Given Data
Formation of NO(g) from elements:
N2(g)} + O2(g) NO(g) = +90kJ mol}^{-1}
Oxidation of NO(g) to NO₂(g):
NO(g) + O2(g) – > NO2(g),= -74 kJ mol
Interpretation of Enthalpy Changes
- Formation of NO from N2 and O2 is endothermic (+90 kJ mol⁻¹). This means NO has a higher enthalpy (is less stable) than its constituent elements in their standard states.
- Conversion of NO to NO₂ is exothermic (–74 kJ mol⁻¹). NO₂ lies at a lower enthalpy than NO, so NO tends to get oxidised further to NO₂ in the presence of oxygen.
Comment on Stability
- Because its formation from N2 and O2 absorbs heat, NO(g) is thermodynamically unstable relative to the mixture of N2(g) and O2(g).
- Also, NO(g) can spontaneously (exothermically) convert to NO₂(g); therefore NO is less stable than NO₂ as well.
- Conclusion: NO(g) is a thermodynamically unstable species, existing mainly because it can be formed under high‑temperature conditions and then persists kinetically, but it is not the most stable form of N–O combination.