Question

The elements nitrogen and oxygen are diatomic gases in their standard states. Therefore, the standard enthalpies of formation of these elements are zero. When nitrogen and oxygen gases react to form one mole of NO2, the enthalpy of the reaction is also zero.

-true
-false

Answer 1

The statement is false because although the standard enthalpies of formation for elemental nitrogen and oxygen are zero, the formation of NO2 from these elements results in a nonzero enthalpy change of 33.2 kJ/mol.

Step-by-step explanation:

The statement that the enthalpy of the reaction for the formation of one mole of NO2 from nitrogen and oxygen gases is zero is false. The standard enthalpies of formation (ΔH°f) of the elements in their standard states (N2(g) and O2(g)) are indeed zero. However, the formation of NO2 from these elements involves a change in enthalpy. The reaction for forming NO2 is:

N2(g) + 2 O2(g) → 2 NO2(g)

Since NO2 is a compound and not an element in its standard state, it has a nonzero ΔH°f. Specifically, the standard enthalpy of formation for NO2(g) is 33.2 kJ/mol. Thus, when nitrogen and oxygen react to form NO2, there is a release or absorption of heat, and the enthalpy change is not zero.