Iron as an element is a solid. That's it's most stable state at room temperature and pressure, right, and so on. And then we can figure out heats of formation.

铁单质是固态,这是他在室温和常压下,最稳定的状态，然后我们,可以算出生成热，现在这个。

麻省理工公开课 - 热力学与动力学课程节选

A molar enthalpy of or whatever, iron, as a solid at 298.15 Kelvin is zero.

98。15K下固态铁的，摩尔焓是零。

麻省理工公开课 - 热力学与动力学课程节选