Question

Question 6.21 Comment on the thermodynamic stability of NO(g), given

N2(g) + O2(g)→NO(g) ; ΔrHθ = 90 kJ mol–1

NO(g) +O2(g)→NO2(g) : ΔrHθ= –74 kJ mol–1

Class XI Thermodynamics Page 184

Answer 1

            NO(g) is unstable , because formation of NO is endothermic ( when heat is absorbed )  see 1st reaction , heat is positive it means formation of NO is endothermic reaction. 
similarly, NO2(g) is stable , because formation of NO2 is exothermic reaction ( when heat is released ). see 2nd reaction, heat is negative means exothermic reaction.

therefore, unstable NO(g) converts into NO2(g) stable gas.