Why is deltaG of formation 0 for diatomic molecules?






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sofiakavanaugh
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Why is deltaG of formation 0 for diatomic molecules?

Postby sofiakavanaugh » Sun Feb 11, 2018 9:18 pm

Why is deltaG(formation) zero for diatomic molecules again? and same with enthalpy values? (But not for entropy values?)

Vincent Tse 1K
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Re: Why is deltaG of formation 0 for diatomic molecules?

Postby Vincent Tse 1K » Sun Feb 11, 2018 9:23 pm

Heat of formation of a certain element is defined to be 0 for that element in its standard state (which is usually the state it is found in naturally, the diatomic one).

I suppose ΔG has a similar definition and entropy doesn't, but I'm not too entirely sure about those.

Jason Liu 1C
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Re: Why is deltaG of formation 0 for diatomic molecules?

Postby Jason Liu 1C » Sun Feb 11, 2018 9:37 pm

ΔS isn't 0 for diatomic molecules because there is always entropy. S is only 0 when T = 0K.

Anna Li 2E
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Re: Why is deltaG of formation 0 for diatomic molecules?

Postby Anna Li 2E » Sun Feb 11, 2018 9:50 pm

The heat of formation is 0 only for certain diatomic molecules. This occurs for the elements where their natural/most common state is a diatomic. These elements are H2, N2, F2, O2, L2, Cl2, Br2.

This is due to the fact that standard enthalpy of formation is defined as enthalpy change of a substance when it is formed from the most stable form of itself in standard condition (P=1 bar and T=25C). As these diatomics are already in their most stable form, their standard enthalpy of formation is zero. This is also true for Gibbs free energy (where G=0) as each diatomic molecule is in its standard state.


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