All of the following have a standard enthalpy of formation value of zero at 25°C except

Question

Quizplus · Accepted Answer

The standard enthalpy of formation value of zero at 25°C is assigned to pure elements in their standard states. C(s) as graphite, Ne(g) as a noble gas, and Fe(s) as solid iron are all in their standard states. However, CO(g) (carbon monoxide gas) is a compound, not a pure element, so it does not have a standard enthalpy of formation value of zero. The choice D, "F₂(g)," appears to be a formatting error or nonsensical entry and does not correspond to a recognized chemical substance in standard state; however, the principle that pure elements in their standard states have an enthalpy of formation of zero applies, making CO(g) the correct answer as the exception.

All of the Following Have a Standard Enthalpy of Formation