In a system at equilibrium, if the reaction is exothermic and the temperature is increased, what happens to the equilibrium constant K?

• A. K increases
• B. K decreases
• C. K remains the same
• D. K changes unpredictably

Answer: (B)

For an exothermic reaction, heat behaves like a product. When you raise the temperature, the system shifts to offset the added heat, moving the equilibrium toward the reactants. That shift lowers the product-to-reactant ratio at equilibrium, so the equilibrium constant K decreases (since K = [products]/[reactants]). A quick thermodynamic note: the van’t Hoff relation d(ln K)/dT = ΔH°/(R T^2) with ΔH° negative (exothermic) means increasing T makes ln K smaller, hence K smaller. Thus, increasing temperature causes K to decrease for an exothermic reaction.