Enthalpy

The enthalpy is the total energy of a thermodynamic system. During phase changes like melting, sublimation, crystallization and evaporation but also solid-solid phase changes (interconversion of polymorphic structures) energy (as heat and/or work) is exchanged with the environment and enthalpy of the system changes. If energy is absorbed from the environment the process is called endothermic, otherwise it’s called exothermic.

Chemical reactions can be exothermic or endothermic. Melting, evaporation and sublimation are endothermic while the reverse process is exothermic (crystallization, condensation etc.). Chemical reactions and dissolutions can be either exothermic or endothermic.

Scientific definition

The total energy of a thermodynamic system is called enthalpy H. As sum of the internal energy U and the product of Volume V and pressure p it’s a thermodynamic state function:

H = U + pV

At constant pressure the enthalpy corresponds to the transferred energy in form of heat and/or work other than expansion work.

Enthalpy has the dimension of energy: Joule. In chemical transformations, the dimension energy/mole is used.

Measuring methods

Only enthalpy changes can be measured. This can be done by so called differential scanning calorimetry (DSC).

Two DSC techniques are known: so called heat flux method and energy compensated technique. In heat flux devices, which are the most common devices, sample and reference (inert material undergoing no transformations in the temperature range) are positioned in one furnace and the temperature difference is measured. Whenever energy is consumed or liberated by the sample, the temperature difference regarding the reference will suddenly change. Peak surface is proportional to the enthalpy (for constant pressure) when experimental conditions allow quantifying the energy flux to or from the sample. Surface is calibrated by analyzing melting enthalpies of standard materials (In, Sn, Zn, Pb etc.).

In so called power compensated devices, sample and reference are in two separated furnaces which are regulated in such a way to maintain the temperature difference as small as possible. In this case, the difference in heating power of the furnaces corresponds to the enthalpy. This latter technique is rarely used.

With DSC instruments also the heat capacity Cp can be measured. More detailed information about this method can be found in the dedicated chapter.