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Enthalpy and thermodynamics

We touched upon the subject of thermodynamics and entropy in a previous blog post, this week will continue the topic of thermodynamics and entropy's closely related cousin of enthalpy.


Whilst entropy is more concerned with the physical aspects of a reaction or substance, enthalpy is more concentrated upon the effects of temperatures.

Defined as a thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume.

The easiest way to discuss an introduction of enthalpy is to look at gasses, due to the ease at which pressure and volumes can be visualised. These two other physical properties have a direct correspondence to temperature through the ideal gas law, where thermal energy is correlated to increases in pressure and volume, or a reduction in density because as we know, as a gas increases in temperature, its density decreases.


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In a sealed system, as temperature increases the latent pressure on the sealed barrier increases due to lowered density, so it is known that pressure is a form of energy directly related to the temperature of the system, and pressure also directly relates to the volume of a system, as a hotter system would require a larger space to account for the lower density to maintain the same pressure of a gas of a lower temperature, which brings us to combine all three aspects of heat volume and pressure into a single value known as enthalpy. Mathematically it is defined as the internal energy of a system, plus the total of pressure multiplied by the volume of a system.

This also is expressed in solids. As temperatures in a metal or other solid increase, the energy causes more vibrations. These vibrations are physical movements which would increase the bond length between two molecules, and thus as a block of metal increases in temperature, the size of the metal block would increase slightly.


When a system contains multiple species, the enthalpy of the system refers to the sum total of all enthalpies of the individual constituents, and the individual constituents' enthalpies of a multi species system is affected the presence of other species in the system.