Reaction quotient Q uses which of the following to predict the direction of a reaction?

• A. Temperature and pressure only.
• B. Current concentrations of reactants and products.
• C. Equilibrium concentrations only.
• D. Activation energies only.

Answer: (B)

The idea being tested is how the reaction quotient tells us where the system is relative to equilibrium. Q is formed from the actual concentrations of reactants and products present at that moment (each raised to its stoichiometric power). Because it uses current conditions, Q shows whether the system still needs to shift toward products or toward reactants to reach equilibrium.

If the current state has fewer products (Q is small) compared with the equilibrium position, the forward reaction is favored to produce more products; if there are too many products (Q is large), the reverse reaction is favored to form more reactants; when Q equals the equilibrium constant, the system is already at equilibrium. Activation energies and temperatures/pressures influence rates or equilibrium constants, but Q itself is defined by the present concentrations (or activities) of reactants and products, not by those other factors.