Standard Enthalpies of Formation of Diatomic Molecules

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Chris Lamb 1G
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Standard Enthalpies of Formation of Diatomic Molecules

Postby Chris Lamb 1G » Fri Jan 12, 2018 9:09 am

Why is the standard enthalpy of formation of O2 gas 0kJ/mol?
(This is from example 8.11 on page 296).

Julie Steklof 1A
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Re: Standard Enthalpies of Formation of Diatomic Molecules

Postby Julie Steklof 1A » Fri Jan 12, 2018 9:14 am

All elements in their natural states have an enthalpy equal to zero because there is no change involved in their formation.

Meredith Steinberg 2E
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Re: Standard Enthalpies of Formation of Diatomic Molecules

Postby Meredith Steinberg 2E » Fri Jan 12, 2018 9:56 am

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.

Chris Lamb 1G
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Re: Standard Enthalpies of Formation of Diatomic Molecules

Postby Chris Lamb 1G » Fri Jan 12, 2018 10:01 am

So, this rule would only count for the naturally diatomic elements: H, O, F, Cl, Br, I, N?

miznaakbar
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Re: Standard Enthalpies of Formation of Diatomic Molecules

Postby miznaakbar » Fri Jan 12, 2018 10:11 am

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.

Ashley Garcia 2L
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Re: Standard Enthalpies of Formation of Diatomic Molecules

Postby Ashley Garcia 2L » Fri Jan 12, 2018 5:26 pm

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.

David Zhou 1L
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Re: Standard Enthalpies of Formation of Diatomic Molecules

Postby David Zhou 1L » Sat Jan 13, 2018 12:29 am

It's also of note that standard enthalpy refers to when the pressure is at 1 atm and 25 degrees C (298 K).


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