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Le Chatelier's principle

Push on a system at equilibrium and it pushes back. Le Chatelier's principle predicts which way — but you have to know which stresses move the position and which actually change K.

§1

Systems that push back.

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Le Chatelier's principle: a system at equilibrium responds to a stress by shifting to partly counteract it. Add a reactant and it shifts toward products; remove a product and it shifts toward products; add a product and it shifts back toward reactants.

Changing volume (pressure) shifts a gaseous equilibrium toward the side with fewer moles of gas when the volume is decreased. But a concentration or volume stress shifts the position without changing K itself.

Two things do not shift the equilibrium as students expect: a catalyst (it speeds both directions equally, reaching the same position faster), and, for K, any concentration change. Only temperature changes 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
Fig. 7.9.1 Le Chatelier's principle: a system 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. Only temperature changes K.
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Predicting a shift.

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Name the stress, then the counteracting shift.

  1. Identify the stress. Added/removed species, changed volume/pressure, or changed temperature.
  2. Shift to counteract it. Add reactant → shift right; remove product → shift right; decrease volume → shift toward fewer gas moles.
  3. Decide whether K changes. Concentration and volume changes shift the position but leave K unchanged; only temperature changes K.
  4. Remember catalysts. A catalyst does not shift the equilibrium; it just reaches it faster.
§3

The pieces you'll meet.

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Stress in, counteracting shift out.

Le Chatelier
Le Chatelier
Shift to partly counteract a stress.
add reactant
Add reactant
Shifts toward products.
volume
Volume/pressure
Decreasing volume shifts toward fewer gas moles.
K unchanged
Concentration/volume
Shift the position but do not change K.
temperature
Temperature
The one stress that changes K.
catalyst
Catalyst
Does not shift equilibrium; reaches it faster.
§4

Worked example: apply the principle.

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Stress 1 — add reactant. Adding more A to A + B ⇌ C shifts the equilibrium right to consume the added A. K is unchanged.

Stress 2 — decrease volume. For a gaseous reaction, compressing the system shifts toward the side with fewer gas moles. K is unchanged.

Stress 3 — add a catalyst. No shift — the catalyst speeds both directions equally, so the same equilibrium position is reached faster.

Stress 4 — change temperature. This is the special case: it actually changes K, shifting the equilibrium to a genuinely new position.

§5

Mistakes that cost real points.

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Pitfall · 01

"A catalyst shifts the equilibrium toward products."

A catalyst speeds the forward and reverse reactions equally, so it reaches the same equilibrium position faster but does not shift it. The equilibrium amounts are identical with or without a catalyst.

Fix. Treat a catalyst as changing only the speed to equilibrium, never the position or the amounts.

Pitfall · 02

"Adding more reactant (a concentration stress) changes K."

A concentration change shifts the position of equilibrium but does not change K. The system shifts so that Q returns to the unchanged K. Only a temperature change alters K itself.

Fix. Distinguish position from K: concentration and volume stresses shift the position; only temperature changes K.

Pitfall · 03

"Decreasing the volume shifts a gas equilibrium toward more moles of gas."

Decreasing the volume (raising pressure) shifts toward the side with fewer moles of gas, to reduce the pressure — the opposite direction. Shifting toward more moles would worsen the stress.

Fix. Compress → shift toward fewer gas moles; expand → shift toward more gas moles.

§6

Skill Check.

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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.

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