What type of reaction is characterized by an increase in internal energy of the products?

The increase in internal energy of the products is a key characteristic of an endergonic reaction. In these reactions, energy is absorbed from the surroundings, resulting in products that contain more free energy than the reactants. This is often associated with processes that require energy input, such as photosynthesis, where plants convert light energy into chemical energy stored in glucose molecules.

In contrast, exergonic reactions release energy, leading to a decrease in the free energy of the products compared to the reactants. Reversible and irreversible reactions focus more on the ability of the reaction to proceed in both directions or how they progress towards equilibrium, rather than the energy changes associated with the formation of products. Therefore, understanding that endergonic reactions involve a net gain of internal energy helps clarify why this option is the correct choice.