Heat

Discussion in 'Physics & Math' started by Bird11dog, Jun 21, 2016.

  1. Bird11dog Registered Member

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    Is the vibrations that cause matter to have heat due to the entire atom vibrating or just the electron vibrating?
     
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  3. origin Heading towards oblivion Valued Senior Member

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    In a solid the bond length between the atoms increase and decrease (vibrate) so that the whole atom moves. The amplitude of the movement increases with an increase in temperature. In a gas the individual atoms have a velocity that increases with temperature.
     
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  5. timojin Valued Senior Member

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    Wouldn't that be depend of the solid ?
     
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  7. exchemist Valued Senior Member

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    At temperatures below those at which visible radiation is emitted, it is the motion of whole atoms. There are three types of this motion: motion of whole molecules ("translational" motion), rotation of whole molecules, and vibration of the bonds between atoms in a molecule (or in a "giant structure" -type solid). This can be seen in the way that the specific heat capacity of gases increases in steps, if the temperature is slowly raised from close to absolute zero. At very low temperature, only translational modes of motion can participate, then, once the energy of the molecules becomes enough to overcome the gaps between rotational energy levels, the rotations start to participate, becoming an extra reservoir for heat and thus leading to an increase in heat capacity. Then at a higher temperature again, the bigger gaps that exist between successive vibrational levels can be jumped and vibrations join in, providing a further heat reservoir and leading to a further increase in heat capacity.

    When you get to very high temperatures indeed, the very big gaps in energy between electronic orbitals can be jumped, and at that point electrons start to be excited by the thermal motion, leading to emission of visible light, as in materials that are said to be "red-hot" or "white-hot". If you continue to raise the temperature, eventually the electrons can be excited to energy levels above the limit at which they cease to be bound to the atom at all - in other words ionisation takes place. You then have a plasma.

    So the strict answer to your question is that it depends on the temperature regime you are talking about: the forms of motion (degrees of freedom) amongst which thermal energy is shared depends on whether the energy of the molecules is enough to jump the energy gaps that are characteristic of each type of motion. But the electrons are the last to participate, because the energy gaps are largest in their case.
     
  8. origin Heading towards oblivion Valued Senior Member

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    Wouldn't what depend on the solid?
     
  9. timojin Valued Senior Member

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    I would think polymers and inorganic molecules explanation would be some how different.
     
  10. origin Heading towards oblivion Valued Senior Member

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    No, same principle.
     
  11. timojin Valued Senior Member

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    Let say ICE , Ice have a larger volume than water as it melts into liquid it reduces the volume . Vibration is an energy to increase volume . How do you apply the same principle ?
     
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  12. ajanta Registered Senior Member

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    And I knew that water has top density at the temperature of 4 degree celsius. It's really interesting to me.

    Does pressure set this physical state

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    Last edited: Jun 23, 2016
  13. exchemist Valued Senior Member

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    When a substance freezes, its molecules no longer have enough energy to overcome intermolecular bonding and so the molecules settle down into an ordered pattern that minimises the energy by allowing the strongest possible bonds to form. It happens that in the case of water, the ordered pattern with the lowest energy, i.e with the strongest bonding between the molecules, occupies a bit more space than the liquid state. The density of ice at 0C is thus lower than that of water at 0C.

    One important consequence is that by applying pressure, you can alter the thermodynamic equilibrium between ice and water, in favour of water. In other words, if you apply pressure to water at 0C, the molecules have to do more work to expand against this pressure - and that can mean that forming ice at 0C no longer creates a lower energy state than water at 0C. So the water will then not freeze. Applying pressure to ice melts it - which is why ice is slippery. When you try to walk on it, you apply pressure and a film of water appears under the pressure point, acting as a lubricant.

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    P.S. Rereading your post I realise you may have been asking something slightly different, viz. why the maximum density of water is at 4C and not at 0C, immediately after melting. My understanding is that this is due to the progressive development of order, due to hydrogen bonds starting to form. There is a good description in the Wiki article on properties of water, which I quote below:

    "The solid form of most substances is denser than the liquid phase; thus, a block of most solids will sink in the liquid. However, a block of ice floats in liquid water because ice is less dense. Upon freezing, the density of water decreases by about 9%.[26] This is due to the 'cooling' of intermolecular vibrations allowing the molecules to form steady hydrogen bonds with their neighbors and thereby gradually locking into positions reminiscent of the hexagonal packing achieved upon freezing to ice Ih. Whereas the hydrogen bonds are shorter in the crystal than in the liquid, this locking effect reduces the average coordination number of molecules as the liquid approaches nucleation. Other substances that expand on freezing are acetic acid, silicon, gallium, germanium, antimony, bismuth, plutonium and also chemical compounds that form spacious crystal lattices with tetrahedral coordination. "
     
    Last edited: Jun 23, 2016
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  14. exchemist Valued Senior Member

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    See my reply to Ajanta.

