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Q and Le Chatelier

Le Chatelier is really just Q chasing K. A stress knocks Q away from K, and the system shifts to bring it back — with one exception, where the stress moves K itself.

§1

Shifts explained by Q and K.

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Every Le Chatelier shift can be explained with Q and K. A stress (adding or removing a species, changing volume) suddenly moves Q away from K. The system then shifts in the direction that brings Q back to K.

For concentration and volume stresses, K stays constant — the shift restores Q to the same, unchanged K. Add reactant, Q drops below K, and the reaction runs right until Q climbs back to K.

A temperature change is the exception: it changes K itself. The equilibrium moves to a genuinely new position because the target K has moved, not just because Q was displaced.

UNIT 7 TOPIC 7.10 • REACTION QUOTIENT AND LE CHATELIER'S PRINCIPLE SHIFT EXPLAINER CONCENTRATION STRESS K stays constant STRESS add A INSTANTLY Q < K SHIFT right NEW EQ. Q = K A + B ⇌ C Q = [C] [A] [B] ← add A raises the denominator Adding A raises the denominator of Q, so Q drops below K. The reaction shifts right, forming products until Q = K again. K is UNCHANGED — a concentration stress never changes the value of K. TEMPERATURE STRESS K itself changes K temperature (T) K rises as T rises → endothermic A concentration or pressure stress changes Q first, and Q settles back to the same K. A temperature change is different — it changes the value of K itself. Here K rises with T, so heat acts like a reactant: an endothermic case. CED ANCHOR Le Chatelier shifts are explained with Q and K: most stresses change Q (K is unchanged); a temperature change moves K itself. AP Chemistry · Unit 7 · Equilibrium
Fig. 7.10.1 A Le Chatelier shift is a Q-versus-K story. A stress moves Q away from K; the system then shifts to bring Q back to K. K stays constant for concentration and volume changes; only a temperature change alters K.
§2

Reasoning a shift through Q.

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See where the stress puts Q relative to K.

  1. Apply the stress. Note how the stress instantly changes Q (e.g. adding reactant lowers Q).
  2. Compare the new Q to K. Is Q now below K (shift right) or above K (shift left)?
  3. Shift Q back to K. The reaction proceeds in the direction that returns Q to K.
  4. Check if K moved. Concentration/volume leave K fixed; temperature changes K, moving the target.
§3

The pieces you'll meet.

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Q moves; K usually stays.

Q displaced
Q displaced
A stress moves Q away from K.
shift back
Shift restores Q
The system moves Q back to K.
K fixed
K fixed
For concentration and volume stresses.
temperature
Temperature
Changes K, moving the target.
add reactant
Add reactant
Lowers Q below K, so shift right.
remove product
Remove product
Lowers Q below K, so shift right.
§4

Worked example: add reactant, via Q.

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Stress. To a system at equilibrium (Q = K), you add more reactant.

Effect on Q. More reactant increases the denominator of Q, so Q drops below K.

Shift. Since Q < K, the reaction runs right, making product until Q rises back to K (which never changed).

Temperature contrast. Had you instead raised the temperature, K itself would change, and the system would move toward a new equilibrium set by the new K — not merely restore Q to the old K.

§5

Mistakes that cost real points.

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Pitfall · 01

"A temperature change leaves K fixed, like other stresses."

Temperature is the exception: it changes K. Concentration and volume stresses displace Q while K stays fixed, but a temperature change moves the target K itself, so the system settles at a genuinely new position.

Fix. Remember temperature changes K; concentration and volume changes only displace Q from an unchanged K.

Pitfall · 02

"A concentration stress changes K, which is why the system shifts."

A concentration stress shifts the system because it displaces Q from K, not because K changed — K is unchanged. The system moves to restore Q to the same K.

Fix. Attribute a concentration-stress shift to Q being displaced from a fixed K, not to K changing.

Pitfall · 03

"After a stress, the system settles at a different value of Q than before."

For concentration and volume stresses, the system shifts until Q returns to the same K it had before. The equilibrium position (the concentrations) changes, but Q ends back at K.

Fix. Recognize that the shift drives Q back to the unchanged K (for concentration/volume stresses).

§6

Skill Check.

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