⚖️ Skill 3: The Reaction Quotient (Q) and Predicting Direction

The reaction quotient (Q) is calculated using exactly the same expression as Kₑq, but with current concentrations instead of equilibrium concentrations. Q tells you where the system is right now compared to where equilibrium is.

Q has the same formula as Kₑq, but uses concentrations at ANY point in time — not just at equilibrium.

By comparing Q to Kₑq, you can predict which direction the reaction will shift to reach equilibrium:

Worked Example 3

Q: For N₂O₄(g) ⇌ 2NO₂(g), Kₑq = 0.36 at 373 K. A mixture contains [N₂O₄] = 0.10 M and [NO₂] = 0.30 M. Predict the direction of reaction.

Step 1: Calculate Q using the same expression: Q = [NO₂]² / [N₂O₄]

Step 2: Q = (0.30)² / 0.10 = 0.09 / 0.10 = 0.90

Step 3: Compare: Q (0.90) > Kₑq (0.36)

Step 4: Since Q > Kₑq, there are too many products relative to equilibrium. The reaction shifts LEFT (reverse direction) to convert excess NO₂ back to N₂O₄ until Q decreases to equal Kₑq.

💡 Memory Trick for Q vs Kₑq

Q < Kₑq: Think "Q is too low" — not enough products yet. System needs to make MORE products. Shifts RIGHT (forward). →

Q > Kₑq: Think "Q is too high" — too many products. System needs to reduce products. Shifts LEFT (reverse). ←

Q = Kₑq: Already at equilibrium. No shift.