How do the reaction quotient Q and the equilibrium constant K differ?

• A. Q uses current concentrations; K uses the equilibrium concentrations.
• B. Q and K are the same thing.
• C. Q is calculated from partial pressures only.
• D. Q is independent of temperature.

Answer: (A)

The main idea is how Q and K relate to the system’s state. The reaction quotient Q is calculated from the current concentrations (or partial pressures) of all species right now, plugged into the same expression used for the equilibrium constant. The equilibrium constant K, on the other hand, is the value of that expression when the system has actually reached equilibrium at a specific temperature. Because K is tied to temperature, it stays constant for that temperature, while Q can vary as the reaction proceeds and the concentrations change. When the system settles at equilibrium, Q equals K. If Q is less than K, the forward reaction is favored to reach equilibrium; if Q is greater than K, the reverse reaction proceeds. That temperature dependence is why K shifts with temperature, but Q reflects the instantaneous state.