    It is true that increasing amplitude of vibration (via the excitation of higher vibrational energy levels in the bonds) does cause expansion of a solid and this is true of ice just as in anything else. However a phase change occurs when the vibrational excitation exceeds the bond energy of the the intermolecular bonds, i.e. it involves actual breaking of the intermolecular bonds, enabling the molecules to start rotating and translating, as well as vibrating. The structure then changes completely, so the simple idea of increasing vibrational amplitude no longer accounts for what is going on.
     
    Last edited: Jun 23, 2016
  15. wellwisher Banned Banned

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    Water is interesting because water at 4C will expand whether you heat or cool it.

    The explanation for both observations is connected to hydrogen bonding. Hydrogen bonding is a hybrid bond showing both polar and covalent bonding characteristics. Polar bonds, in general, are not directional, but only prefer to be as close as possible to lower charge potential. Covalent bonding is more directional; specific bond angles, and often need more room so the covalent bonding orbitals can overlap properly. The covalent hydrogen bonding is less about the potential between charges, and more about magnetic addition of delocalized electrons; covalent bonds add motion to electrons.

    In liquid water, the hydrogen bonding binary is evident as high and low density water clusters. The high density water has more polar hydrogen bonding, while low density water has more covalent hydrogen bonding. The energy between the two is quite close, so the water can shift back and forth between high and low density, while never having to break any hydrogen bonds. The low density water defines lower energy, lower entropy and higher volume. While the high density defines slightly higher energy, slightly higher entropy and less volume.

    When we form ice, we have less density, less energy and less entropy; covalent hydrogen bonding. The ice crystals make the bonds more directional making the covalent hydrogen bonding preferential.

    If we apply pressure to the water at 0C; freezing point, we can force the low density water nucleation sites, about to freeze, into higher density water clusters. This shifts the hydrogen bonding to more polar. This will delay the freezing of the water because it will inhibit the low density water needed to begin nucleation. Water is anomalous and does things differently.
     
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  16. origin Heading towards oblivion Valued Senior Member

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    AAAAAHHHHHHHH!!
     
  17. timojin Valued Senior Member

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    So far we don't have a good answer and AAAAHHHH is no good
     
  18. origin Heading towards oblivion Valued Senior Member

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    I didn't answer your question because exchemist did. Did you read it? It is a good answer.
     
  19. exchemist Valued Senior Member

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    Surely you mean, "BINGO!!" ?

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  20. timojin Valued Senior Member

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    Paragraph 1 ) If you freeze things remain at the present state so water and Ice have the same density . The fact is the volume increase at constant pressure . The expansion force to increase volume is stronger the the tensile strength if iron or brass. That means there is a geometric rearrangement to favor volume increase . I like to think There is no relationship between phase change of water and vibration energy
    2 ) Molecule don't do work, I believe they respond to the environment . If you increase pressure you should reduce volume at constant temperature . But here comes the anomaly You reduce the temp/ and you have an increase in volume
     
  21. exchemist Valued Senior Member

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    1)You seem to be saying two contradictory things in the same sentence. Water and ice do NOT have the same density, BECAUSE there is indeed a geometrical structure created as ice forms, that minimises the energy of the system, even though this does not involve the molecules being as close to one another as possible, i.e. it energetically favours volume increase, as you say. The reason is that the formation of the hydrogen bonds in this structure releases a lot of energy and this outweighs the disadvantage of not having the molecules as densely packed as possible, which is what would give the maximum van der Waals binding.

    In most molecular substances, formation of the solid is driven by van der Waals forces alone, but in water this is not so, hence that somewhat unusual behaviour. (In fact there are several other substances that expand on freezing. Some of them are referred to in the Wiki link I gave, earlier in this thread.)

    2) Of course molecules do work. If pressurised gas expands a balloon the molecules are doing work (what we call "PV work") in pushing back the atmosphere. In most of chemistry we express energy changes in terms of "Enthalpy", H, which is defined as internal energy, U + PV work done, precisely because most chemistry is done under atmospheric pressure. Surely you have come across H = U + PV, haven't you?

    Therefore, if there is a volume change during any chemical reaction, you can alter the thermodyamic favourability of it by altering the pressure.

    When water freezes, the Latent Heat of Fusion is released and this is the Enthalpy change on freezing. https://en.wikipedia.org/wiki/Enthalpy_of_fusion
     
    Last edited: Jun 26, 2016

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