# Enthalpy - Thermodynamics

Enthalpy is a state function of thermodynamics that is symbolized by the letter H. Enthalpy is also known as absolute enthalpy or amount of heat. The variation of the enthalpy of a thermodynamic system allows to express the amount of heat exchanged during an isobaric transformation, that is, at constant pressure.

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

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

As defined, the enthalpy itself encompasses not only the internal energy of the system, but also the energy stored in the environment of the system (surroundings). By decreasing its volume, the enthalpy also integrates a portion of passable energy that will be extracted in the form of heat from that system.

In short, 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 cannot be measured directly, however, the enthalpy variation of a system can be measured.

### Enthalpy Units

Enthalpy is a variable that indicates an amount of energy, therefore, in accordance with the international measurement system, is expressed in Joules. Although the amount of heat can also be expressed in chilocalories (kcal).

## Enthalpy of bond

Enthalpy of bond is defined as the amount of energy needed to create or break a chemical bond between two chemical elements. Link enthalpy is also known colloquially as link energy.

When a stable chemical bond is formed, a certain amount of energy is released, which will be the same amount of energy that must be provided later to break the bond formed. This energy is called bond enthalpy (if it occurs at constant pressure).

In a more formal way, we can define the enthalpy of normal or standard bond as the variation of enthalpy or heat released, under standard conditions of 1 atmosphere and 25 ° C, which accompanies the reaction of formation of one mole of chemical bonds from the Isolated atoms in a gaseous state.

## Enthalpy and solar energy

In the field of solar energy, enthalpy appears in a relevant way in solar thermal energy systems. In all heat exchangers, the elements involved suffer a variation of their enthalpy when heat is transferred from one circuit to another.

In a photovoltaic solar energy installation, the thermodynamic aspect is not as relevant since the energy generated is an electric current. The photovoltaic effect generates a movement of electrons, that is electricity, without heat exchange.

## Origins of enthalpy

The word enthalpy is derived from the ancient Greek verb enthalpein (ἐνθάλπειν), which means heating. Combine the classical Greek prefix ἐν- en-, which means put in, and the verb θάλπειν thalpein, which means to heat. The word enthalpy is often incorrectly attributed to Benoît Paul Émile Clapeyron and Rudolf Clausius through the 1850 publication of their relationship Clausius-Clapeyron. 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 concept of enthalpy did not appear until 1875, when Josiah Willard Gibbs introduced "a function of heat for constant pressure." However, Gibbs did not use the word "enthalpy" in his writings.

The real word first appears in the scientific literature in a 1909 publication by JP Dalton. According to that post, 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 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 enthalpy ΔH is equal to the heat absorbed in constant pressure processes. However, this equality is not true in general (when the pressure varies), so the term heat content is considered misleading and is now deprecated.

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Published: December 19, 2017
Last review: September 25, 2019