# Enthalpy

The enthalpy ($\displaystyle H$) 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:

$\displaystyle \Delta{} H = n \Delta{} H_{x}$.

enthalpy change ($\displaystyle \Delta{} H$)
the energy absorbed from or released to the surroundings when a system’s reactants react to form products, measured in kilojoules (kJ)
moles ($\displaystyle n$)
the molar amount of the substance in question (the system)
molar enthalpy ($\displaystyle \Delta{} H_{x}$)
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: $\displaystyle \Delta{} H_{\text{sol}}$, $\displaystyle \Delta{} H_{\text{vap}}$, $\displaystyle \Delta{} H_{\text{fr}}$); see pages 307 and 799–800 of the textbook for molar enthalpy values of various substances

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? ($\displaystyle \Delta{} H_{\text{vap}} = 58.8 \, \text{kJ}$.)

We can rearrange $\displaystyle \Delta{} H = n \Delta{} H_{\text{vap}}$ and substitute:

$\displaystyle n = \frac{\Delta{} H}{\Delta{} H_{\text{vap}}} = \frac{200.0 \, \text{kJ}}{58.8 \, \text{kJ/mol}} = 3.40 \, \text{mol}$.