Microwave heating

Discussion in 'Chemistry' started by arauca, Jun 17, 2013.

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  1. arauca Banned Banned

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    I am not sure but it is said that microwave in commercial oven is monochromatic. in the microwave region . It is also said that is in the rotational energy , based on the nomenclature "rotation" I can picture myself in an example of organic alcohols primary secondary and tertiary , there should be a difference in rotation between the oxygen bond and carbon , but can not visualize the rotation in a water molecule , yet water apparently absorbs the microwave energy more efficient .
    In essence the question becomes what does rotation means , Is it the molecule as a whole or is it the bond between the two atoms like in water the bond of Hydrogen and Oxygen .
    In my view if it is rotation of a molecule as a whole, then it is hard to believe or accept a microwave photon interaction does have any thing to do with the atomic energy levels .
    Any comments
     
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  3. Buddha12 Valued Senior Member

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    Photons in the microwave frequency range are important in quantum research - for quantum information processors, for example. Now, for the first time, researchers have achieved the controlled production of single photons in the microwave region and successfully detected them with highly sensitive measuring instruments - although they are 100,000 times weaker than the photons emitted by an electric light bulb.

    When photons released by a conventional light or radiation source hit a detector, they trigger an electrical signal comparable to a single “click” of a Geiger counter, which rattles when radioactive particles strike it. However, unlike optical photons, until now there have been no detectors that can detect single photons at particularly low frequencies, such as the microwave frequency range. The intensity of these microwave photons is much too weak for this. The research group led by ETH Zurich Professor Andreas Wallraff from the Department of Physics has now been able to characterise such low-intensity photons even without any “clicking” detectors by using a special apparatus and method. Physicists need techniques of this kind, for example to research the fundamental principles of quantum mechanics or to enable efficient information transmission in optical data communication.

    Read more at: http://phys.org/news/2011-02-breakthrough-photons-microwave-frequency-range.html#jCp
     
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  5. AlexG Like nailing Jello to a tree Valued Senior Member

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    Yes. You have no idea what you're talking about.
     
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  7. arauca Banned Banned

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    Well could you help to make me understand better in your own words
     
  8. Aqueous Id flat Earth skeptic Valued Senior Member

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    Not efficiently, just that the molecules or water are polarized.
    It's acting on the water molecules. They are polar due to the proximity of the two H atoms. They are susceptible to the forces from electromagnetic fields.

    It begins with radiation, which is photons. Photons do not act directly on the polar water molecules. They interact with individual electrons, causing them to jump and drop back to their ground state. Thus the electrons re-radiate the generated frequency. Waves are set up inside the material and the polar molecules wiggle, jiggle and spin wherever there is freedom to move. There may be some translation in (x, y, z) but the most likely degree of freedom is axial, since that requires less energy than pushing other molecules out of the way in order to be able to translate.
     
  9. exchemist Valued Senior Member

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    I don't think this is right. Microwave photons are of the wavelength to be absorbed directly by molecular rotation. Nothing to do with electrons first. You need generally need photons in the visible or UV to region to excite electrons to different orbitals. Microwave photons do not have enough energy to do this (E=hν).

    As I have explained on the concurrent thread about "why do photons make matter hotter", molecular rotation is quantised, so only set speeds of rotation are allowed. The molecule thus will only absorb microwave energy of the right wavelength to excite it to a different rotational level, in the just the same way as for electrons. http://hyperphysics.phy-astr.gsu.edu/hbase/molecule/rotrig.html
     
  10. arauca Banned Banned

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    I follow your view as
    I understand the water molecule is polarized , but it absorbs microwave better then many chemicals . Water have hydrogen bonding , then there are 2 sigma bonds between oxygen and Hydrogen, then here comes photons at x mm intervals and so the in-coming photon is absorbed by (x,y,z. )p and as the electron falls back it releases the enectrgy and that energy breaks temporary the hydrogen bonds in the system . Is that sequence ?

    Now what is your explanation of -OH functional groups in an organic molecule like alcohol, amines , amides were here is called rotation. Is the rotation of the functional group with respect to the attached carbon or is a rotation of Hydrogen with respect to oxygen ?. My original question was by taking primary , secondary and tertiary alcohols to verify as to were the so called rotation takes place . Then the contraversial question was about whet is meant by rotation ,is the whole molecule or just the functional group.
     
  11. exchemist Valued Senior Member

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    Arauca, that's not right, if the photons are of microwave wavelength the have insufficient energy to interact with the electrons in a molecule. It is the direct stimulating of rotation by the microwave photons interacting with the dipole of the molecule. For this reason, diatomic molecules in which both atoms are the same do not have a rotational spectrum! They rotate via collisions of course, but there is no way to excite rotations with electromagnetic radiation, because there is no dipole for it to couple with.

