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For the reaction 2A ⇌ B + C, Kc = 4×10⁻³. At a given time, the composition of reaction mixture is: [A] = [B] = [C] = 2×10⁻³ M. Then, which of the following is correct?
Q_c = [B][C] / [A]² = (2e-3)(2e-3) / (2e-3)² = 1. Since Q_c = 1 >> K_c = 4 × 10⁻³, products are in excess -> Reversible reaction shifts BACKWARD!
For the generalized reversible reaction 2A ⇌ B + C, the reaction quotient Qc is defined using non-equilibrium instantaneous concentrations in the same functional form as the equilibrium constant:
Substituting the instantaneous concentration values [A] = [B] = [C] = 2 × 10⁻³ M into the quotient expression:
Comparing the calculated reaction quotient with the given equilibrium constant: - Qc = 1.0 - Kc = 4 × 10⁻³ = 0.004 Because Qc > Kc, the current concentration ratio of products to reactants is significantly greater than that at equilibrium. To establish dynamic equilibrium, the reaction must proceed from right to left (backward direction) to consume B and C while producing A until Qc decreases to equal Kc.
Diagnose every choice: Know exactly why the right option works and why distractors fail.
Exact thermodynamic deduction since Qc = 1 > Kc = 4 × 10⁻³.
Reversible equilibrium reactions with finite Kc do not go to completion.
Equilibrium requires Qc = Kc; here Qc = 1 ≠ 0.004.
Forward direction occurs only when Qc < Kc.
Equilibrium does NOT mean equal concentrations of all species! Equilibrium means the specific ratio of products to reactants equals Kc.
"If Qc > Kc, the reaction proceeds in the direction of reactants (reverse direction) until Qc decreases to Kc."
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