[1] This enthalpy, if released exothermically, can in an extreme case cause an explosion.
In ideal mixtures, the enthalpy of mixing is null.
Enthalpy of mixing is defined exclusively for the continuum regime, which excludes molecular-scale effects (However, first-principles calculations have been made for some metal-alloy systems such as Al-Co-Cr[4] or β-Ti[5]).
When two substances are mixed the resulting enthalpy is not an addition of the pure component enthalpies, unless the substances form an ideal mixture.
[6] The interactions between each set of molecules determines the final change in enthalpy.
[6] In the case of alcohol and its interactions with a hydrocarbon, the alcohol molecule participates in hydrogen bonding with other alcohol molecules, and these hydrogen bonding interactions are much stronger than alcohol-hydrocarbon interactions, which results in an endothermic heat of mixing.
[7] Enthalpy of mixing is often calculated experimentally using calorimetry methods.
A bomb calorimeter is created to be an isolated system with an insulated frame and a reaction chamber, and is used to transfer the heat of mixing into surrounding water for which the temperature is measured.
Flory-Huggins is useful in calculating enthalpies of mixing for polymeric mixtures and considers a system from a multiplicity perspective.
Calculations of organic enthalpies of mixing can be made by modifying UNIFAC using the equations[8] Where: It can be seen that prediction of enthalpy of mixing is incredibly complex and requires a plethora of system variables to be known.
This explains why enthalpy of mixing is typically experimentally determined.
Among other important thermodynamic simplifications, this means that enthalpy of mixing is zero:
Any gas that follows the ideal gas law can be assumed to mix ideally, as can hydrocarbons and liquids with similar molecular interactions and properties.
[12] Intermolecular forces are the main constituent of changes in the enthalpy of a mixture.
Stronger attractive forces between the mixed molecules, such as hydrogen-bonding, induced-dipole, and dipole-dipole interactions result in a lower enthalpy of the mixture and a release of heat.
[6] If strong interactions only exist between like-molecules, such as H-bonds between water in a water-hexane solution, the mixture will have a higher total enthalpy and absorb heat.