What is Chemical Equilibrium?
Chemical equilibrium is the state a reversible reaction reaches when the forward and reverse reaction rates become equal. Concentrations stop changing, but the reaction never actually stops — it's a dynamic balance, not a frozen one.
Chemical equilibrium is the dynamic state of a reversible reaction where the forward and reverse rates are equal, so concentrations of reactants and products stay constant over time.
Try it: interactive calculator
Step-by-step worked examples
For the reaction A ⇌ B, at equilibrium [A] = 0.40 M and [B] = 1.60 M. Find Kc.
Kc = [B]/[A] Kc = 1.60 / 0.40 Kc = 4.0
For N2(g) + 3H2(g) ⇌ 2NH3(g), at equilibrium [N2] = 0.50 M, [H2] = 0.30 M, [NH3] = 0.20 M. Find Kc.
Kc = [NH3]^2 / ([N2][H2]^3) Kc = (0.20)^2 / (0.50 × (0.30)^3) Kc = 0.04 / (0.50 × 0.027) = 0.04 / 0.0135 Kc ≈ 2.96
For A + B ⇌ C + D, Kc = 10. At a given moment Q = [C][D]/([A][B]) = 4. Which way does the reaction shift?
Compare Q to Kc Q = 4 < Kc = 10 Since Q < Kc, the reaction shifts forward (toward products) until Q rises to equal Kc
Flashcards
Quick quiz
Q1.At equilibrium, the forward reaction rate is __ the reverse reaction rate.
Q2.For 2A ⇌ B, the Kc expression is:
Q3.If Q < Kc, the reaction will:
Q4.Increasing pressure on a gaseous equilibrium shifts it toward the side with:
The full card deck, worked steps and AI-tutor support for “What is Chemical Equilibrium?” are in Notek — study by hand before your exam.
Common mistakes
Equilibrium means reactant and product concentrations are equal. — Correct: Equilibrium means the rates are equal — concentrations can be very different from each other.
A large Keq means the reaction is fast. — Correct: Keq describes the position of equilibrium, not reaction speed — that's kinetics, a separate topic.
Adding a catalyst shifts equilibrium toward products. — Correct: A catalyst speeds up reaching equilibrium but does not shift its position.
Pure solids and liquids are included in the Keq expression. — Correct: Pure solids and liquids are left out of Keq — only gases and dissolved (aqueous) species count.
FAQ
What is chemical equilibrium?
It's the dynamic state of a reversible reaction where forward and reverse rates are equal, so concentrations stay constant.
What is the formula for the equilibrium constant?
Kc = [products]^coefficients / [reactants]^coefficients, evaluated at equilibrium.
How do you calculate Keq from concentrations?
Plug the equilibrium molar concentrations of products and reactants into the Kc expression, raised to their stoichiometric coefficients.
What are examples of chemical equilibrium?
The Haber process (N2 + 3H2 ⇌ 2NH3), dissolved CO2 in soda, and oxygen binding to hemoglobin are all real equilibria.