Final answer:
Changing the system's temperature at equilibrium alters the equilibrium constant value. Increasing temperature generally favors the reverse reaction in exothermic reactions and lowers the equilibrium constant, while decreasing temperature does the opposite.
Step-by-step explanation:
Changing the temperature of a system at equilibrium changes the value of the equilibrium constant. This is different from changing the concentration or pressure, which simply shifts the reaction quotient without affecting the equilibrium constant. According to Le Chatelier's principle, if the temperature of a system at equilibrium is changed, the system responds by shifting the equilibrium to counteract this stress. For instance, when the temperature is increased for an exothermic reaction, the equilibrium shifts towards the reactants, thereby decreasing the equilibrium constant value.
An example of this is the reaction between hydrogen and gaseous iodine, where heat is evolved. If the temperature is raised, the reaction will favor the reactants (H₂ and I₂) and the equilibrium constant will decrease. Conversely, lowering the temperature favors the formation of products and increases the equilibrium constant.