Final answer:
A reaction achieving its theoretical limit is an exothermic reaction, indicated by energy listed as a product, releasing heat. An example is the formation of water from hydrogen and oxygen, which releases energy, making it exothermic.
Step-by-step explanation:
If a reaction achieves its theoretical limit, it is referred to as an exothermic reaction. This can be ascertained because energy is released in the form of heat during the reaction, as indicated by a heat term listed alongside the products in a chemical equation. For instance, the reaction 2H₂(g) + O₂(g) → 2H₂O(l) + 135 kcal shows that 135 kilocalories of energy are released as heat, making it an exothermic reaction. Conversely, if the heat term is listed alongside the reactants, such as in the case of N₂(g) + O₂(g) + 45 kcal → 2NO(g), energy is required for the reaction to occur, making it an endothermic reaction.