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Introduction to Le Chatelier's Principle

Le Chatelier's principle says a system at equilibrium responds to a stress by shifting to partly counteract it. Adding a reactant shifts toward products; removing a product does the same; changing volume shifts toward the side with fewer or more gas moles. A concentration or volume stress shifts the position but does not change K.

UNIT 7 TOPIC 7.9 • INTRODUCTION TO LE CHATELIER'S PRINCIPLE STRESS TESTER A system at equilibrium responds to a stress by shifting to partly counteract it (restoring Q = K). ADD REACTANT A + B ⇌ C Extra A drops Q below K, so the system shifts RIGHT toward products to use up added A. Add product → shifts LEFT. REMOVE PRODUCT A + B ⇌ C Taking C out drops Q below K, so the system shifts RIGHT to replace some product. Remove reactant → shifts LEFT. NO SHIFT CATALYST Speeds both directions equally; equilibrium is reached faster. Ea lower, K unchanged. INERT GAS (const. V) No concentration changes, so the position is unchanged. DECREASE VOLUME / INCREASE PRESSURE 2A(g) ⇌ B(g) Compression raises the pressure, so the system shifts toward the side with FEWER moles of gas. shift right toward B (2 mol → 1 mol) compressed TEMPERATURE STRESS (changes K) ENDOTHERMIC — heat is a reactant heat + A ⇌ B raise T → shift RIGHT, K increases EXOTHERMIC — heat is a product A ⇌ B + heat raise T → shift LEFT, K decreases Only temperature actually changes the value of K. CED ANCHOR · TAKEAWAY A stress moves Q away from K (or, for temperature, changes K itself). The system shifts in the direction that partly counteracts the stress until Q = K again. Volume shifts need unequal gas moles. AP Chemistry · Unit 7 · Equilibrium
Le Chatelier's principle: a system at equilibrium responds to a stress by shifting to partly counteract it. Adding reactant, removing product, or changing volume shifts the position — but a concentration or volume stress does not change K itself. Only temperature changes K.
Stress Tester · Open the sandbox →

Two traps: thinking a catalyst shifts the equilibrium (it only speeds both directions, reaching the same position faster), and thinking a concentration stress changes K (it shifts the position; K is unchanged by concentration). Only temperature changes K.

The work

3 ways in · any order
Lesson
Le Chatelier's Principle ›

A stress shifts an equilibrium to counteract it; concentration and volume change the position but not K. The lesson applies Le Chatelier and the K rules, then closes with a ten-scenario check.

Skill check · 10 scenarios
Open lesson →
Diagnostic
10-item topic check ›

Ten items spanning the Topic 7.9 misconceptions: a catalyst thought to shift equilibrium, and a concentration stress thought to change K.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
Diagnostic first