The enthalpy (`H`) of a system is an important quantity studied by chemists. It is defined as the total heat content of a system. It is different from heat because it is a property of a system, while heat is a transport phenomenon (a system does not possess heat, but heat may transfer to or from it).
We don’t use enthalpy in our calculations, but we do use enthalpy change:
`Delta H = nDelta H_x`.
`Delta H` and `Delta H_x` are usually measured in kilojoules (or sometimes megajoules), but when they are placed in an equation with `q`, which is measured in joules, they all must be converted to the same unit.
As already mentioned, heat always has a positive value. Enthalpy change (and molar enthalpy) is negative for exothermic reactions and positive for endothermic reactions.
What amount of ethylene glycol (in moles) would vaporize while absorbing 200.0 kJ of heat? (`Delta H_"vap"=58.8\ "kJ"`.)
We can rearrange `Delta H = nDelta H_"vap"` and substitute:
`n = (Delta H)/(Delta H_"vap") = (200.0\ "kJ")/(58.8\ "kJ/mol") = 3.40\ "mol"`.