Standard Enthalpy of Solid/Liquid

[victoria alvarez 3c]

In lecture, professor mentioned that standard state for a pure liquid or solid is the pure liquid or solid and the standard state for an element is its most stable phase. Can someone explain the difference between the two?

[Mona Reddy Kurra 1J]

Hi! Based on my understanding, standard state of a pure liquid or solid can also apply to compounds, such as H2O. For these, the standard reaction enthalpies are measured when the compounds are in this state. On the other hand, elements can differ in their standard state based on the element. For example, the most stable phase of Cl2 at 1 atm is gas while graphite is the most stable form of carbon, which is a solid. Therefore these respective phases are considered the standard states. I hope that helps!