Reaction Quotient and Equilibrium Constant
Write Q and K for the stated species and phases; pure solids are conditions, not variable terms.
AP Chemistry Topic 7.3 · CED printed p. 130, 131 / PDF p. 137, 138. This teaching, practice and feedback were added for the AP edition; original author material remains separately traceable.
Before you start
- Balanced equations
- Molarity and partial pressures
The reaction quotient is a snapshot of a mixture. For aA + bB ⇌ cC + dD, . Products go on top, reactants underneath, and the balanced coefficients become powers. Square brackets mean mol L−1. For gases you can also write Qp using partial pressures in the same pattern. Gases may appear in Qc too: n/V is a perfectly good gas concentration. The suffix tells you what measurement to use, not which phase is allowed to exist.
Pure solids and pure liquids do not appear as variable terms. Adding more of a pure solid does not change its composition or activity, so that constant factor is already built into K. Water used as the solvent is treated this way in dilute aqueous equilibria. Water vapor is a gas and does appear. Omitting a solid from the expression does not mean it has no physical role; the relevant solid phase must be present for a heterogeneous saturated equilibrium.
Use the current concentrations for Q. Use the equilibrium concentrations at the given temperature for K. Their forms match, but their jobs differ. Kc and Kp should not be interchanged numerically; conversion between them is outside AP assessment. The rigorous quotient uses dimensionless activities relative to standard states. Activity is an effective concentration that accounts for interactions among particles; a standard state is its agreed reference condition. For an ideal dilute solute, concentration divided by the 1 M reference approximates activity; an ideal gas uses pressure divided by its stated reference pressure. Pure solids and liquids have activity one in their own standard states. AP problems normally give dilute-solution concentrations or near-ideal gas pressures, so use that stated model consistently rather than attaching arbitrary units to a thermodynamic K. Calculations for dissolved gas in equilibrium with its gas phase are also outside AP assessment.
Let's work through it
- For H2(g) + I2(g) ⇌ 2HI(g), identify all three species as gases.
- Write Qc = [HI]2/([H2][I2]) or Qp = (PHI)2/(PH2PI2). Pick the form matching the supplied data.
- The coefficient 2 on HI becomes a square. It is not a factor of 2 multiplying [HI].
- For a measured equilibrium state at the stated temperature, replace Q by the corresponding K without changing the expression.
Try it from memory
Pause & retrieve
Try it before you peek.
Write your answer and the reason from memory. Then compare with the worked answer. Your self-assessment is a study decision, not an automatically graded score.
Question 1
Does H2O(g) disappear from a quotient?
See every recall question with its answer
These references preserve each prompt and its answer for direct evidence links. Try the interactive questions above before opening the worked reasoning.
Retrieval 1
Does H2O(g) disappear from a quotient?
Worked answer and reason
No. Its variable gas concentration or partial pressure belongs in the expression.
The omission applies to a pure condensed phase, not to the chemical name water.
Retrieval 2
Why omit a pure solid?
Worked answer and reason
Its activity stays constant while that pure phase exists.
Changing the mass does not create an independent concentration variable.
Catch the tempting mistake
- A coefficient is multiplied by its concentration in Q.
- A coefficient becomes an exponent in the law-of-mass-action expression.
Put it to work
Give the chemical reason as well as the calculation. Worked solutions are on a separate page.
- Unit 7 problem 3 · ap application · symbolic
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For the teacher
- Use one gas reaction and one heterogeneous reaction in the same check; students must explain both state and exponent choices.
Source objectives and scope
Source edition: ap-chemistry-ced-fall2024-local-b5dfe867. See this topic in the course outline →
- 7.3.A
- Represent the reaction quotient Q_c or Q_p, for a reversible reaction, and the corresponding equilibrium expressions K_c = Q_c or K_p = Q_p.
- 7.3.A.1
- The reaction quotient Q_c describes the relative concentrations of reaction species at any time. For gas phase reactions, the reaction quotient may instead be written in terms of partial pressures as Q_p. The reaction quotient tends toward the equilibrium constant such that at equilibrium K_c = Q_c and K_p = Q_p. As examples, for the reaction a A + b B ⇌ c C + d D the law of mass action indicates that the equilibrium expression for (K_c, Q_c) is EQN: K_c = [C]^c[D]^d/([A]^a[B]^b) and that for (K_p, Q_p) is EQN: K_p = (P_C)^c(P_D)^d/((P_A)^a(P_B)^b)
- 7.3.A.2
- The reaction quotient does not include substances whose concentrations (or partial pressures) are independent of the amount, such as for solids and pure liquids.
- Conversion between K_c and K_p will not be assessed on the AP Exam. Students should be aware of the conceptual differences and pay attention to whether K_c or K_p is used in an exam question.
- Equilibrium calculations on systems where a dissolved species is in equilibrium with that species in the gas phase will not be assessed on the AP Exam.
Science references for this teaching
- AP Chemistry Course and Exam Description, effective Fall 2024 — Required topic-numbered learning objectives and essential knowledge, all continuations and AP exclusions; supplied PDF SHA256 b5dfe8677ef3d88c613865d2e2a3e8d6125d652e2b24c71ef1e8ce4e011094f0.
- OpenStax Chemistry 2e §13.2, Equilibrium Constants — Pressure/concentration quotients, pure-phase activities, and the distinction between composition and rate.
Glossary and deeper reading · 8 refreshers
These editorial refreshers supplement the reading. Definitions and examples are added guidance; original source wording and recorded answers remain in their original context.
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Molarity
Molarity is the amount of a solute in moles divided by the final solution volume in liters. It describes concentration rather than the total amount of solute. For example, 0.10 mole dissolved to make 0.50 liter of solution gives a molarity of 0.20 mole per liter.
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Partial pressure
A gas's partial pressure is the pressure it would exert alone in the mixture's volume at the same temperature. In an ideal mixture, the component partial pressures add to the total pressure. For example, a component with mole fraction 0.20 contributes 0.20 of the total ideal pressure.
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Reaction quotient
The reaction quotient describes a mixture's current composition using the equilibrium-expression form. It uses species activities, approximated by concentrations or partial pressures in suitable models, with powers from balanced coefficients. For example, a quotient below the equilibrium constant favors net forward reaction toward equilibrium.
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Mixture
A mixture contains two or more substances whose proportions can vary. Its components retain their chemical identities until a chemical reaction changes them. For example, water and ethanol can form mixtures with many different compositions.
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Solvent
A solvent is the component used as the dissolving medium in a solution. It is often present in the greater amount, though the designation also depends on the mixture's context. For example, water is the solvent in a dilute aqueous sugar solution.
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Dilution
Dilution lowers solute concentration by adding solvent while conserving the solute amount when no reaction or loss occurs. Concentration calculations use the final solution volume rather than simply the volume of solvent added. For example, increasing a solution's volume from 100 to 200 milliliters halves its concentration if its solute amount is unchanged.
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Chemical equilibrium
Chemical equilibrium is a dynamic state in which forward and reverse reaction rates are equal. Macroscopic composition remains constant in a closed system under fixed conditions, but reactant and product concentrations need not be equal. For example, a reversible reaction can reach equilibrium with much more reactant than product.
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Standard state
A standard state is a defined reference for a substance's thermodynamic properties. It specifies activities or reference forms and does not by itself fix the temperature at 25 °C. For example, gases commonly use a one-bar reference pressure while the table states temperature separately.
Read more: Gibbs Free Energy and Thermodynamic Favorability →