# Definition of ionisation energy

My lecturer has defined ionization energy or ionization potential of a species A as the molar internal energy change, ΔU, for the reaction: $$\ce{A (g) -> A+ (g) + e-}$$ at $$\pu{0 K}$$, with reactants and products being in their standard states.

How can reactants and products be in their standard states when they must be gaseous?!

He then goes on to say that we often assume $$\Delta U (\pu{0 K})\approx \Delta H (\pu{298 K})$$. I presume he means that the molar internal energy change of the above process at $$\pu{0 K}$$ is approximately equal to the enthalpy change of the above process at $$\pu{298 K}$$? Why is this approximation valid?

• I am wondering how some teachers can teach simple topics in a dry, complicated and "bookish" way. chem.libretexts.org/Bookshelves/Inorganic_Chemistry/…. They explain it here "This energy change is generally considered equivalent to the enthalpy change at 298 K (ΔH298 K). Estimates of the error suggest < 10 kJmol-1 which when compared to typical IE values often in their thousands, is insignificant." – M. Farooq Apr 18 at 13:56