🎓 Prepared by students from Boğaziçi University

What is the Equilibrium Constant?

The equilibrium constant K quantifies the ratio of product to reactant concentrations at equilibrium. A larger K means products are strongly favored; a smaller K means reactants dominate. K is temperature-dependent and fixed for a specific reaction.

Short answer

The equilibrium constant K = [products]/[reactants] (each raised to stoichiometric coefficients) at equilibrium. Its value is fixed for a given reaction at a specific temperature and determines which side the reaction favors.

Concentration change toward equilibrium

x: Time · y: Concentration (M)Reactant [A]Product [C]

Try it: interactive calculator

Equilibrium constant Kc

0.08(depends on stoichiometry)

= (0.5*0.3)/(2*1)

[A] reactant (M)2 M[B] reactant (M)1 M[C] product (M)0.5 M[D] product (M)0.3 M

Step-by-step worked examples

For the reaction N₂ + 3H₂ ⇌ 2NH₃, equilibrium concentrations are [N₂] = 0.5 M, [H₂] = 1.5 M, [NH₃] = 2 M. Calculate Kc.

Kc = [NH₃]² / ([N₂][H₂]³)
Kc = (2)² / (0.5 × (1.5)³)
Kc = 4 / (0.5 × 3.375) = 4 / 1.6875 = 2.37

A reaction A ⇌ B has Kc = 4 at 25°C. If at equilibrium [A] = 0.1 M, find [B].

Kc = [B] / [A]
4 = [B] / 0.1
[B] = 0.4 M

For 2A ⇌ B + C, Kc = 9 at 300K. At equilibrium [A] = 1 M, [B] = 1.5 M. Find [C].

Kc = [B][C] / [A]²
9 = (1.5)[C] / (1)²
[C] = 9 / 1.5 = 6 M

Flashcards

Quick quiz

Q1.For A ⇌ 2B, Kc = [B]²/[A]. At equilibrium, [A] = 0.2 M and [B] = 0.8 M. Calculate Kc.

Correct answer: D. Kc = (0.8)² / 0.2 = 0.64 / 0.2 = 3.2. Wait, recalculate: (0.8)² = 0.64; 0.64/0.2 = 3.2. The index should be 0 but let me verify: (0.8)^2 = 0.64, divided by 0.2 = 3.2. Correct answer is 3.2, index 0.

Q2.Which statement is true about Kc?

Correct answer: B. Kc is temperature-dependent. It does not change with concentration, catalyst, or pressure (for gases).

Q3.A reaction has Kc = 0.1. At equilibrium, are products or reactants favored?

Correct answer: B. Kc < 1 means reactants are favored; very little product forms.

Q4.If Kc = 100, the equilibrium position favors:

Correct answer: B. Kc > 1 means products are strongly favored at equilibrium.

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Common mistakes

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Kc changes with concentration. — Correct: Kc is constant at a given temperature, independent of concentration.

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Kc tells you how fast equilibrium is reached. — Correct: Kc tells you the ratio at equilibrium; rate of reaching equilibrium is separate.

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Stoichiometric coefficients are not used in Kc. — Correct: Each substance is raised to its stoichiometric coefficient: Kc = [C]^c[D]^d / [A]^a[B]^b.

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Only products appear in Kc numerator. — Correct: Both products (numerator) and reactants (denominator) appear, each raised to their coefficient.

FAQ

What is the equilibrium constant formula?

For aA + bB ⇌ cC + dD: Kc = [C]^c[D]^d / [A]^a[B]^b, where [X] is molar concentration.

How does temperature affect Kc?

Kc changes with temperature; each temperature has its own Kc value. Kc does not change with concentration or pressure.

What does Kc > 1 mean?

Products are favored at equilibrium; the reaction strongly produces products.

Can Kc be negative or zero?

No, Kc is always positive because it is a ratio of concentrations (all positive).