Final answer:
In the context of explosives, heat is the thermal energy released from the chemical reaction of the explosive material during combustion, typically under constant volume, changing the internal energy of the system.
Step-by-step explanation:
In the context of explosives, heat typically refers to the thermal energy that is released when a chemical reaction takes place, particularly during direct combustion. This energy release occurs because the reactants, the explosive materials, transform into products, usually gases like CO₂ and H₂O at high temperatures, which cause a rapid expansion translating into an explosion. The combustion reactions of explosives often occur under circumstances where the volume is constant, as in a sealed environment, which leads to heat being the change in internal energy (AU), not enthalpy (AH). Activation energy is an important concept in understanding explosives, as it is the minimum amount of energy required to initiate these combustion reactions - energy that is often provided in the form of applied heat.