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Lesson 7 of 914 minutes

Dynamic Equilibrium, Q, K, and Response

Start with the lesson question, connect the representations, and test the model with evidence.

dynamic equilibriumequilibrium expressionreaction quotientle chatelier

Learning objectives

  • Explain equilibrium as equal forward and reverse rates, not stopped reactions.
  • Construct and interpret equilibrium expressions.
  • Use Q and K to predict the direction of net change.
Lesson flowHook, model, explanationShow guidance

Inspect the opening phenomenon

Predict what changes, then name the evidence.

Apply in the lab

Name the evidence before reading the answer.

Read only what helps

Then use the lab and recall check.

More when needed

Transcript and resources stay available below.

Course progress

AP Chemistry · Equilibrium · Lesson 7

Dynamic Equilibrium, Q, K, and Response

In progress

Decision challenge

Observe the phenomenon. Then connect the representations.

Use the opening example to make a prediction, identify evidence, and explain which model supports it.

Q vs K—Which Way Will Equilibrium Move? | AP Chemistry

Predict the net direction when Q is smaller than K.

Compare the current reaction quotient with the equilibrium constant to predict net direction.

Before

Predict the net direction when Q is smaller than K.

During

Track whether Q must rise or fall to equal K.

After

Explain why a catalyst changes the time to equilibrium but not K.

Reference drawerTranscript, source notes, scripts, and package status stay tucked away until you need them.7 files

Lesson reading

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14 min

Video script

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Transcript fallback

available

courses/ap-chemistry/modules/07-equilibrium/lessons/01-dynamic-equilibrium-q-k-and-response/video-transcript.md

A Reversible Color Equilibrium

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1 hr

Mastery check

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6 questions / 15 min

Book section:courses/ap-chemistry/modules/07-equilibrium/lessons/01-dynamic-equilibrium-q-k-and-response/book-section.md
Transcript for accessibility and fallback

# Dynamic Equilibrium: Q versus K Equilibrium does not mean stopped. It means the forward and reverse reaction rates are equal, so concentrations remain constant while particles keep reacting. To predict the next net change, calculate Q using the same expression as K. If Q is less than K, the mixture makes products. If Q is greater than K, it makes reactants. If Q equals K, it is at equilibrium. Adding a catalyst gets the system to equilibrium faster, but changes neither K nor the equilibrium composition. Remember: direction comes from comparing Q with K, not from guessing which side looks crowded. Retrieval pause: if extra product makes Q greater than K, which direction follows? Net reverse reaction. Continue the free AP Chemistry lesson at EduQuest.

Reading lab

Core explanation

Connect the lesson's words, diagrams, graphs, evidence, and equations.

Driving question

How can a mixture keep reacting while its macroscopic composition remains constant?

Equilibrium is dynamic

For a reversible reaction, equilibrium occurs when the forward and reverse reaction rates are equal. Both reactions continue. Constant concentrations do not imply equal concentrations; they imply no net macroscopic change.

Forward and reverse rates approach the same nonzero value at dynamic equilibrium

For aA+bB⇌cC+dDaA+bB\rightleftharpoons cC+dD, Kc=[C]c[D]d[A]a[B]b.K_c=\frac{[C]^c[D]^d}{[A]^a[B]^b}. Pure solids and pure liquids are omitted because their activities are effectively constant. Gases may be represented with partial pressures in KpK_p.

Q answers “where are we now?”

The reaction quotient QQ has the same algebraic form as KK, but uses current, not necessarily equilibrium, values.

  • Q<KQ<K: the net reaction proceeds forward, raising the product-to-reactant ratio.
  • Q>KQ>K: the net reaction proceeds in reverse, lowering that ratio.
  • Q=KQ=K: the system is at equilibrium.

This comparison predicts direction, not speed. A very slow system can have Q≠KQ\ne K, and a catalyst can speed its approach without changing KK.

Decision map showing net forward reaction for Q below K, equilibrium for Q equal to K, and net reverse reaction for Q above K

Disturbances as a Q-versus-K problem

At constant temperature, changing a concentration or partial pressure immediately changes QQ while KK stays fixed. The system then changes in the direction that restores Q=KQ=K. Temperature is different: changing temperature can change KK, because heat transfer changes the thermodynamic preference of the reaction.

For N2O4(g)⇌2NO2(g)\mathrm{N_2O_4(g)\rightleftharpoons 2NO_2(g)}, Qc=[NO2]2[N2O4].Q_c=\frac{[NO_2]^2}{[N_2O_4]}. Adding NO2NO_2 makes QcQ_c larger. If Qc>KcQ_c>K_c, net reverse reaction consumes NO2NO_2 until equilibrium is re-established.

Particle and rate model

A correct explanation connects three levels: the imposed change alters particle populations, collision frequencies change the forward or reverse rate, and composition evolves until the two rates are equal again. “The system shifts to oppose change” is a shortcut, not a causal explanation.

Retrieval challenge

  1. Why can concentrations remain constant while reactions continue?
  2. If Q<KQ<K, which net direction follows?
  3. Does a catalyst change KK? Explain.
  4. Why are pure solids omitted from heterogeneous equilibrium expressions?

Sources

Practice labA Reversible Color EquilibriumOpen this when you are ready to apply the model, collect evidence, and check your explanation.1 hr

Objective

How do concentration and temperature disturbances affect an equilibrium mixture?

Materials

  • Teacher-approved microscale iron(III)-thiocyanate mixtures or a nonhazardous digital simulation
  • Spot plate and transfer pipettes
  • Colorimeter or standardized image-analysis setup
  • Splash goggles and appropriate gloves

Steps

  1. Prepare a control and several matched microscale samples.
  2. Change one variable at a time as directed by the instructor.
  3. Record color intensity quantitatively.
  4. Interpret each immediate disturbance as a change in Q.
  5. Explain the net response as the direction required to restore Q = K.

Safety

Wear splash goggles and appropriate gloves; use microscale quantities in a ventilated laboratory; avoid skin and eye contact. If exposure occurs, notify the instructor immediately and flush the affected area with water for at least 15 minutes. Collect all iron/thiocyanate mixtures in the instructor-designated hazardous aqueous-waste container; do not pour them down the drain or return reagents to stock containers. A teacher-supervised nonhazardous simulation is the lower-risk alternative, but it does not replace required supervised AP laboratory experience.

Expected Result

For each trial, state the disturbance, predicted change in Q, net reaction direction, rate-level explanation, observation, and uncertainty. Distinguish concentration effects (Q changes while K stays fixed) from temperature effects (K may change).

Reflection Questions

  1. Which disturbance changed Q without changing K?
  2. How did the particle-level explanation support the observed color change?
  3. What evidence distinguishes a rate change from a change in equilibrium composition?

Extension Challenge

Design a quantitative calibration that converts color intensity to equilibrium concentration, then calculate an experimental K and propagate measurement uncertainty.