Question

If qp​ and qv​ are the heat of formation of nitric oxide at constant pressure and constant volume in calories at 27 degrees, then:

a) qp​=qv​
b) qp​>qv​
c) qp​d) The relationship between qp​ and qv​ cannot be determined from the information provided.

Answer 1

The correct answer is b) qp > qv because at constant pressure, additional work is involved when the gas expands during the formation of nitric oxide, which is not required at constant volume.

Step-by-step explanation:

The student asks about the relationship between the heat of the formation of nitric oxide at constant pressure (qp) and constant volume (qv) in calories at 27 degrees Celsius. The heat of formation at constant pressure is associated with enthalpy change, while the heat of formation at constant volume is related to the internal energy change.

Looking at the thermodynamic relationships, we find that at constant pressure, the heat flow (qp) and enthalpy change (ΔH) for a process are equal. This includes any PV work done by the system as it expands or contracts. In contrast, at constant volume, no work is done (as there is no change in volume), so qv relates purely to the change in internal energy (ΔU).

Since forming a gas often involves work against atmospheric pressure (expansion), qp tends to be greater than qv because qp includes this additional work. Thus, the correct answer is b) qp > qv. This is because forming NO from its elements involves expansion, which requires work at constant pressure that is not required at constant volume.