Equilibrium at the Particle Level: How to Draw and Explain Shifts With Le Châtelier in AP Chemistry

Ever stared at a brown gas turning colorless and wondered what particles are doing behind the scenes? That’s equilibrium in action. At equilibrium, reactions still happen in both directions, but at equal rates, so the counts of particles stay constant. In this guide, you’ll learn to predict and draw shifts using Le Châtelier’s principle for gases, acids, and ions in solution. We will use K and Q to justify every move, show step-by-step particle sketches, and share AP exam tactics you can trust.

Ready to think like particles do?

Key Takeaways

• At equilibrium, forward and reverse rates are equal, so concentrations stay constant while reactions continue.
• Use Q vs K to predict direction: Q < K shifts right to products, Q > K shifts left to reactants.
• Changing concentration or pressure shifts the position, but only temperature changes K.
• For gases, volume down or pressure up shifts toward fewer moles; volume up shifts toward more moles.
• Draw shifts in three panels: at equilibrium, immediately after the stress, and after re-equilibration, with counts that match the predicted change.

What Equilibrium Looks Like at the Particle Level (K, Q, and Dynamic Balance)

Dynamic equilibrium in simple words

Dynamic equilibrium means the forward and reverse reactions happen at the same rate. The counts of reactants and products stay constant, but particles keep colliding and reacting. Picture two rooms with students walking back and forth at the same rate. The rooms look steady even though people are moving.

On a particle chart over time, you would see counts level off into flat lines. Concentrations become constant, not zero. A catalyst speeds up both directions equally, so it helps the system reach that flat region faster, but it does not change K or the final equilibrium state.

K vs Q: predict the direction of shift fast

• K is the ratio of products to reactants at equilibrium, each raised to their coefficients.
• Q is the same ratio, but at any moment.

If Q < K, the system makes more products to catch up. If Q > K, it makes more reactants. For gases:

• N2O4(g) ⇌ 2 NO2(g). If you add NO2, Q grows larger than K, so the system shifts left to make N2O4.

For solutions:

• HA(aq) ⇌ H+(aq) + A−(aq). If you add H+, Q increases and the system shifts left, forming more HA.

Only temperature changes K. Changing concentration or pressure shifts the system but does not change K.

How to read and draw particle diagrams that match K

• Use a simple key. Circles for reactants, squares for products. Dark shade for ions like A−, lighter shade for HA.
• Use three frames: before stress, right after stress, and after re-equilibration.
• Keep counts tidy, about 15 to 30 particles total.
• Label states: (g) in a box for gases, (aq) as particles spread across water.
• When K > 1, show more product particles at equilibrium. When K < 1, show more reactant particles.

For gases, pack particles into a box with spacing that suggests collisions. For solutions, scatter ions evenly.

Le Châtelier With Gases: Concentration, Pressure, and Temperature Shifts

Change concentration of a gas: N2O4 and NO2 color change example

Reaction: N2O4(g) ⇌ 2 NO2(g). NO2 is brown, N2O4 is colorless.

• Add NO2. Q > K, so the system shifts left. More N2O4 forms, and the mixture becomes less brown.
• Remove NO2. Q < K, so it shifts right. More NO2 forms, and the brown color deepens.

Particle drawing plan:

• Panel 1: Equilibrium mix. For example, 4 N2O4 circles, 8 NO2 squares.
• Panel 2: After adding NO2, double the squares to show the stress.
• Panel 3: After re-equilibration, pairs of NO2 combine into N2O4, so squares drop and circles rise.

Change volume or pressure: count gas moles to predict the shift

When volume decreases (pressure rises), the equilibrium shifts to the side with fewer moles of gas. When volume increases (pressure falls), it shifts to the side with more moles.

Example: N2(g) + 3 H2(g) ⇌ 2 NH3(g). Left side has 4 total moles of gas, right side has 2. Higher pressure pushes the system to the right.

