of moles of gas.
- 4Increasing pressure by decreasing volume shifts equilibrium towards the side with fewer moles of gas; decreasing pressure by increasing volume shifts towards the side with more moles of gas.
- 5For exothermic reactions (ΔH < 0), K decreases with increasing temperature, shifting equilibrium left. For endothermic reactions (ΔH > 0), K increases with increasing temperature, shifting equilibrium right.
⚠️ Common Mistakes
- ✕Mistaking the effect of a catalyst (speeds up attainment of equilibrium) for shifting the equilibrium position (it does not).
- ✕Incorrectly calculating `Δn_g` by including moles of solids/liquids, or making sign errors (e.g., reactants - products instead of products - reactants).
- ✕Confusing the effect of adding inert gas at constant volume versus at constant pressure, or assuming it always shifts the equilibrium.
- ✕Believing that Kp or Kc changes with changes in concentration, pressure, or volume; K is constant at a given temperature.
📝 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