We explain what enthalpy is, the types that exist according to chemical and physical phenomena, and its difference with entropy.
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. In physics and chemistry, this magnitude is usually represented with 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 brought into play under conditions of pressure constant depending on whether the system receives or contributes energy.
According to this, any process or transformation can be classified into two types:
- Endothermic Those that consume heat or energy from the environment.
- Exothermic Those that release heat or energy into the environment.
Depending on the type of matter involved in the system (for example, chemicals 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.
See also: Endothermic reactions
Types of enthalpy
The different types of enthalpy can be classified according to:
- Enthalpy in chemical phenomena:
- Enthalpy of formation It is the amount of energy necessary to form one mole of a compound from its constituent elements under standard pressure and temperature conditions, that is, 1 atmosphere and 25º C.
- Decomposition enthalpy Conversely, it is the amount of energy absorbed or released when a complex substance becomes simpler substances.
- Combustion enthalpy It is the energy released or absorbed by the burning of 1 mole of substance, always in the presence of gaseous oxygen.
- Neutralization enthalpy It involves the energy released or absorbed whenever an acidic and a basic solution are mixed, 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 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.
- Enthalpy of dissolution It is that of the mixture of a solute and a solvent, understandable 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 Law of Thermodynamics which states that every system in equilibrium is at its point of maximum entropy.
This principle translates 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 the lowest possible exchange, that is, the lowest recordable enthalpy.
This means that entropy and enthalpy are inversely proportional: At the maximum point of entropy, the enthalpy will be minimum, and vice versa.
