We explain what enthalpy is, the types that exist according to chemical and physical phenomena, and its difference with entropy.

Enthalpy is the amount of energy that is put into play in processes at constant pressure.

What is enthalpy?

Enthalpy is the amount of Energy that a thermodynamic system exchanges with its environment under conditions of constant pressure, that is, the amount of energy that the system absorbs or releases to its environment in processes in which the pressure does not change. Inphysical Ychemistry, this magnitude is usually represented by the letter H and it is measured in joules (J).

Taking into account that every object can be understood as a thermodynamic system, enthalpy refers to the amount of heat that is put into play in conditions ofPressure constant, depending on whether the system receives or supplies power.

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

  • Endothermic. Those that consume heat or energy from theenvironment.
  • Exothermic. Those that release heat or energy into the environment.

Depending on the type of matter that intervenes in the system (for example, chemical substances in a reaction), the degree of enthalpy will be different.

The Dutch physicist Heike Kamerlingh Onnes, discoverer of superconductivity and winner of the Nobel Prize in Physics in 1913, was the first to use this term.

Enthalpy types

The enthalpy of combustion is the energy released or absorbed by burning 1 mole of substance.

The different types of enthalpy can be classified according to:

  • Enthalpy in chemical phenomena:
    • Formation enthalpy. It is the amount of energy required to form a mole of a compound from its constituent elements under pressure conditions and temperature standards, that is, 1 atmosphere and 25º C.
    • 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 burning 1 mole of substance, always in the presence of gaseous oxygen.
    • Neutralization enthalpy. It implies the energy released or absorbed whenever a acid solution and a basic they mix, that is, when bases and acids neutralize each other.
  • Enthalpy in physical phenomena:
    • Phase change enthalpy. It involves the absorption or release of energy when 1 mole of substance passes from a State of aggregation to another, that is, fromgas tosolid or toliquid, etc. It is subdivided into: enthalpy of vaporization, enthalpy of solidification and enthalpy of fusion.
    • Enthalpy of dissolution. It is the proper mixing of a solute and a solvent, comprehensible in two phases: reticular (absorbs energy) and hydration (releases energy).

It should be noted that all the processes described are at constant pressure.

Enthalpy and entropy

The concepts of enthalpy and entropy (which is the degree or tendency of disorder of systems) are related from the Second Principle of Thermodynamics, which states that every system in equilibrium is at its point of maximum entropy.

This principle is translated into the Principle of Minimum Enthalpy, which says that no equilibrium can be achieved as long as the exchange of energy with the system is abundant or exceeds certain limits; the equilibrium must be the state of lowest possible exchange, that is, of the lowest recordable enthalpy.

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

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