Gas/liquid...

Discussion in 'Chemistry' started by Beer w/Straw, Jul 22, 2018.

  1. sideshowbob Sorry, wrong number. Valued Senior Member

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  3. exchemist Valued Senior Member

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    As a change of state is a bulk property, you can't talk of a molecule expanding or contracting when changing state. I presume you mean to ask if there are other molecular substances that do this, besides water.

    I don't know the answer to that, offhand, but I would not be surprised. Water expands on freezing due to the energy released when a fixed network of hydrogen bonds is formed, which happens to make a slightly more open structure than liquid water. There may be other hydrogen-bonded substances that do this.

    However water is a bit special in that each molecule has 2 H atoms and 2 lone pairs of electrons, enabling every lone pair and H atom to participate in H-bonding. So the effect of H-bonding is particularly strong in water.
     
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  5. Beer w/Straw Transcendental Ignorance! Valued Senior Member

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    So, it's about the electrons you mean and an atoms nucleus would have nothing to do with it?
     
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  7. Beer w/Straw Transcendental Ignorance! Valued Senior Member

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  8. sideshowbob Sorry, wrong number. Valued Senior Member

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    Yeah, weird stuff.

    I remember reading an article about plutonium many years ago, in Scientific American maybe. Besides the radioactivity and heavy metal poisoning difficulties, apparently it also expands and contracts at various temperatures, making it rather difficult to work with.
     
  9. exchemist Valued Senior Member

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    Indeed. It's all about chemical bonding and intermolecular forces (hydrogen bonding sits somewhere in between the two).....which are all about the electrons in the atom. The nucleus has nothing to do with this directly, save that it is the presence of more than one +ve charged nucleus that creates potentials of the shape that cause the electrons to go into molecular rather than atomic orbitals - and thus form chemical bonds.

    For example in a diatomic molecule, the electrons are confined, not just by a single, spherically symmetrical potential well, but by a pair of them with a saddle in between. So their wavefunction changes, to occupy molecular orbital states. One of these will have lower energy than the original atomic orbitals and that will be filled first, making a lower energy configuration than was possible in the atoms - which is what we call a bond. But if you have more than 2 electrons, the extra ones will have to go into the other molecular orbital (due to the Pauli Exclusion Principle), which is of higher energy than the original atoms, and is called an "antibonding" orbital. This will cancel out the effect of the bonding orbital and lead to no net bonding. This is why, although hydrogen and helium both have electrons in the 1s shell, hydrogen forms a diatomic molecule but helium doesn't.

    Hydrogen bonds however are funny things, with a bond strength about 1/10 of a normal covalent chemical bond and about 10x that of Van der Waals intermolecular forces. They require a hydrogen atom and a pair of unbonded "spare" electrons on a neighbouring atom - a so-called "lone pair". One could write a nice essay about them, but they are still not well modelled in modern chemistry, so far as I know.

    (At this point, Wellwisher usually comes out of the woodwork, attracted by the smell of water and H bonds, to give us one of his diatribes of mystical ballocks about water, entropy and liberals.

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    So I'd better quit while I'm ahead...)
     
    Last edited: Dec 13, 2018
  10. exchemist Valued Senior Member

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    This is because below that temperature, local groups of molecules are progressively starting to take up, in a fleeting way, the hydrogen bonded lattice structure that you find in ice, with its wider intermolecular spacing.
     
    Last edited: Dec 13, 2018
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  11. origin In a democracy you deserve the leaders you elect. Valued Senior Member

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    That makes a lot of sense. I knew about the change in density near 32F and wondered why that was. Your post elicited a slap in the forehead and an 'e-duh'.
     
  12. exchemist Valued Senior Member

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    Nice to see that, even on a scientifically pretty moribund forum like this, we can still occasionally learn from one another.

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  13. Beer w/Straw Transcendental Ignorance! Valued Senior Member

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    How about a hydrogen dipole molecule?
     
  14. origin In a democracy you deserve the leaders you elect. Valued Senior Member

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    Hydrogen is not a dipole molecule. Are you referring to the fact that hydrogen on Earth is typically a molecule in the form of 2 hydrogen atoms? Hydrgen gas is in the form of H2. Solid and liquid hydrogen is not H2.
     
  15. Beer w/Straw Transcendental Ignorance! Valued Senior Member

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    Not necessarily at Earth pressure and temperature, but are you saying H2 can't exist as a liquid or solid?
    Is metallic hydrogen a single element?

    :EDIT:

    It was a simple molecule.
     
  16. exchemist Valued Senior Member

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    Actually, both liquid and solid hydrogen are composed of H2 molecules, just like the gas. The intermolecular forces between H2 molecules are extremely weak because H2 is not a dipole. They arise from mutually induced "flickering, fleeting dipoles" in the electron clouds of adjacent molecules - causing what is known as London forces: https://en.wikipedia.org/wiki/London_dispersion_force. , one type of what are collectively called van der Waals forces.

    As they are so weak, the boiling point of H2 is only 33K and the melting point 14K.

    Metallic hydrogen, which of course is not composed of molecules any more, is a phase predicted to exist at very high pressure, but I don't think it has been definitively confirmed by experimental synthesis.
     
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  17. origin In a democracy you deserve the leaders you elect. Valued Senior Member

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    Thanks for the setting me right. No idea what I was thinking on that....
     
  18. Beer w/Straw Transcendental Ignorance! Valued Senior Member

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    To clarify, maybe, I'm thinking of gravity like that of Jupiter's core.

    Where gravity would be a factor.
     
    Last edited: Dec 15, 2018
  19. exchemist Valued Senior Member

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    Gravity as a cause of the very high pressures theoretically needed? It seems this is one theory: https://science.nasa.gov/science-news/science-at-nasa/2011/09aug_juno3/

    That article says that metallic hydrogen has actually been synthesised, for a few microseconds at least. However when I made my previous comments about this, it was after reading this Wiki article: https://en.wikipedia.org/wiki/Metallic_hydrogen, which is considerably more circumspect. But it ought to exist, so maybe it does inside Jupiter. It looks to me as if the conviction that it does is linked to the observation of Jupiter's considerable magnetic field, which strongly suggests a conducting fluid in its core.

    Hydrogen is, from its electronic structure, a Group I element in the Periodic Table and as such could be thought of as the first of the Alkali Metals. However, its ionisation energy is far too high for it to take part in metallic bonding under terrestrial conditions. (The 1st ionisation energies fall as one descends any given group of the Periodic Table, as the outermost ("valence") electrons are in orbitals that are progressively further out from the nucleus.)

    To put it another way, the metal/non-metal diagonal that runs through the Periodic Table (and which arises due to the pattern of ionisation energies) cuts the Group I elements between H and Li.
     
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