Standard Enthalpies of Formation Enthalpies

[Yijia_ZHOU]

In class we talked that the standard enthalpy of formation for one mole of a substance in its most stable form is zero, however I only saw that being the case for diatomic. Do we only assume zero for diatomic? Because for an example in class for the standard enthalpy of formation for combustion of methane, CO2(g) in its most stable form has a value of -394 not zero. When do you exactly assume zero?

[Sanjana Tottemp 3C]

Hello!

The bond enthalpy for diatomic molecules is always going to be zero because when discussing bond enthalpy you are looking at the most stable for the bond. In this case, when having a diatomic molecule, for example, O2, its most stable form is in the form O2, which is also the original form the bond enthalpy, causing there to be no net change between the original form and the stable form, making the bond enthalpy zero.

Hope that Helps!

[Julian Wang]

Hi Yijia. Something important to note when comparing, say, O₂ and CO₂ is that diatomic oxygen is only comprised of one element, while carbon dioxide is comprised of multiple. Therefore, O₂ can be considered pure and in its standard (gaseous) state, while CO₂ must form from elemental carbon and oxygen. In the case of specifically carbon dioxide, the reaction of elemental carbon and gaseous oxygen to produce carbon dioxide is significantly exothermic, hence the large release of energy as indicated by the negative enthalpy of reaction. Just to reiterate, the reasoning behind O₂ having an enthalpy of formation of zero is that it happens to be the same molecule and in the same phase as the "standard" for oxygen. For example, ozone, or O₃, does NOT have an enthalpy of zero because it is not the standard form of oxygen. Hope this helped!

[Natalya Cummings 2E]

Since methane is a compound, there is a reaction that takes place to form it. It isn't a pure element. Diatomic compounds are in their pure state, since they are only one element, and they exist naturally in their most stable form that way. Thus, their enthalpies of formation will always be zero, while the most stable version of a molecule comprised of more than one element will usually be negative.

[Viviana Hsiao 1G]

You assume 0 when the molecule is in its elemental form. For example, O2 would be 0 since it is in its most basic elemental form.