In what situation would a catalyst not be useful?

A catalyst is a substance that increases the rate of a chemical reaction by lowering the activation energy barrier without being consumed in the process. However, there are certain conditions under which a catalyst would not be beneficial.

In scenarios where reactions do not proceed at all, a catalyst cannot facilitate a reaction that is fundamentally unfavorable or impossible. This might occur due to reasons such as an unfavorable thermodynamic profile where the reactants lack sufficient energy to reach the transition state or when the reactants do not interact at all due to lack of appropriate conditions (like concentration, pressure, or temperature).

On the other hand, reactions with high activation energy barriers can still benefit from a catalyst, as the catalyst works to lower that barrier. Similarly, reactions at ambient temperature or in exothermic systems can also see a positive impact from the presence of a catalyst, depending on other conditions such as the activation energy and concentrations of reactants. Thus, the unique aspect of completely unreactive conditions highlights why a catalyst would not be useful in that specific situation.