How is 'enthalpy' defined in thermodynamics?

Enthalpy is a thermodynamic property that quantifies the total heat content of a system, taking into account both its internal energy and the energy related to the pressure and volume of the system. It is defined mathematically as the sum of the internal energy of the system plus the product of its pressure and volume. This means that enthalpy not only reflects the energy contained within the substance but also incorporates the energy associated with its expansion against the surrounding pressure, which is crucial in processes such as chemical reactions or phase changes.

Understanding this concept is vital in various applications within thermodynamics, such as analyzing heat transfer during processes at constant pressure. By focusing on the total heat content, engineers and scientists can predict how systems will behave under different conditions, making it an essential factor in energy calculations for engineering projects.

The other choices do not accurately capture the multifaceted nature of enthalpy. Temperature is a fundamental property but does not encompass heat content. Similarly, representing energy lost and pressure within a system does not relate to the comprehensive definition of enthalpy, which fundamentally connects internal energy with pressure and volume.