Question

Which of the substances have a standard enthalpy of formation, delta H∘ₜ, equal to zero?

a) F₂(8)
b) C(diamond)
c) Hg(1)
d) NH₃(g)

Answer 1

Elements in their most stable form have a standard enthalpy of formation of zero. F₂(g) fits this criteria, while C(diamond), Hg(l), and NH₃(g) do not.

Step-by-step explanation:

The substances that have a standard enthalpy of formation, ΔH°ₜ, equal to zero are elements in their most stable form at 25°C and 1 atm pressure. In this case, F₂(g) is the most stable form of fluorine, thus, it has a ΔH°ₜ of zero. On the other hand, C(diamond) is not the most stable form of carbon; graphite is, so diamond does not have a ΔH°ₜ of zero. Hg(l) and NH₃(g) are not elemental forms; mercury is an element but not in the gas phase, and NH₃(g) is a compound, so neither fit the criteria for a ΔH°ₜ of zero.