    Regarding more complex molecules, yes there are many modes of internal rotation of parts of the molecule, which can be excited in the same way, provided the part in question has a dipole. I am sure you can look up information about rotational spectra of alcohols and find some detailed answers, though my guess is that interpreting the spectrum will become fiendishly complicated once you more than about 5 atoms present.
     
  12. Brett Registered Member

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    If the whole molecule was rotating, you could take it out of the microwave and observe them doing so through your lens and stuff. if it is just the 'functional group,' as you say, then you would need to isolate them and see.

    How do you know if anything is rotating? well, if you were to measure it one second, then after you are finished measuring it, wait about a minute, then check again. if they are in the same places, then there is no rotation?

    So, if you don't have a minute, you should try to prove they rotate by observing the mixture. if it is full of polarised molecules, then there has to be some rotation.
     
  13. exchemist Valued Senior Member

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    Eh? "...through you lens and stuff"?

    Have you any idea of the size of a molecule? And stuff.
     
  14. Brett Registered Member

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    very small? like my pinky!
     
  15. exchemist Valued Senior Member

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    Yup. ~6 x 10²³ of them in 18cc of water. Far smaller - by about a factor of 1,000 - than the wavelength of visible light, in fact. Which means no optical microscope has any chance of seeing them, I'm afraid.
     
  16. arauca Banned Banned

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    I believe a photon is a unit and there is not a photon for microwave and an other for IR , visible UV. ete. for me is how many come in a unit of time , and that is were the word frequency are produced . Since the delivery of photon is slow , the electron have enough time to fall back to the orbital from were it was elevated by the photon and so the energy remitted will go to heat up the water and as a consequence breakage of hydrogen bonds will take place in bulk liquid .
    Microwave spectrophotometry is used in weather forecasting by measuring Oxygen which is a symmetrical unit

    Microwave absorption and emission of the troposphere

    The principal sources of atmospheric microwave emission and absorption are water vapor, oxygen, and cloud liquid (Fig. 1). In the frequency region from 20 to 100 GHz, water-vapor absorption arises from the weak electric dipole rotational transition at 22.235 GHz and the so-called continuum absorption of water vapor most likely to arise from the far wing contributions of higher-frequency resonances that extend into the infrared region. It should be noted that a much stronger water vapour line exists at 183.31 GHz that is used at dry conditions [Cimini et al., 2009]. Oxygen absorbs due to a series of magnetic dipole transitions centered around 60 GHz and the isolated line at 118.75 GHz. Because of pressure broadening, i.e. the effect of molecular collisions on radiative transitions, both water vapor and oxygen absorption extend outside of the immediate frequency region of their resonant lines. There are also resonances by ozone that are important for stratospheric sounding [Klein and Gasiewski, 2000]. In addition to gaseous absorption, scattering, absorption, and emission also originate from hydrometeors in the atmosphere
    http://wiki.eg-climet.org/index.php?title=MWR_Fundamentals
     
  17. origin Trump is the best argument against a democracy. Valued Senior Member

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    Well what you believe is terrible at odds with what is experimentally seen. So you do not think that a single photon has frequency? Quite frankly that is really bonkers. If you have a source of radiation that is emitting only about 1 gama ray per second you can measure the wavelength of those single photons. Does this help to realize you are wrong?
     
  18. exchemist Valued Senior Member

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    Arauca thanks for bringing oxygen to my attention. My remarks about no rotational spectrum for diatomic molecules with 2 identical atoms apply in fact to electric dipole induced transitions, which are the norm. Oxygen does indeed have a microwave spectrum, excited in this special case by the fact that its ground state, unusually, is a triplet state, i.e. it is paramagnetic. Consequently there is a magnetic dipole which can couple with the microwave photons. However for more typical diatomic elemental gases such as nitrogen, there is not.

    http://en.wikipedia.org/wiki/Rotational_spectroscopy

    Apologies for not including this qualification in my previous post.
     
  19. Layman Totally Internally Reflected Valued Senior Member

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    I would think that the rotation is just the rotational kinetic energy is just the amount of work the electrons can do from rotating around the nucleus of the atom. It can be described as being Newtonian as it derived the same way as in Newtonian physics using Newton's second law of motion as shown from hyperphysics website.

    Rotational Kinetic Energy

    The electrons create the force that is the actual surface of the atoms that we feel that prevent movement into the atoms themselves. So then if the surface was to rotate then it would just be the rotation of electrons around the atom, that is described to be known electron behavior.

    Then if an electron absorbs a photon it changes its energy that alters it's wavelength, it then changes orbitals because of a change in frequency. That is how I learned it in chemistry and how it was described to me.

