If the standard enthalpy of formation is defined as

the energy change when 1 mole of a substance is formed from its elements in their standard states

and forming chemical bonds is an exothermic process, then does this mean all standard enthalpies of formation are non-positive?

Why do some compounds such as $\ce{NO}$, $\ce{NO2}$ have positive standard enthalpies of formation? Is this because to form $\ce{NO}$, we must first break the bonds in $\ce{N2}$ and $\ce{O2}$ (endothermic) to form the bond in $\ce{NO}$ (exothermic, smaller energy produced than the energy required to break the bonds of original standard state elements)?

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    $\begingroup$ Well, you've pretty much answered your own questions. $\endgroup$ Commented Apr 23, 2016 at 6:15
  • $\begingroup$ Yes i agree right answer is no $\endgroup$
    – Fatima
    Commented May 20, 2021 at 2:36
  • $\begingroup$ All combustion reaction are??????? $\endgroup$
    – Fatima
    Commented May 20, 2021 at 2:41
  • $\begingroup$ Another good example is ozone (O3), whose $\Delta H^{\circ}_f = 142.7 \,\mathrm{ kJ/mol}$. It's also interesting to consider monatomic N(g), which has $\Delta H^{\circ}_f = 473 \,\mathrm{ kJ/mol}$. That's because, for every mole of N(g) you form, you have to break a half mole of N≡N triple bonds, which have a bond dissociation energy of 2 x 473 = 946 kJ/mol, and you don't form any bonds to compensate. $\endgroup$
    – theorist
    Commented May 20, 2021 at 5:36
  • $\begingroup$ @Fatima Please note your comments did not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review $\endgroup$
    – Buck Thorn
    Commented May 20, 2021 at 5:42

1 Answer 1


You'v answered the question yourself.

In general, any enthalpy of reaction is the measure of the energy remainder after breaking the bonds (positive) and what energy we got back from the formation of the bonds (negative).

The net sum can be positive or negative, and it depends on what the reactants and products were. As you correctly said yourself, $\ce{NO}$ happens to have a positive standard enthalpy of formation because, as experimentally determined, more energy went into breaking the bonds of reactants than what we got back from forming the products.


Is the standard enthalpy of formation always non-positive?

The answer is no.


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