A LITTLE LIGHTBULB MOMENT
General chemistry and equilibrium
Reaction quotient predicts direction, not equilibrium size
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Q checks where the mixture is headed now
The idea behind the cue
The reaction quotient Q has the same form as K but uses current, not equilibrium, concentrations or pressures. If Q < K, the system shifts toward products; if Q > K, it shifts toward reactants. Q tells the direction of change, not the equilibrium position itself.
Practice this flashcard →Make it memorable
Short & simple
Q checks where the mixture is headed now
Acronym
QKC — Quirky Kangaroos Cartwheel.
Story
A train station scoreboard showed where every train stood right now. A conductor compared that snapshot to the timetable and knew which platform would get crowded next, without changing the schedule itself.
Connect the pieces
- Q (Quirky) → Quotient: Q has the same form as K, but uses the current concentrations or pressures instead of equilibrium values.
- K (Kangaroos) → Keq: K is the equilibrium constant for the balanced reaction at that temperature.
- C (Cartwheel) → Compare: Compare Q and K at the same temperature: Q < K shifts toward products, Q > K shifts toward reactants.
- Quirky · Quotient: Q has the same form as K, but uses the current concentrations or pressures instead of equilibrium values.
- Kangaroos · Keq: K is the equilibrium constant for the balanced reaction at that temperature.
- Cartwheel · Compare: Compare Q and K at the same temperature: Q < K shifts toward products, Q > K shifts toward reactants.
Know the limits
Q and K must be written for the same balanced reaction and compared at the same temperature.
Further reading · OpenStax science textbooks ↗Check your understanding
A reaction mixture has Q < K. What will happen spontaneously?
A. It is already at equilibrium.
B. The value of K decreases immediately.
C. It shifts toward products.
D. It shifts toward reactants.
Reveal explanation
It shifts toward products.
When Q is smaller than K, the mixture has too few products relative to equilibrium, so it proceeds forward to make more products.