Particle drawing plan:

• Panel 1: Normal box size at equilibrium.
• Panel 2: Smaller box for decreased volume. Immediately after, count stays the same but particles are closer.
• Panel 3: After re-equilibration, reduce particles on the higher-mole side and increase on the lower-mole side to match the shift.

Change temperature: endothermic vs exothermic and what happens to K

Treat heat as a reactant for endothermic, and as a product for exothermic.

• Exothermic: A + B ⇌ C + heat. Increasing temperature adds heat, so the system shifts left, and K decreases.
• Endothermic: heat + D ⇌ E. Increasing temperature shifts right, and K increases.

Drawing tip: mark a tiny flame for exothermic on the product side, or a snowflake next to the reactant side for endothermic, as a visual reminder. For exam writing, justify K change using “heat acts like a reactant or product” and avoid saying temperature changes Q only. Temperature changes K.

For a model of how graders value temperature justifications, skim the College Board’s scoring notes where an exothermic reaction shifts left when heated, lowering K: AP Chemistry sample scoring commentary.

Draw the shift for gases: before, stress, after

Use three labeled panels:

1. At equilibrium: include a legend, like R = circle, P = square.
2. Immediately after stress: add or remove particles, resize the box for volume changes, or mark heat change.
3. After re-equilibration: show new counts matching the predicted shift.

Under each panel, add one sentence:

• Because Q > K after adding NO2, the reaction shifts left.
• Because volume decreased, the system shifts to the side with fewer moles.
• Because the reaction is endothermic and temperature increased, K increased and the reaction shifts right.

If you want guided practice with equilibrium problems and visuals, see these focused lessons on Le Chatelier’s principle in AP Chem lessons: Online AP Kimya Kursları | AP Kimya Özel Ders.

Acids and Ions in Solution: pH and Common Ion Effects at the Particle Level

Weak acid equilibrium (HA ⇌ H+ + A−): add acid or base and show the shift

Start with HA(aq) ⇌ H+(aq) + A−(aq). If Ka is small, show mostly HA with a few ions.

• Add strong acid. [H+] rises, so Q > K, and the system shifts left. More HA forms, fewer free A− particles remain.
• Add strong base. OH− removes H+ to form water, which lowers [H+]. Now Q < K, and the system shifts right. HA dissociates more, so you see more A−.

Drawing plan:

• Panel 1: Many HA, a few H+ and A−.
• Panel 2: After adding H+, sprinkle extra H+ ions.
• Panel 3: After shift left, combine A− with H+ into HA, so A− count drops.

For adding base, include a few OH− that pair off with H+ and “disappear” into H2O, then show HA splitting to restore H+ and A−.

Buffers and the common ion: add acetate or H+ to acetic acid

Use the classic buffer pair: HC2H3O2(aq) and C2H3O2−(aq).

• Adding acetate (the common ion) increases [A−], so the equilibrium shifts left. The buffer resists a big pH change because HA forms slightly.
• Adding H+ shifts the balance right, as A− soaks up H+ to form HA. Again, the pH changes only a little.

Particle drawing plan:

• Panel 1: Show both HA and A− present in similar amounts.
• Panel 2: Stress with extra A− or H+.
• Panel 3: Small adjustments back to near the original ratio, not a huge swing.

Solubility and Ksp: common ion and precipitation (AgCl with NaCl)

AgCl(s) ⇌ Ag+(aq) + Cl−(aq). Adding NaCl raises [Cl−], so Q > Ksp, and the system shifts left. More solid forms, and the solution loses some dissolved ions.

If ions are removed, for example by forming a complex, Q can drop below Ksp, and more solid dissolves.

Drawing plan:

• Panel 1: Show a small cluster for solid AgCl and a few free ions.
• Panel 2: After adding Cl−, add many Cl− ions to the solution.
• Panel 3: The cluster grows larger, and fewer ions remain free in solution.

