Equilibrium Constants Study Pack
Kibin's free study pack on Equilibrium Constants includes a 6-section study guide, 25 quiz questions, 30 flashcards, and 5 open-ended Explain review questions. 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 27, 2026
Equilibrium Constants Study Guide
Master the equilibrium constant expression, from writing Kc and Kp to interpreting Q vs. K and predicting reaction direction. Covers stoichiometric exponents, the Kp = Kc(RT)^Δn relationship, and rules for reversed or combined reactions.
Key Takeaways
- •The equilibrium constant (K) is a dimensionless ratio that expresses the relationship between product and reactant concentrations at equilibrium for a given reaction at a specific temperature.
- •For a general reaction aA + bB ⇌ cC + dD, the equilibrium constant expression is K = [C]^c[D]^d / [A]^a[B]^b, where brackets denote molar concentrations and exponents match stoichiometric coefficients.
- •Kc uses molar concentrations while Kp uses partial pressures of gases; 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 remain effectively constant throughout the reaction.
- •The magnitude of K indicates the position of equilibrium: K >> 1 means products predominate at equilibrium, while K << 1 means reactants predominate.
- •When a reaction is reversed, its K becomes the reciprocal of the original; when reactions are added together, their K values are multiplied.
- •The reaction quotient Q has the same algebraic form as K but uses non-equilibrium concentrations, and comparing Q to K predicts the direction a reaction will shift to reach equilibrium.
What the Equilibrium Constant Represents
At equilibrium, the forward and reverse rates of a reversible reaction are equal, producing a stable ratio of products to reactants that is captured mathematically by the equilibrium constant.
Defining Chemical Equilibrium
- •A reversible reaction reaches equilibrium when the rate of the forward reaction equals the rate of the reverse reaction, so the concentrations of all species remain constant over time.
- •Equilibrium is dynamic, not static — individual molecules continue to react in both directions, but the macroscopic concentrations do not change.
- •The equilibrium state depends on temperature; changing the temperature produces a different value of K for the same reaction.
The Equilibrium Constant K
- •K is a single number that characterizes the equilibrium position of a reaction at a given temperature, regardless of the initial concentrations used.
- •Because K is derived from a ratio of concentrations raised to powers, it is dimensionless — units cancel through the convention of dividing each concentration by a standard reference concentration of 1 M.
- •K is specific to the reaction as written, including the physical states and stoichiometric coefficients shown in the balanced equation.
Writing Equilibrium Constant Expressions
The algebraic form of the equilibrium constant expression follows directly from the stoichiometry of the balanced chemical equation, with specific rules about which species are included.
The Law of Mass Action
- •For a reaction aA + bB ⇌ cC + dD, the equilibrium expression is Kc = [C]^c[D]^d / [A]^a[B]^b, where square brackets indicate molar concentration and the exponents are the stoichiometric coefficients from the balanced equation.
- •Products appear in the numerator and reactants in the denominator — this convention is fixed and not interchangeable.
Excluding Pure Solids and Pure Liquids
- •Pure solids and pure liquids have constant densities and are not included in equilibrium expressions because their 'concentrations' do not change as the reaction proceeds.
- •Water acting as a solvent is omitted from Kc expressions for the same reason, even though dissolved species in water are included.
- •When a pure solid or liquid appears in a reaction, its activity is defined as 1, so including it would not change the numerical value of K.
Heterogeneous vs. Homogeneous Equilibria
- •A homogeneous equilibrium involves all species in the same phase (e.g., all aqueous or all gaseous), and every species appears in the K expression.
- •A heterogeneous equilibrium involves species in more than one phase; only gases and dissolved solutes appear in the expression, while solids and pure liquids are excluded.
Kc and Kp: Concentration vs. Pressure Forms
For reactions involving gases, the equilibrium constant can be expressed in terms of either molar concentrations (Kc) or partial pressures (Kp), and the two forms are mathematically related.
Defining Kp
- •Kp is written using the partial pressures of gaseous reactants and products in place of molar concentrations, with each partial pressure raised to the power of the corresponding stoichiometric coefficient.
- •Partial pressures are typically expressed in atmospheres or bars, but the ratio convention makes Kp dimensionless when pressures are referenced to a standard pressure of 1 bar.
Converting Between Kc and Kp
- •The relationship between the two forms is Kp = Kc(RT)^Δn, where R is the ideal gas constant (0.08206 L·atm/mol·K), T is the absolute temperature in kelvins, and Δn is the change in the total number of moles of gas (moles of gaseous products minus moles of gaseous reactants).
- •When Δn = 0, Kp equals Kc because the (RT)^Δn factor equals 1.
- •When Δn is positive, Kp > Kc; when Δn is negative, Kp < Kc.
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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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Question 1 of 25
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For the general reaction aA + bB ⇌ cC + dD, which expression correctly represents the equilibrium constant Kc?
Card 1 of 30
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Concept 1 of 5
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Chemical Equilibrium
Explain what it means for a chemical reaction to reach equilibrium. Why is equilibrium described as 'dynamic' rather than 'static,' and what role does temperature play?
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