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Enthalpy

Enthalpy

Enthalpy is a term linked to the field of thermodynamics. The enthalpy symbol is H.

Enthalpy is also known as absolute enthalpy or amount of heat.

We define enthalpy as a physical quantity defined in the field of classical thermodynamics so that it measures the maximum energy of a thermodynamic system theoretically capable of being eliminated from it in the form of heat or thermal energy.

Enthalpy is particularly useful in the understanding and description of isobaric processes: the constant pressure, the enthalpy change are directly associated with the energy received by the system in the form of heat, these are easily measurable in calorimeters.

As defined, the enthalpy encompasses not only the internal energy of the system, but also the energy stored in the environment set-system (surroundings), which absorbs through the system through work, carried out by the surroundings in the thermodynamic processes that implies decreasing its volume, also integrates a portion of passable energy that will be extracted in the form of heat from that system. The enthalpy measures from all the energy in some way  linked to the system  - including not only this energy blocked in the system, but also connected to the energy system by virtue of which the relationship with your neighborhood is established.

According to the saying, enthalpy is the sum of the internal energy of matter and the product of its volume multiplied by pressure. Enthalpy is a quantifiable state function, although the total enthalpy of a system can not be measured directly, however, the enthalpy variation of a system can be measured.

Origins of enthalpy

The word enthalpy is derived from the ancient Greek verb enthalpein (ἐνθάλπειν), which means to warm up. It combines the classical Greek prefix ἐν- en-, which means to put in, and the verb θάλπειν thalpein, which means to warm up. The word enthalpy is often incorrectly attributed to Benoît Paul Émile Clapeyron and Rudolf Clausius through the 1850 publication of their Clausius-Clapeyron relationship. This misconception was popularized by the 1927 publication of The Mollier Steam Tables and Diagrams. However, neither the concept, the word nor the enthalpy symbol existed until long after Clapeyron's death.

The first writings containing the enthalpy concept did not appear until 1875,  when  Josiah Willard Gibbs  introduced "a heat function for constant pressure". However, Gibbs did not use the word "enthalpy" in his writings.

The real word appears for the first time in the scientific literature in a 1909 publication by JP Dalton. According to that publication,  Heike Kamerlingh Onnes  really coined the word.

Over the years, scientists used many different symbols to denote enthalpy. In 1922, Alfred W. Porter proposed the symbol  as a standard,  thus ending the terminology still in use today.

In the past, enthalpy was sometimes called  heat content . The reason for this name is the fact that the change in the enthalpy ΔH is equal to the heat absorbed in the processes at constant pressure. However, this equality is not true in general (when the pressure varies), so the term  heat content  is considered deceptive  and is now in  disuse .

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Last review: December 19, 2017