Definition: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: Reactions can proceed by themselves if they are exergonic or exothermic, that is if they release energy. The associated free energy of the reaction is composed of two different thermodynamic quantities, enthalpy and entropy. Enthalpy is a measure of the total energy of a thermodynamic system. The change in enthalpy is positive in endothermic reactions, and negative in exothermic processes. Answer: thermodynamic
Output:
Enthalpy is a measure of the total energy of what kind of system?