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Equilibrium appeared 36 times across 3 years — 4.2% of Chemistry. This question is from Le Chatelier's Principle and Equilibrium Constant.

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Consider the following gaseous equilibrium in a closed container of volume "V" at T(K). P _ 2 (g) + Q _ 2 (g) leftharpoons 2 PQ (g) 2 moles each of P₂(g) , Q₂(g) and PQ (g) are present at equilibrium. Now one mole each of ' P₂ ' and ' Q₂ ' are added to the equilibrium keeping the temperature at T(K). The number of moles of P₂ , Q₂ and PQ at the new equilibrium, respectively, are -

Solution & Explanation

Core Logic
P₂(g) + Q₂(g) leftharpoons 2PQ(g)

Initially at equilibrium (t = teq): 2 mole, 2 mole, 2 mole.

Calculate Keq:

Keq = [PQ]²[P₂][Q₂] = 2²2 · 2 = 1

(Since Δ ng = 0, volume V cancels out in the expression.)

Now 1 mole of each P₂ and Q₂ is added. The reaction will move in the forward direction. t = t'eq : Moles of P₂ = 2 + 1 - x = 3 - x Moles of Q₂ = 2 + 1 - x = 3 - x Moles of PQ = 2 + 2x

Step 1: Solve for x
Kc = 1 = (2 + 2x)²(3 - x)(3 - x)

Taking the square root on both sides:

(2 + 2x)/(3 - x) = 1

2 + 2x = 3 - x

3x = 1 x = (1)/(3)
Step 2: Final Moles

At the new equilibrium: Moles of P₂ = 3 - (1)/(3) = (8)/(3) 2.67 Moles of Q₂ = 3 - (1)/(3) = (8)/(3) 2.67 Moles of PQ = 2 + 2((1)/(3)) = (8)/(3) 2.67

Pattern Recognition

For reactions where Δ ng = 0, concentration can be directly substituted with moles in the Kc expression because the volume terms cancel entirely.

Chapter Mix

Class 11 Chemistry: Equilibrium

Reference Study Guides

Equilibrium

More Equilibrium Previous-Year Questions — Page 8

Q62 jee_main_2024_31_jan_morning Equilibrium Constant
For the given reaction, choose the correct expression of KC from the following :- Fe(aq)³⁺ + SCN(aq)⁻ leftharpoons (FeSCN)(aq)²⁺
  • A. KC = [FeSCN²⁺][Fe³⁺][SCN⁻]
  • B. KC = [Fe³⁺][SCN⁻][FeSCN²⁺]
  • C. KC = [FeSCN²⁺][Fe³⁺]²[SCN⁻]²
  • D. KC = [FeSCN²⁺]²[Fe³⁺][SCN⁻]

Solution

Related Formula
KC = [Products][Reactants]
Core Logic
KC = Products ion conc.Reactants ion conc. KC = [FeSCN²⁺][Fe³⁺][SCN⁻]
Chapter Mix

Class 11 Chemistry: Equilibrium

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