When Q equals K, what is true?

• A. The system is at equilibrium.
• B. The rates of the forward and reverse reactions are not equal.
• C. The reaction is spontaneous.
• D. The temperature must be changed to reach equilibrium.

Answer: (A)

Think of Q as the current snapshot of the system and K as the snapshot when things have settled at a given temperature. When Q equals K, the current concentrations match those at equilibrium, so the forward and reverse reactions run at the same rate and there’s no net change in the amounts of reactants and products. In other words, the system is in equilibrium at that temperature. There’s no need to change the temperature to reach equilibrium, and the idea of the reaction being spontaneous isn’t relevant here because spontaneity is tied to a driving force (delta G) that is zero at equilibrium.