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Direction of reversible reactions

A reversible reaction does not have a fixed direction — it depends on where you start. Compare the reaction quotient Q to the equilibrium constant K and the arrow points itself.

§1

Q versus K sets the direction.

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A reversible reaction can run either way, and the direction it will go depends on the current mixture. Compute the reaction quotient Q from the current concentrations and compare it to the equilibrium constant K.

If Q < K, there is too little product relative to equilibrium, so the reaction shifts right (toward products). If Q > K, there is too much product, so it shifts left (toward reactants).

If Q = K, the system is already at equilibrium and does not shift. Q is the moving indicator that tells the system which way to go to reach the fixed target K.

UNIT 7 TOPIC 7.2 • DIRECTION OF REVERSIBLE REACTIONS Q VS. K COMPASS REACTION QUOTIENT Q aA + bB ⇌ cC + dD Q = [C]c [D]d [A]a [B]b Q uses current, non-equilibrium values. Compare Q to K to predict the direction of shift. DIRECTION DECISION Q < K ⇌ SHIFT RIGHT make products Q = K ⇌ NO SHIFT at equilibrium Q > K ⇌ SHIFT LEFT make reactants GASES CAN USE Kp Kp uses partial pressures For gases, brackets may be replaced by partial pressures. Pure solids and liquids are still omitted. Example: CaCO₃(s) ⇌ CaO(s) + CO₂(g) Kp = P(CO₂) DO NOT INCLUDE liquid solid Omit pure solids and pure liquids from Q and K expressions — their activity is fixed at 1. TAKEAWAY Compare Q with K. Q < K → products form (shift right). Q > K → reactants form (shift left). Q = K → net rates equal, so the system is at equilibrium (no shift). AP Chemistry · Unit 7 · Equilibrium
Fig. 7.2.1 Comparing Q (from current concentrations) to K tells you the direction. Q < K shifts right (toward products); Q > K shifts left (toward reactants); Q = K means the system is at equilibrium.
§2

Predicting the shift.

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Build Q, compare to K, read the arrow.

  1. Compute Q from current values. Use the same expression as K, with the current (non-equilibrium) concentrations.
  2. Compare Q to K. Is Q less than, greater than, or equal to K?
  3. Read the direction. Q < K shifts right; Q > K shifts left; Q = K is at equilibrium.
  4. Remember reactions go both ways. A reversible reaction can run forward or backward depending on the starting mixture.
§3

The pieces you'll meet.

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Q is the indicator; K is the target.

Q
Reaction quotient
Built from current concentrations, like K's expression.
K
Equilibrium constant
The value of the quotient at equilibrium.
Q
Q < K
Shifts right, toward products.
Q>K
Q > K
Shifts left, toward reactants.
Q=K
Q = K
At equilibrium; no net shift.
reversible
Reversible
Can run in either direction.
§4

Worked example: which way does it go?

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Given. A reaction has K = 10. In the current mixture, the quotient works out to Q = 2.

Compare. Q (2) is less than K (10).

Direction. Q < K, so the reaction shifts right, toward products, raising Q until it reaches K.

Contrast. If instead Q had been 50 (> K), the reaction would shift left toward reactants. Same reaction, opposite direction — set by Q versus K.

§5

Mistakes that cost real points.

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Pitfall · 01

"A given reaction always runs in one direction."

A reversible reaction can run either way; the direction depends on the current mixture compared to equilibrium. Starting with excess product, the same reaction runs backward. Direction is set by Q versus K, not fixed.

Fix. Determine direction by comparing Q to K for the current mixture, not by assuming a fixed one-way reaction.

Pitfall · 02

"Q is always equal to K."

Q equals K only at equilibrium. Away from equilibrium, Q differs from K, and that difference is exactly what tells the system which way to shift. Q is a variable; K is a fixed target (at a given temperature).

Fix. Treat Q as changing with the current concentrations; it equals K only when the system is at equilibrium.

Pitfall · 03

"If Q < K, the reaction shifts left."

Q < K means too little product, so the reaction shifts right (toward products) to raise Q up to K. It is Q > K that shifts left. Mixing up the two reverses every direction prediction.

Fix. Use: Q < K → shift right (make product); Q > K → shift left (make reactant).

§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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