Equilibrium Constants Study Pack

Kibin's free study pack on Equilibrium Constants includes a 3-section study guide, 8 quiz questions, 10 flashcards, and 1 open-ended Explain review question. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.

Last updated May 21, 2026

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Equilibrium Constants Study Guide

Master the math and meaning behind equilibrium constants, including how to write expressions for Kc and Kp, apply the relationship Kp = Kc(RT)^Δn, and exclude pure solids and liquids correctly. This pack also covers interpreting K values to identify product- or reactant-favored reactions, using the reaction quotient Q to predict equilibrium shifts, and manipulating K algebraically when reactions are reversed or rescaled.

Key Takeaways

  • •The equilibrium constant (K) is a dimensionless ratio that expresses the relative concentrations (or pressures) of products to reactants at equilibrium for a given reaction at a specific temperature.
  • •Kc uses molar concentrations while Kp uses partial pressures; the two are related by the equation Kp = Kc(RT)^Δn, where Δn is the change in moles of gas.
  • •Pure solids and pure liquids are excluded from equilibrium expressions because their concentrations do not change during a reaction.
  • •A large K (much greater than 1) indicates that products are favored at equilibrium, while a small K (much less than 1) indicates that reactants are favored.
  • •The reaction quotient Q has the same mathematical form as K but is calculated using non-equilibrium concentrations; comparing Q to K predicts the direction a reaction will shift to reach equilibrium.
  • •Reversing a reaction inverts K, and multiplying the stoichiometric coefficients by a factor n raises K to the power n; these relationships allow K values to be combined algebraically.

What Equilibrium Constants Represent

When a reversible reaction reaches equilibrium, the forward and reverse rates are equal and concentrations stop changing — but the mixture is rarely half products and half reactants. The equilibrium constant K captures exactly how far toward products or reactants the system settles.

The Nature of Chemical Equilibrium

  • •At equilibrium, both the forward and reverse reactions continue to occur, but at identical rates, so macroscopic concentrations remain constant.
  • •Equilibrium is a dynamic state, not a static one — molecules keep reacting, but no net change in composition occurs.
  • •A given reaction reaches the same equilibrium ratio of products to reactants regardless of whether you start with pure reactants, pure products, or a mixture, as long as temperature is held constant.

Defining the Equilibrium Constant K

  • •K is defined as the ratio of product concentrations to reactant concentrations, each raised to the power of their stoichiometric coefficients in the balanced equation.
  • •For the general reaction aA + bB ⇌ cC + dD, the equilibrium expression is K = [C]^c[D]^d / [A]^a[B]^b.
  • •K is dimensionless and has a fixed value for a specific reaction at a specific temperature; changing the temperature changes K.

Interpreting the Magnitude of K

  • •When K >> 1 (for example, K = 10^5), the numerator dominates and products are strongly favored at equilibrium.
  • •When K << 1 (for example, K = 10^-5), the denominator dominates and the reaction barely proceeds — reactants are favored.
  • •When K ≈ 1, significant amounts of both reactants and products coexist at equilibrium.

Writing Equilibrium Expressions: Kc and Kp

The form of an equilibrium expression depends on whether concentrations or partial pressures are used, and certain species are excluded from the expression by convention.

Kc: Equilibrium Constant in Terms of Concentration

  • •Kc uses molar concentrations (mol/L) for all dissolved or gaseous species in the equilibrium expression.
  • •Brackets [ ] denote molar concentration; for example, the decomposition of N2O4(g) ⇌ 2NO2(g) gives Kc = [NO2]^2 / [N2O4].

Kp: Equilibrium Constant in Terms of Partial Pressure

  • •Kp applies specifically to gas-phase reactions and uses the partial pressure of each gaseous species in place of concentration.
  • •For the same N2O4 / NO2 equilibrium, Kp = (P_NO2)^2 / (P_N2O4), where pressures are expressed in atmospheres or bar.

Relationship Between Kc and Kp

  • •Kp and Kc are related by Kp = Kc(RT)^Δn, where R is the ideal gas constant (0.08206 L·atm/mol·K), T is absolute temperature in Kelvin, and Δn is the change in the total moles of gas (moles of gaseous products minus moles of gaseous reactants).
  • •When Δn = 0, Kp = Kc because the RT term equals 1.

Excluding Pure Solids and Pure Liquids

  • •Pure solids and pure liquids are omitted from equilibrium expressions because their concentrations are constant and do not shift with reaction progress.
  • •For example, in CaCO3(s) ⇌ CaO(s) + CO2(g), the expression reduces to Kp = P_CO2 because both solids are excluded.
  • •The solvent in a dilute aqueous solution is also treated as a pure liquid and excluded from Kc expressions.

About this Study Pack

Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.

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