Enthalpy

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.

enthalpy change (Delta H)
the energy absorbed from or released to the surroundings when a system’s reactants react to form products, measured in kilojoules (kJ)
moles (n)
the molar amount of the substance in question (the system)
molar enthalpy (Delta H_x)
the enthalpy change associated with one mole of the substance, measured in kilojoules per mole (kJ/mol); the subscript is a letter or combination of letters that indicate the type of change that is occurring (ex: Delta H_"sol", Delta H_"vap", Delta H_"fr"); see pages 307 and 799–800 of the textbook for molar enthalpy values of various substances

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.

Example

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".