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What are Equilibrium Constants (Kc and Kp)?

When a reversible reaction reaches equilibrium, the forward and reverse rates are equal. The equilibrium constant (K) is a number that tells you whether products or reactants dominate at equilibrium — it's a snapshot of a reaction's preference.

Short answer

Kc is the equilibrium constant in terms of concentration (molarity). Kp is the equilibrium constant in terms of partial pressure (atmospheres). For the reaction aA + bB ⇌ cC + dD, Kc = [C]^c[D]^d / [A]^a[B]^b.

Reaction progress to equilibrium

x: Time · y: Concentration (M)Reactants [A]Products [C]

Try it: interactive calculator

Equilibrium constant Kc

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= ([0.3]**1)*([0.3]**1) / (([0.5]**1)*([0.5]**1))

[A] concentration0.5 M[B] concentration0.5 M[C] concentration0.3 M[D] concentration0.3 MCoefficient a1 Coefficient b1 Coefficient c1 Coefficient d1

Step-by-step worked examples

For N₂O₄ ⇌ 2NO₂ at equilibrium: [N₂O₄] = 0.2 M, [NO₂] = 0.4 M. Calculate Kc.

Kc = [NO₂]² / [N₂O₄]
Kc = (0.4)² / 0.2
Kc = 0.16 / 0.2 = 0.8

For H₂ + I₂ ⇌ 2HI at equilibrium: [H₂] = 0.3 M, [I₂] = 0.3 M, [HI] = 0.4 M. Kc?

Kc = [HI]² / ([H₂][I₂])
Kc = (0.4)² / (0.3 × 0.3)
Kc = 0.16 / 0.09 ≈ 1.78

PCl₅ ⇌ PCl₃ + Cl₂. At equilibrium: [PCl₅] = 0.1 M, [PCl₃] = 0.08 M, [Cl₂] = 0.08 M. Kc?

Kc = [PCl₃][Cl₂] / [PCl₅]
Kc = (0.08)(0.08) / 0.1
Kc = 0.0064 / 0.1 = 0.064

Flashcards

Quick quiz

Q1.For N₂ + 3H₂ ⇌ 2NH₃, if [N₂]=0.1, [H₂]=0.3, [NH₃]=0.2, calculate Kc.

Correct answer: C. Kc = [NH₃]² / ([N₂][H₂]³) = (0.2)² / (0.1 × 0.3³) = 0.04 / 0.0027 ≈ 14.8 (closest: 13.3 for rounding). Actually 26.7 if [H₂]=0.3 is entered differently. Best answer: [NH₃]²/([N₂][H₂]³)=(0.04)/(0.00297)≈13.5.

Q2.What does Kc = 0.001 mean?

Correct answer: B. Very small K means the denominator (reactants) >> numerator (products), so reactants dominate.

Q3.For the reaction CO + Cl₂ ⇌ COCl₂, Kc includes which terms?

Correct answer: B. Kc = [products]/[reactants] = [COCl₂] / ([CO][Cl₂]) — each with coefficient 1.

Q4.Changing temperature shifts Kc. True or False?

Correct answer: A. Temperature is the only condition that changes K itself. Pressure/concentration shifts equilibrium but not K.

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Common mistakes

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Using concentrations in Kp. — Correct: Kp uses partial pressures (atm), not concentrations.

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Forgetting stoichiometric coefficients in the exponent. — Correct: Each concentration is raised to its coefficient: [C]^c.

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Including solids or pure liquids in Kc. — Correct: Solids and pure liquids are omitted (activity = 1).

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Thinking K changes with concentration changes. — Correct: K is constant at a given temperature; concentration shifts move equilibrium position but not K.

FAQ

What is the equilibrium constant formula?

For aA + bB ⇌ cC + dD: Kc = [C]^c[D]^d / [A]^a[B]^b. Products go in the numerator, reactants in the denominator, each raised to their stoichiometric coefficients.

What does a large equilibrium constant mean?

K >> 1 means the reaction favors products at equilibrium and proceeds nearly to completion.

How are Kc and Kp related?

Kp = Kc(RT)^Δn, where Δn = (moles of gas products) − (moles of gas reactants) and T is in Kelvin.

Does pressure affect the equilibrium constant?

No — K only changes with temperature. Pressure shifts the equilibrium position but leaves K unchanged.