For a lab context that ties pressure and dissolution ideas to Le Châtelier, see this AP Chem lab note on gas and solubility effects: Applications of Le Châtelier’s Principle.

AP Exam Tactics: Draw, Justify, and Avoid Common Mistakes

3-step plan: identify the stress, use Q vs K, then explain with collisions

• Step 1: Name the stress clearly. Add or remove a species, change volume, or change temperature.
• Step 2: Compare Q to K, or count gas moles for pressure changes.
• Step 3: Explain the why. Use collision frequency for pressure changes, or treat heat as a reactant or product for temperature shifts.

Close with a one-line claim: The system shifts right or left to reduce the stress.

Sentence starters and keywords graders look for

• “Because Q < K, the reaction makes more product.”
• “Because the volume decreased, the system shifts to the side with fewer gas molecules.”
• “Because the reaction is exothermic, raising temperature shifts the equilibrium to reactants, and K decreases.”
• Keywords to include: endothermic, exothermic, common ion, dynamic, constant concentrations, temperature changes K, concentration or pressure do not change K.

Pair each claim with a mini-diagram note, like “product squares decrease” or “reactant circles increase.”

For more structured practice aimed at AP expectations, this short lesson summary is handy: Introduction to Le Châtelier’s Principle | AP Chemistry.

If you want a quick refresher on the core idea, see this clear walkthrough of Le Châtelier for AP Chem with examples and practice from Save My Exams. You can also practice with structured notes on Fiveable’s intro to Le Châtelier’s principle.

Common pitfalls to avoid in particle diagrams

• Do not erase or create all particles at once. Show the immediate change, then the gradual re-balance.
• Do not violate conservation of atoms. Keep stoichiometry and counts consistent.
• Do not change K when you change pressure or concentration.
• Do not forget states. Use a box for (g) and spread-out ions for (aq).
• Keep particle ratios consistent with coefficients in the balanced equation.

Conclusion

Equilibrium is a balance of rates, not a stop. Use K and Q to predict direction, then show the story with clean particle drawings in three panels. Practice with a gas system like N2O4/NO2 for color and pressure, a weak acid like HA for pH shifts, and a Ksp case like AgCl for precipitation.

Quick checklist:

• Identify the stress.
• Compare Q and K, or count gas moles.
• Place heat on the correct side.
• Draw before, stress, after.
• Write one clear claim with a short justification.

Keep your diagrams simple, your words precise, and your logic tight. You’ve got this.

Frequently Asked Questions About Le Châtelier’s Principle and Particle-Level Equilibrium

What is dynamic equilibrium?

Dynamic equilibrium means the forward and reverse reactions occur at the same rate. Concentrations appear steady, but particles keep colliding and reacting. A catalyst only helps reach equilibrium faster, it does not change K or the final state.

How do I use Q and K to predict the direction of shift?

K is the products-to-reactants ratio at equilibrium. Q is the same ratio at any moment. If Q < K, the system forms more products. If Q > K, it forms more reactants. Example: adding NO2 to N2O4(g) ⇌ 2 NO2(g) makes Q > K, so it shifts left to form N2O4.

Does changing concentration or pressure change the equilibrium constant K?

No. Concentration and pressure changes shift the position of equilibrium, but K stays the same. Only temperature changes K. Treat heat as a reactant for endothermic systems and as a product for exothermic systems.

How does temperature affect both K and the shift?

Heating an exothermic system shifts left and decreases K because heat acts like a product. Heating an endothermic system shifts right and increases K because heat acts like a reactant. Cooling has the opposite effect in each case.

How should I draw particle diagrams that match the predicted shift?

Use three panels: at equilibrium, immediately after the stress, and after re-equilibration. Keep counts tidy, include a legend, and match stoichiometry. For gases, adjust box size for volume changes. For solutions, show ions spread in water. Make particle counts reflect K, Q, and mole counts on each side.