What is enthalpy?

We explain what enthalpy is, the types of enthalpy according to their transformation processes and their difference with entropy.

1. What is enthalpy?

When talking about enthalpy, reference is made specifically to the amount of energy that a thermodynamic system exchanges with its environment , that is, the amount of energy that the system absorbs or releases to its environment. In physics  and chemistry , this magnitude is usually represented with the letter H and measured in joules (J).

Taking into account that every known object can be understood as a thermodynamic system, the enthalpy refers to the amount of heat that is put into play under conditions of constant pressure , depending on whether the system receives or rather provides energy.

According to this, any process or transformation can be classified into two types:

• Endothermic . Those who consume heat or energy from the environment .
• Exothermic . Those that release heat or energy to the environment.

Depending on the type of matter involved in the system (for example, chemical substances in a reaction), the degree of enthalpy of the system will depend.

The first time this term was used in that sense was done by Dutch physicist Heike Kamerlingh Onnes, discoverer of superconductivity and winner of the Nobel Prize in Physics in 1913. Another name for the same is heat content.

2. Types of enthalpy

The types of enthalpy depend on the transformation processes involved in the system, and can be:

• Enthalpy of formation . The amount of energy that is absorbed or released when one mole of substances react to form a compound substance.
• Enthalpy of decomposition . Conversely, it is the amount of energy absorbed or released when a complex substance becomes simpler substances.
• Enthalpy of combustion . It is the energy released or absorbed by the burning of 1 mole of substance, always in the presence of gaseous oxygen.
• Enthalpy of neutralization . It implies the energy released or absorbed whenever an acidic solution and a basic solution are mixed, that is, when bases and acids are neutralized reciprocally.

These types of enthalpy are of paramount importance to chemistry, since they understand reactions as thermodynamic systems. Therefore they are known as chemical enthalpy. On the other hand, we can talk about their physical equivalents, which are:

• Enthalpy of phase change . That which implies the absorption or release of energy when a substance passes from one state of aggregation to another, that is, from gas to solid or liquid , etc. It is subdivided into: enthalpy of vaporization, enthalpy of solidification and enthalpy of fusion, mainly.
• Enthalpy of dissolution . That of the mixture of a solute and a solvent , understandable in two phases: cross-linking (absorbs energy) and hydration (releases energy).

3. Enthalpy and entropy

Enthalpy and entropy (which is the degree or tendency of disorder of the systems) are related from the Second Principle of Thermodynamics , which states that every system in equilibrium is at its maximum entropy point, since it is in the that more things can tend to disorder.

Well, that principle will become with respect to enthalpy in the Principle of Minimum Enthalpy, which says that no balance can be achieved while the exchange of energy with the system is abundant or exceeds certain limits; the balance must be the state of least possible exchange, that is, of the lowest recordable enthalpy.

This means that entropy and enthalpy are inversely proportional : at the maximum entropy point, enthalpy will be minimal, and vice versa.