    I don't see how that could translate into an increase of work being able to be done by the valence electrons when they would change to different orbitals looking at it from the same perspective that a lot of theory is based on in chemistry that is based on the Bohr Model of the atom. It would seem that it would have to be replaced by an electron of the same frequency that was then able to do more work.

    I have never heard of photon's being a known catalyst in chemistry (I would be hesitant in saying for sure that it was a known catalyst), yet microwaving stuff causes chemical reactions. It was only stated that electrical current itself is responsible for these types of chemical reactions and that is actually the catalyst that is affecting it. It would then seem that the electromagnetic energy would then have to produce an electrical current in order to cause a chemical reaction and that it would not be the actual electromagnetic energy itself.

    To do more work that would have to be an increase in wattage. Wattage is defined as being an increase in the number of electrons of the electrical current.
     
  20. Aqueous Id flat Earth skeptic Valued Senior Member

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    Not only is the Hydrogen bond broken, but the liberated dipole is then free to gyrate like a nanomotor, in sympathy with the 2.4 GHz field oscillation. Except the dipole is not really liberated, is it? -- since it has to contend with gyrating neighboring dipoles which would otherwise clamp into the tetrahedral structure that existed before the power was turned on. Now consider the extensive network of tetrahedrons affected by the very long waves that are disturbing their rest state. It's quite a large number of molecules per wavelength, regardless if whether we model this at the dimer distance or take the mean for the all orientations of a tetrahedron. But this effect, in which the power source is working against the Hydrogen bonds, is the source of the heat. It's a bulk effect, much the same as the group effect that produces heat in a wire when current passes through it. There is a difference between these two cases, of course, since here the molecules themselves go into rotation. There are some parallels in acoustic resonance of cavities, or resonance in mechanical strutures which are network (or system) effects as well. For this reason microwave heating is sometimes described as a resonance, which is sometimes incorrectly associated with a molecule's fundamental resonance, while forgeting that this is a network effect.

    I think I answered the last part of that above. As for the rest of this, it's not clear to me what you are comparing. Are you addressing the optical spectra of polar compounds in general? As a chemist you are aware that the sensitivity of a material to radiation has different causes in different bands. Just as you would not expect to see the optical spectrum of a given isomer repeated in the X-ray band, it obviously is not repeated in the microwave band. This speaks to what causes the various spectra observed, the level of interaction, and the reason we subdivide the electromagnetic spectrum into bands in the first place. That being said, it remains to be seen how we might address the absorption spectra of the compounds you mention in the microwave band. Are you wondering about a way to heat materials in general using radio waves, or are you just wondering about molecular rotation in general?

    I notice since you posted this you've started researching some more, so maybe you are converging on something tangible.

    Another thing - as Origin pointed out, photons radiate frequency information regardless of band. As Einstein pointed out, this is localized into quantized packets of energy that are proportional to frequency, and which radiate waves at that frequency. Given what I said above, that the frequency bands are just a human convention for separating the level of interactions taking place, it should be obvious that a photon has no need for our conventions. It just radiates at the frequency equivalent of the amount of energy given to it by the electron that created it.
     
  21. Aqueous Id flat Earth skeptic Valued Senior Member

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    Except here the rotation is at the molecular scale, and in such a way that conserves energy on a much larger scale - that of the relatively long wave coming from the oven.

    The most important contribution of Newton to this is the concept of superposition, which is a play on several scales, involving different kind of interactions.

    ???

    ???

    It relates to the physics Origin was referring to, namely, energy quantizes into packets in an amount equal to the Planck constant times frequency.

    When electrons jump orbital, bonds between atoms tend to split. This is in specific measurable amounts of bond energy. And of course the reverse is true when bonds are formed - the same amount of energy is given up, either by heat, electricity, or some other form.

    This is not a chemical reaction. This is the addition of energy to a chemical without altering its molecular structure. Catalysts are chemicals that temporarily enter into a reaction to accelerate it.

    Like all machines, microwave ovens are lossy. At least a third of the supplied energy is wasted.

    Watts are the units of power in all forms, electrical or otherwise. Quantities of electrons have units of charge (Coulombs). An increase in something specifies a positive sense, designated by a plus sign (+). Hence I can say "the engine changed its power by -5 MW when the throttle was reduced by 10%.
     
  22. Read-Only Valued Senior Member

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    Something's gone *badly* wrong in this thread. Microwaves are non-ionizing radiation. There are NO molecular bonds being broken.
     
  23. Aqueous Id flat Earth skeptic Valued Senior Member

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    To clarify, the references to splitting the Hydrogen bond refer to the electromagnetic disruption of the Van der Waals forces between molecules, forces which arise from the dipolar nature of water molecule geometry, and do not involve ionization. The molecules remain intact but gyrate as their dipoles are propelled like rotors in the strong electromagnetic field of the typical microwave oven.
     
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