If a reactant concentration is suddenly doubled and the system is allowed to re-establish equilibrium at the same temperature, what happens to the value of the equilibrium constant K?

• A. K increases
• B. K decreases
• C. K remains the same
• D. K becomes undefined

Answer: (C)

The value of the equilibrium constant is determined by temperature, not by how much reactant you start with. At the same temperature, K is fixed for a given reaction. If you suddenly double the reactant concentration, the system shifts to re-establish equilibrium (Le Chatelier’s principle), producing more products or adjusting the mix until the ratio of products to reactants, each raised to their stoichiometric powers, matches the same K as before. So, once equilibrium is re-established, K remains unchanged. If the temperature were changed, then K would change accordingly; at constant temperature, it stays the same.