Question

What are the conditions for standard enthalpy of formation?

Does `\Delta H_f^{\circ}` mean the reaction takes place at a specific temperature and pressure?

Answer 1

Yes! The reaction by convention is tabulated at `25^@ "C"` and `"1 atm"` pressure by definition of the thermodynamic standard temperature and pressure.

Also, enthalpy of formation is defined for the formation reaction, i.e. the reaction of elements in their elemental states to form `"1 mol"` of the compound in question.

So for `"H"_2"O"(l)`:

`"H"_2(g) + 1/2"O"_2(g) -> "H"_2"O"(l)`

For `"NH"_3(g)`:

`1/2"N"_2(g) + 3/2"H"_2(g) -> "NH"_3(g)`

For `"NH"_4"Cl"(s)`:

`1/2"N"_2(g) + 2"H"_2(g) + 1/2"Cl"_2(g) -> "NH"_4"Cl"(s)`

and so on. Notice how the elemental state just means how it is found in nature at `25^@ "C"` and `"1 atm"`.

Thus, the enthalpy of formation of elements in their elemental state is zero.