rickman said:
Why would that create rectification? Can't electrons flow in both
directions?
Most likely, the difference is in electron affinity of various radicals in
the flame, and the work function (and temperature) of the electrodes.
Chemically, a flame is made of three things: gasses, radicals and ions.
In air, the gasses are mostly N2, plus whatever combination of O2, fuel,
CHx, CO, CO2 and H2O is produced during combustion.
Radicals are fuel molecules in varying states of destruction and
oxidation. If you start with methane, CH4, and ignite it in air,
collisions start knocking off hydrogen atoms (which quickly form OH
radicals and H2O), resulting in CH3, CH2, CH and C2 radicals. All these
release characteristic photons when excited (not necessarily ionized),
which can be identified:
http://en.wikipedia.org/wiki/File:Spectrum_of_blue_flame_-_intensity_corrected.png
When you see the cool blue glow of a gas flame, you are watching transient
molecular species crying out in pain.
There aren't many ions in a flame, since even a very hot flame is much
cooler than a cool plasma. (Plasma itself is not usually very ionized --
most plasmas are predominantly unionized* gas. It just happens that the
ions are the most exciting part, so once it's past some level, we call the
whole thing plasma.)
*Chemists pronounce "unionized" with four syllables. Given the political
bent here, I thought I would clarify.
You can also introduce a wide variety of things into a flame. Sodium is
an absolutely pervasive contaminant, and is responsible for the orangish
color of a flame in contact with anything hot. The sodium glow is so
intense that a hot surface needn't have any apparent sodium residue on
it -- your gas stove's metal grate will pick up enough dust (which is
mostly composed of dead skin, which includes dried sweat, therefore
containing a trace of sodium) to do this. Sodium in a flame is monatomic
(something like NaCl is broken without too much energy, forming Na. + Cl.
radicals, both of which are not very high energy, as radicals go).
Sodium, of course, is well renouned for its ability to give up an electron
with little energy expense, forming Na+, so a sodium-doped flame would be
reasonably conductive.
I don't know any data offhand regarding the electron affinity (or
ionization energy) of these molecules. It's likely that both kinds of
ions will form. For example, the OH. radical may gain a companion for its
unpaired electron, forming the hydroxyl ion, OH-, while the CH3. radical
might lose an electron, forming the methyl carbocation**, CH3+.
**Carbo-cat-ion, not 'carbokayshun'. Yes, chemists are like that.
Tim
