Definition of Enthalpy
In thermodynamics, the enthalpy (H) of a closed system is defined as:
(1) H = U + PV
where:
H = enthalpy
U = internal energy of system
P = pressure
V = volume
The internal energy of system U is made up of heat and any energy stored using internal mechanism to the system in question
(2) U = Q + Einternal
where:
Q = total energy in the form of heat
Einternal = total non-heat energy stored in system
Combining (1) and (2) gives us:
(3) H = Q + Einternal + PV
When we use the term enthalpy chemistry the Einternal is usually the energy absorbed, or released by the formation of chemical bonds. The term "enthalpy of Formation" (ΔHf°) is more simply the energy absorbed in the process of forming the chemical bonds within the compound.
(3a) H = Q + ΔHf° + PV
Enthalpy of Formation
In chemistry the " Enthalpy of Formation" is the energy absorbed in the formation of the existing chemical bonds. The assumption is made that the enthalpy of formation for the elements in uncombined form is zero, which means that for chemical compounds this term refers to: the amount of energy (per mole) that would be absorbed in the process of preparing the compound starting from its constituent elements. Note that often we see values of ΔHf° that are negative since the formation of these chemical compounds involves the release of energy, which is in fact the opposite, or the negative of, "the absorption of energy."
Hess' Law tells us that enthalpy of formation for a particular chemical compound will always be the same, regardless of the path taken to get from the raw elements to the final compound in question.
The following is a table showing some values for the Enthalpy of Formation for some common compounds.
Enthalpy of Formation for some Common Compounds.
| Compound | ΔHf° | Compound | ΔHf° |
| CH4 | -74.9 | SO2 | -297 |
| H2S | -20.1 | NaF | -571 |
| CO | -110.5 | KCl | -436 |
| CO2 | -393.5 | CCl4 | -139 |
| NO | +90.4 | FeCl2 | -341 |
| NO2 | +33.5 | CaCO3 | -1207 |
| Al2O3 | -1674 | H2SO4 | -811 |
| SiO2 | -908 | P2O5 | -1492 |
| HCl | -92.1 | CuO | -155 |
| MgO | -599 | H3PO4 | -1281 |
| NH3 | -46.2 | SO3 | -395 |
| H2O | -285.9 | CaO | -643 |
| HF | -268.6 | Fe2O3 | -821 |
| H2O2 | -188 | NaCl | -413 |
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