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Standard Enthalpies of Formation of Diatomic Molecules

Posted: Fri Jan 12, 2018 9:09 am
by Chris Lamb 1G
Why is the standard enthalpy of formation of O2 gas 0kJ/mol?
(This is from example 8.11 on page 296).

Re: Standard Enthalpies of Formation of Diatomic Molecules

Posted: Fri Jan 12, 2018 9:14 am
by Julie Steklof 1A
All elements in their natural states have an enthalpy equal to zero because there is no change involved in their formation.

Re: Standard Enthalpies of Formation of Diatomic Molecules

Posted: Fri Jan 12, 2018 9:56 am
by Meredith Steinberg 2E
O2 is the most stable form of oxygen and therefore has an enthalpy equal to zero. This is the same for other diatomic elements, such as nitrogen (N2), chlorine (Cl2), etc.

Re: Standard Enthalpies of Formation of Diatomic Molecules

Posted: Fri Jan 12, 2018 10:01 am
by Chris Lamb 1G
So, this rule would only count for the naturally diatomic elements: H, O, F, Cl, Br, I, N?

Re: Standard Enthalpies of Formation of Diatomic Molecules

Posted: Fri Jan 12, 2018 10:11 am
by miznaakbar
No this rule applies to all elements, because in their standard state they are in their most stable state, so enthalpy is zero. For instance, Carbon's standard state is a solid as graphite, so it's enthalpy would be zero. However, Carbon in its solid state as a diamond would have a nonzero enthalpy because it is not the standard state.

Re: Standard Enthalpies of Formation of Diatomic Molecules

Posted: Fri Jan 12, 2018 5:26 pm
by Ashley Garcia 2L
The standard enthalpy of formation for diatomic molecules such as O2 (g) equals 0 because no change occurs in the reaction O2 (g) --> O2 (g). The definition of the standard enthalpy of formation is that the standard enthalpy of formation of an element in its most stable form equals 0. O2 (g) is the most stable form of oxygen, and no change occurs in the reaction, therefore the standard enthalpy of the reaction equals 0.

Re: Standard Enthalpies of Formation of Diatomic Molecules

Posted: Sat Jan 13, 2018 12:29 am
by David Zhou 1L
It's also of note that standard enthalpy refers to when the pressure is at 1 atm and 25 degrees C (298 K).