Comment on the thermodynamic stability of `NO_((g))`, given:
`(1)/(2) N_(2(g)) , + (1)/(2) O_(2(g)) to NO_((g)), Delta_(r) H^( Theta ) = 90 "kJ mol"^(-1)`
`NO_((g)) + (1)/(2) O_(2(g)) to NO_(2(g)), Delta_(r) H^( Theta ) = -74"kJ mol"^(-1)`

Explanation

Heat is absorbed in first reaction. So, `NO_((g))` unstable. In second reaction, heat is released, So, `NO_(2(g))` is stable.
So, unstable `NO_((g))` is converted into stable `NO_(2(g))`.