Question

Which substance has ΔHf defined as 0 kJ/mol?

A) H2O (s)
B) Ne (g)
C) F2 (g)
D) CO2 (g)

Answer 1

The substance with ΔHf defined as 0 kJ/mol is Ne (g), since the standard enthalpy of formation for a pure element in its stable form at standard conditions is zero.

Step-by-step explanation:

The substance with ΔHf defined as 0 kJ/mol is B) Ne (g). The standard enthalpy of formation (ΔH°f) is defined as the heat change that results when one mole of a compound is formed from its elements at a pressure of 1 atm. The ΔH°f for a pure element in its stable form at 1 atm and 25 °C is zero. For example, as given for oxygen gas, AH°f, O₂(g): 0 kJ/mol, this is also true for other elemental gases like Neon (Ne) in its most stable form, which is gaseous.