The positive value of ∆rH indicates that heat is absorbed during the formation of NO(g).
This means that NO(g) has higher energy than the reactants (N2 and O2). Hence, NO(g) is unstable.
The negative value of ∆rH indicates that heat is evolved during the formation of NO2(g) from NO(g) and O2(g). The product, NO2(g) is stabilized with minimum energy.
Hence, unstable NO(g) changes to unstable NO2(g).