Question

For which one of the following is ΔHfo zero?

A. O (g)
B. Na (s)
C. CO2(s)
D. N2(l)

Answer 1

The answer is most likely C

Answer 2

The standard enthalpy of formation is zero for B and D since by definition, the standard enthalpy of formation for an element at its standard state is zero. The monatomic oxygen is not the standard state of oxygen while carbon dioxide is already a molecule.