[kirushanth]

What is the standard enthalpy of formation of D2?

[sjb]

What is the standard enthalpy of formation of D₂?

At what temperature, pressure, state?

[Borek]

Zero? But perhaps not.

At what temperature, pressure, state?

Why should it matter? It is standard after all.

[sjb]

At what temperature, pressure, state?

Why should it matter? It is standard after all.

Hmm, my understanding was that it was zero if the pure element was in its standard state under those conditions, but e.g. the formation of diamond is not - different allotropes etc. So, if we were talking liquid deuterium at room temperature and pressure, that may not be zero

[Enthalpy]

Since D₂ can only be formed from D₂, I'd have said zero.
Provided both are taken in the same state, which I expect to be the case.

More interesting: D₂ has different chemical properties from H₂, an exception among the elements, because D vibrates with a smaller displacement than H does, even at the lowest vibration energy (the nucleus quantum delocalization). This shifts the boiling and melting points of D₂O by several degrees, and enables to separate D₂O by chemical means.

[fledarmus]

The standard state for an element is its most stable form at 1 atm and room temperature, and the enthalpy of formation for that element at those conditions is zero. So the standard enthalpy of formation for D₂ is zero.

For carbon, the most stable form at 1 atm and room temperature is graphite, not diamond - diamond contains an additional 1.8 kcal/mol. So the standard energy of formation of carbon is defined as zero for the graphite structure.

[Borek]

Question is - is D₂ and "hydrogen" the same thing? When we say "hydrogen" we mean a mixture of isotopes with molar mass 1.0079 g/mol.

[fledarmus]

No, deuterium is not hydrogen, or nitrogen, or oxygen, which all have a standard enthalpy of formation of zero. To calculate the enthalpy of formation of, for example, D₂O would require the enthalpy of an oxygen-deuterium bond rather than an oxygen-hydrogen bond, and although both hydrogen and deuterium gas have a standard enthalpy of formation of zero, the bond strengths between oxygen-hydrogen and oxygen-deuterium are different.

[Enthalpy]

The difference between 1g and 1.0079g comes marginally from deuterium, which adds 0.0001g
http://www.webelements.com/hydrogen/isotopes.html
and mainly from the nuclei binding energy in heavier atoms.

That is, the mole is defined from ¹²C where nucleons hold together firmly hence weigh less than individually. The 6 neutrons each slightly heavier than a proton+electron would work the opposite way and the electrostatic repulsion as well but the strong force is more important. Carbon's deep electrons being attracted by more protons make only 1µg/mol difference.

The ¹²C reference gives masses closer to integers for (mono-isotopic) nuclides near C, N, O but farther for hydrogen and iron. Carbon is also nearly mono-isotopic, so it's one possible convenient compromise. ¹⁶O would be one other, and Silicon may become one day the new kilogram standard.

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I've read occasionally "protium" for hydrogen free of deuterium.

And, my apologies for the digression: sea water contains no tritium for nuclear fusion reactors.

[juanrga]

What is the standard enthalpy of formation of D2?

According to a NIST database citing [1] as reference, the enthalpy of formation at 0K is the same that for H2: 0.0.

[1] Chase, M.W., Jr.; Davies, C.A.; Downey, J.R., Jr.; Frurip, D.J.; McDonald, R.A.; Syverud, A.N., JANAF Thermochemical Tables (Third Edition), J. Phys. Chem. Ref. Data,Suppl. 1, 1985, 14, 1.