What Is Equilibrium, Really?
Equilibrium does not mean the reaction has stopped. It means the forward and reverse reactions proceed at the same rate, so concentrations remain constant. This distinction alone saves students from answering 30% of questions incorrectly.
Kc vs Kp: When to Use Which
Kc — Concentration
Used for reactions in solution or when concentrations are given. Kc = [Products]ⁿ / [Reactants]ⁿ at equilibrium. Temperature is the ONLY factor that changes Kc.
Kp — Partial Pressure
Used for gaseous reactions when pressures are given. Kp = Kc × (RT)^Δng, where Δng = moles of gaseous products − moles of gaseous reactants.
Le Chatelier's Principle — The Shortcut
Increase concentration of reactant: Equilibrium shifts RIGHT (toward products).
Increase pressure (gaseous system): Shifts toward the side with fewer moles of gas.
Increase temperature: Shifts toward the endothermic direction (absorbs heat).
Add catalyst: Does NOT shift equilibrium — it only speeds up reaching it.
Reaction Quotient Q: The Navigator
Q is calculated the same way as K, but uses current (not equilibrium) concentrations. Compare Q with K to predict direction:
Q < K
Shifts RIGHT → more products form
Q = K
Already at equilibrium — no shift
Q > K
Shifts LEFT → more reactants form
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