Final answer:
A. the external temperature. A reaction is classified as exothermic or endothermic based on the overall energy change during bond breaking and formation, which is reflected in the change in enthalpy of the reaction.
Step-by-step explanation:
Whether a reaction is exothermic or endothermic is determined by the overall loss and gain of energy when bonds break and reform. This is related to the change in enthalpy (ΔH) of a reaction, which reflects the energy involved in breaking and forming chemical bonds. In an exothermic reaction, energy is released into the surroundings as the reaction proceeds; typically because the bonds formed are stronger than those broken. Conversely, an endothermic reaction absorbs energy from the surroundings, often due to the new bonds being weaker than the original bonds removed.
The temperature change experienced during a chemical reaction is a manifestation of this energy transfer. For example, during the burning of methane in a kitchen burner, an exothermic reaction occurs, and energy is released in the form of heat making the area around the burner feel warm. On the other hand, during photosynthesis, energy from sunlight is absorbed to convert carbon dioxide and water into glucose, characterizing it as an endothermic process.
It is crucial to understand that the reaction type is not determined by the external temperature, the states of the reactants, nor the activation energy. Instead, it is the comparison of energy required to break the reactants' bonds versus the energy released from forming the products' bonds that dictates if heat is absorbed or released during the chemical process.