Short on time? This page is the recap. For explanations, a Q–K diagram and three worked examples, use the full study guide.
Recap
- Q has the same form as K but uses the concentrations (or partial pressures) at any moment.
- At equilibrium Q = K. A stress makes Q ≠ K, so the system is no longer at equilibrium.
- The system reacts until Q = K again. The new equilibrium has different concentrations.
- Most stresses change Q. A temperature change alters K.
Key relationships
| Relationship | Meaning | Use it to |
|---|---|---|
| Q < K | Too few products | Predict a net forward reaction |
| Q > K | Too many products | Predict a net reverse reaction |
| new Q = s^Δn × K | Every concentration scaled by s; Δn = product coefficients − reactant coefficients (gases, or dissolved species) | Handle volume changes (halve V: s = 2) and dilution (double V: s = ½) |
| Exothermic: T up → K down | Heating favours the reverse reaction | Predict the effect of temperature on K |
| Endothermic: T up → K up | Heating favours the forward reaction |
Assumptions behind the numbers
- The system was at equilibrium before the stress, so Q = K at the start.
- The temperature is constant unless the question changes it; K is only fixed at a fixed temperature.
- Solids, pure liquids and solvent water are left out of Q, just as they are left out of K.
Mistakes to avoid
- Reversing the rule. Q > K means reverse, not forward.
- Changing K when you add a species. Only temperature changes K.
- Changing Q when you heat a rigid flask. The concentrations stay the same at that instant; K moves.
- Forgetting to raise terms to their coefficients when you scale concentrations.
- Expecting old concentrations to return. Only the ratio in Q returns to K.
Quick self-check
- A mixture has Q = 0.80 and K = 0.25. Which way does it react? (Reverse: Q > K.)
- For N₂O₄(g) ⇌ 2NO₂(g) at equilibrium, the volume is halved. What is the new Q in terms of K? (Q = 2K, so the reaction goes in reverse.)
- For A(aq) ⇌ 2B(aq) at equilibrium, the solution is diluted to twice its volume. What is the new Q? (Q = ½K, so the reaction goes forward.)
Next: practice questions.