page_title: Enthalpy
section_title: Diagrams
hierarchical_section_title: Enthalpy / Diagrams
caption_attribution_description: English: Ts diagram of nitrogen with points indicated
context_page_description: Enthalpy is a property of a thermodynamic system, that is a convenient state function preferred in many measurements in chemical, biological, and physical systems at a constant pressure. It is defined as the sum of the system's internal energy and the product of its pressure and volume. The pressure-volume term expresses the work required to establish the system's physical dimensions, i.e. to make room for it by displacing its environment. As a state function, enthalpy depends only on the final configuration of internal energy, pressure, and volume, and not on the path taken to achieve it.
  The unit of measurement for enthalpy in the International System of Units is the joule. Other historical conventional units still in use include the British thermal unit and the calorie.
  The total enthalpy of a system cannot be measured directly, because the internal energy contains components that are unknown, not easily accessible, or are not of interest in thermodynamics. In practice, a change in enthalpy is the preferred expression for measurements at constant pressure, because it simplifies the description of energy transfer.
context_section_description: The enthalpy values of important substances can be obtained using commercial software. Practically all relevant material properties can be obtained either in tabular or in graphical form. There are many types of diagrams, such as h–T diagrams, which give the specific enthalpy as function of temperature for various pressures, and h–p diagrams, which give h as function of p for various T. One of the most common diagrams is the temperature–specific entropy diagram (T–s diagram). It gives the melting curve and saturated liquid and vapor values together with isobars and isenthalps. These diagrams are powerful tools in the hands of the thermal engineer.
caption: T–s diagram of nitrogen.[20] The red curve at the left is the melting curve. The red dome represents the two-phase region with the low-entropy side the saturated liquid and the high-entropy side the saturated gas. The black curves give the T–s relation along isobars. The pressures are indicated in bar. The blue curves are isenthalps (curves of constant enthalpy). The values are indicated in blue in kJ/kg. The specific points a, b, etc., are treated in the main text.
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