AP Chemistry Flashcards: Reaction Quotient and Equilibrium Constant
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: May 2026
Review key ideas with interactive flashcards. This set includes 10 cards to help you master important concepts.
Write the general equilibrium expression (Kc) for the reaction aA + bB ⇌ cC + dD.
The equilibrium expression is Kc = [C]^c[D]^d / [A]^a[B]^b.
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Write the general equilibrium expression (Kc) for the reaction aA + bB ⇌ cC + dD.
The equilibrium expression is Kc = [C]^c[D]^d / [A]^a[B]^b.
What principle defines the equilibrium expression?
The law of mass action defines the equilibrium expression, which relates the concentrations or partial pressures of products and reactants at equilibrium.
How are the expressions for the reaction quotient (Qc) and the equilibrium constant (Kc) related?
The expression for the reaction quotient Qc has the same mathematical form as the equilibrium constant expression Kc, but Qc can be calculated at any time, not just at equilibrium.
Distinguish between Kc and Kp.
Kc is the equilibrium constant expressed in terms of molar concentrations of species, while Kp is the equilibrium constant expressed in terms of the partial pressures of gaseous species.
What is the reaction quotient (Q)?
The reaction quotient (Qc for concentrations, Qp for partial pressures) describes the relative concentrations or partial pressures of species in a reaction at any given time.
What is the relationship between the reaction quotient (Q) and the equilibrium constant (K) when a system is at equilibrium?
When a chemical reaction reaches equilibrium, the value of the reaction quotient (Q) becomes equal to the equilibrium constant (K).
If a reaction includes the species Fe(s), should its concentration be included in the expression for Qc?
No, Fe(s) should not be included because it is a solid, and the concentrations of solids are constant and omitted from the reaction quotient expression.
What types of substances are excluded from reaction quotient (Q) and equilibrium constant (K) expressions?
Solids and pure liquids are excluded because their concentrations are considered constant and independent of the amount present.
For a reaction involving H2O(l) as a solvent, why is it omitted from the Kc expression?
As a pure liquid, the concentration of H2O(l) is constant and independent of the amount, so it is omitted from the equilibrium expression.
What does Qp represent in the context of a chemical reaction?
Qp is the reaction quotient for a reversible reaction involving gases, calculated using the partial pressures of the reactants and products at any given moment.