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Topic: What precisely can be heated with microwave?  (Read 1923 times)

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Offline Zensation

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What precisely can be heated with microwave?
« on: November 03, 2014, 01:55:21 PM »
Within the context of microwave chemistry. I can't find much of a clear answer on this.

So I understand that molecules with a dipole movement can be heated, polar molecules. Many inorganic reactions seem to benefit tremendously from microwave heating, why? Some question I cannot find straight answers for:

1) Can all metals absorb microwaves? Is it just the metal portion of the ion that can absorb microwaves, or both the anion and the cation? I thought it was curious that water insoluble coordination complexes containing a metal atom can absorb microwaves and accelerate reactions.
2) Would non-metal ionic compounds like PTC's absorb MWs, in spite of being water insoluble?
3) IIRC, Halogens can absorb microwaves, correct? Does this only go for free halogen molecules, or do halogens in  non-polar molecules like alkyl halides also absorb microwaves due to being partially polar?
4) Can any molecules that even slightly dissolve in water absorb MW's? IE any molecules with a slight dipole movement?

And yes I have searched and no this is not a homework question, I am simply curious for my own hobby-research.

Offline Enthalpy

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Re: What precisely can be heated with microwave?
« Reply #1 on: November 05, 2014, 07:38:52 PM »
Microwave absorption is not very touchy about individual atoms, ions nor even molecules.

You probably use microwaves at few GHz, for instance 2.45GHz, and heat liquids, don't you? Then absorption results from imperfect conduction and from sluggish polarization. Ions permit conduction losses, polarized molecules permit dielectric losses. So while an alkane would absorb little, you can expect useable absorption from any aqueous solution.

In addition, ovens (I suppose the chemistry microwave heaters differ little) are designed electrically to deliver heat to watery objects over a wide range of conductivity. Too little absorption doesn't happen with water, even pure, and too much absorption results only in heat being superficial.

In a liquid and for few GHz, resonances play no role. The time between shocks is much shorter than the microwave's half-period, so resonances observed in gases at very low pressure are flattened out by the shocks.

So in a kitchen oven, the microwaves are not selective and give just heat to the complete item, nothing subtle acting on specifiic atoms no ions. Would that be very different for chemical reactions?

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