Reaction Quotient — Q vs K
Chemical Equilibrium
5
JEE Qs
8%
Hard
60
min
Master the interpretation of Q vs K values to quickly determine the direction of a reaction and whether a system is at equilibrium, a fundamental skill for solving equilibrium problems.
🧮 Key Formulas
✅ Key Points for JEE
- 1The Reaction Quotient (Q) has the same mathematical form as the Equilibrium Constant (K) but is calculated using non-equilibrium (initial or current) concentrations or partial pressures of reactants and products.
- 2Comparing Q with K predicts the direction a reversible reaction will proceed to reach equilibrium: if Q < K, the reaction proceeds forward; if Q > K, the reaction proceeds backward; if Q = K, the system is at equilibrium.
- 3For heterogeneous reactions, pure solids and liquids are excluded from the expression of Q and K, as their concentrations/activities are considered constant.
- 4K is a constant for a given reaction at a specific temperature, while Q is a variable that changes as the reaction progresses towards equilibrium.
- 5The initial state where only reactants are present yields Q = 0 (assuming no products initially), while a state with only products yields Q = infinity (assuming no reactants initially).
⚠️ Common Mistakes
- ✕Using equilibrium concentrations/pressures to calculate Q, or non-equilibrium values to calculate K.
- ✕Incorrectly interpreting the Q vs K comparison, leading to wrong prediction of reaction direction (e.g., Q > K implies forward reaction).
- ✕Including pure solids or liquids in the reaction quotient or equilibrium constant expressions for heterogeneous equilibria.
- ✕Failing to balance the chemical equation or using incorrect stoichiometric coefficients in the exponent of the concentration/pressure terms.
📝 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