Chemistry

The equilibrium constant changes dramatically with temperature—this is your clue to the reaction's enthalpy. When $K$ decreases as temperature increases, the equilibrium shifts toward reactants at higher $T$, which only happens in an exothermic reaction (one that releases heat). Use Le Chatelier's principle: raising temperature favors the endothermic direction, so if the forward reaction is exothermic, higher $T$ pushes equilibrium backward, shrinking $K$.

The equilibrium constant changes dramatically with temperature—this is your clue to the reaction's enthalpy. When $K$ decreases as temperature increases, the equilibrium shifts toward reactants at higher $T$, which only happens in an exothermic reaction (one that releases heat). Use Le Chatelier's principle: raising temperature favors the endothermic direction, so if the forward reaction is exothermic, higher $T$ pushes equilibrium backward, shrinking $K$.