What does the reaction quotient Q represent in the Nernst equation?

• A. Temperature
• B. Pressure
• C. The ratio of product activities to reactant activities
• D. The number of electrons transferred

Answer: (C)

The reaction quotient Q in the Nernst equation expresses the current state of the reaction by comparing how much product activity there is to how much reactant activity there is. It isn’t about temperature or pressure itself; those factors enter the equation separately (through E° and the RT/nF term), while Q captures the actual composition of the system at that moment.

For a balanced reaction, Q is the product of the activities of the products, each raised to its stoichiometric power, divided by the product of the activities of the reactants, each raised to its stoichiometric power. In dilute solutions, activities are well approximated by concentrations; for gases, by partial pressures; for pure solids/liquids, activities are about 1. The Nernst equation uses this Q via E = E° − (RT/nF) ln Q, so as the system moves toward more products or more reactants, Q changes and the cell potential shifts accordingly. When Q equals the equilibrium quotient K, the potential equals the standard potential E°. The key point is that Q is the ratio of product activities to reactant activities, not temperature, pressure, or the number of electrons transferred.