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What is Chemical Equilibrium and the Equilibrium Constant?

Q: What does a small Kc mean?

Kc << 1 means equilibrium favors reactants; little product forms at equilibrium.

Q: How does temperature affect Kc?

Kc changes with temperature. Exothermic reactions: higher T → smaller Kc. Endothermic reactions: higher T → larger Kc.

Chemical equilibrium is reached when forward and reverse reactions occur at equal rates, so concentrations stay constant. The equilibrium constant (Kc or Kp) quantifies how far a reaction proceeds before equilibrium.

Short answer

For aA + bB ⇌ cC + dD at equilibrium: Kc = [C]^c [D]^d / [A]^a [B]^b (concentration-based), Kp = P(C)^c P(D)^d / P(A)^a P(B)^b (pressure-based). Large K favors products; small K favors reactants.

Reaction progress to equilibrium

x: Time (s) · y: Concentration (mol/L)[A] reactant[C] product

Try it: interactive calculator

Equilibrium constant Kc

3.33varies (dimensionless here)

= (0.5 * 0.4) / (0.2 * 0.3)

Equilibrium conc. of reactant A0.2 mol/LEquilibrium conc. of reactant B0.3 mol/LEquilibrium conc. of product C0.5 mol/LEquilibrium conc. of product D0.4 mol/L

Step-by-step worked examples

For H₂ + I₂ ⇌ 2HI at equilibrium: [H₂]=0.1, [I₂]=0.1, [HI]=0.8 (all mol/L). Find Kc.

H₂ + I₂ ⇌ 2HI
Kc = [HI]² / ([H₂][I₂])
Kc = (0.8)² / (0.1 × 0.1) = 0.64 / 0.01 = 64

For N₂ + 3H₂ ⇌ 2NH₃ at equilibrium: [N₂]=1, [H₂]=1, [NH₃]=2. Calculate Kc.

Kc = [NH₃]² / ([N₂][H₂]³)
Kc = (2)² / (1 × 1³) = 4 / 1 = 4

If Kc = 10 for A ⇌ B, does the equilibrium favor products or reactants?

Kc = 10 >> 1 (much greater than 1)
→ Equilibrium heavily favors products
Most A converts to B at equilibrium

Flashcards

Quick quiz

Q1.For 2NO ⇌ N₂ + O₂, if [NO]=0.1, [N₂]=0.05, [O₂]=0.05 at equilibrium, Kc=?

Correct answer: C. Kc = [N₂][O₂] / [NO]² = (0.05 × 0.05) / (0.1)² = 0.0025 / 0.01 = 0.25. Wait, that's option 0. Let me recalculate: (0.05×0.05)/(0.01) = 0.0025/0.01 = 0.25. So correct is 0.

Q2.Which Kc value indicates equilibrium favors products?

Correct answer: C. Large Kc >> 1 favors products. Kc = 1000 is very large.

Q3.For A ⇌ 2B, if at equilibrium [A]=0.2 and [B]=0.8, Kc=?

Correct answer: D. Kc = [B]² / [A] = (0.8)² / 0.2 = 0.64 / 0.2 = 3.2. Wait, let me check: 0.64/0.2 = 3.2. Actually that's option 2. Hmm, (0.8)^2 = 0.64, 0.64/0.2 = 3.2. So correct is 2.

Q4.Kc and Kp are equal when…

Correct answer: B. Kp = Kc(RT)^Δn. When Δn = 0 (moles gas equal), Kp = Kc.

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Common mistakes

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Forgetting to apply stoichiometric exponents in Kc formula. — Correct: Kc = [C]^c [D]^d / [A]^a [B]^b (use coefficients as exponents).

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Including pure solids or liquids in Kc. — Correct: Kc uses only aqueous/gaseous species; omit solids and pure liquids (activity = 1).

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Thinking Kc changes when conditions change. — Correct: Kc depends only on temperature. Pressure or concentration changes shift equilibrium but don't change Kc.

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Confusing Kc = 1 with no reaction. — Correct: Kc = 1 means products and reactants have comparable concentrations, not no reaction.

FAQ

What is the equilibrium constant formula?

For aA + bB ⇌ cC + dD: Kc = [C]^c [D]^d / [A]^a [B]^b (concentration basis) or Kp (pressure basis).

What does a large Kc mean?

Kc >> 1 means equilibrium strongly favors products; the reaction goes nearly to completion.

What does a small Kc mean?

Kc << 1 means equilibrium favors reactants; little product forms at equilibrium.

How does temperature affect Kc?

Kc changes with temperature. Exothermic reactions: higher T → smaller Kc. Endothermic reactions: higher T → larger Kc.