Expert: Daniel Mazur - 2/16/2008

Question
Hi:

What exactly are "flames"?

From what I've read about, it's where atoms are rearranging, with a net release of energy, typically causing increased kinetic energy (heat) and sometimes radiation (the blue glow of a stoichiometric flame such as methane). But where does the increase in kinetic energy start?

Where does the heat start?

The atoms move and electron cloud wavefunctions overlap.  When the wavefunctions subtract, no energy is gained, but when they add, energy can be released (compared to the atoms being at some distance from each other). If this energy is greater than the energy contained in the input (for instance, methane and oxygen molecules are both quite stable on their own), they will prefer to be together compared to the previous arrangement, and release that gain of energy by releasing a photon or increasing a vibrational or rotational state.

How does being "together compared to the previous arrangement" cause production of heat.

AFAIK, the heat of a flame is neither radiant nor conductive. It is purely convective.

If a "flame" is a gas that is hot enough to glow (incandesce), then why can't a flame be generated by heating an unreactive gas [such as nitrogen] to the the point where it glow? If nitrogen gas is sufficiently heated to the point where it reaches it's "glowing point", then why wouldn't this be a flame?

If a "flame" is a gas that is hot enough to glow (incandesce), them why not heat any random gas or air molecules to their incandescent point and refer to them as "flames"?


Thanks,

Green

Answer
Hi Mr. Green,

Flames only accompany highly exothermic chemical reactions such as burning, which is in essence a violent oxidation of gases. The "glow" is caused by photons released in the reaction region. Not all photons come from the oxidation reactions themselves, but as long as there are energy quanta flying around with energy of some electronvolts, then you get cascades of excitations, some of them radiative... and that's how it happens. The sentence "it's where atoms are rearranging, with a net release of energy" could refer to any gas relaxing to its lowest-energy state. But in practice gases already *are* in their ground state ever since their production (or as far as they can, given the temperature of their surroundings). The only "rearranging, with a net release of energy" that can produce flames is when chemical reaction takes place, chemical bonds are broken and others are made.

If you want to think about it in terms of electronic wavefunctions, you need to see that the total valence wavefunction of a compound AB is NOT the same as superposition (adding) of A's and B's individual wavefunctions. Hence, the energy state of the compound can be higher or lower than the sum of energy states of the components. If DeltaE=E(AB)-E(A)-E(B)<0, the reaction is of the kind that releases energy. When DeltaE corresponds to an energy of a visible photon, we can see the flames. Here you have the answer to another bit of your question: E(AB) is "together" and E(A)+E(B) is the "previous arrangement".

Kinetic energy of atoms and molecules is simply there at any temperature above absolute zero, it does not "start" anywhere. It is simply energy of mechanical motion of the said molecule with respect to some inertial frame. Molecules bounce off each other, they bounce off surfaces of walls of containers. All this keeps them moving. The kinetic energy of a single molecule changes in time, it can be even stopped for a moment. However an ensemble of molecules (container full of gas with 10^23 molecules) maintains its total energy (so-called internal energy U, it also contains the rotational and vibrational kinetic energies) and average kinetic energy per molecule E_Kavg. This average kinetic energy corresponds to the temperature of the system.

During burning the chemical reaction releases energy (DeltaE per reaction) and this energy can either become a photon that we can see (the flame) or can add (by absorption of a photon or by bouncing off another, more energetic atom) to kinetic energy of molecules in the reaction region - and that we feel as warmth, hotness, burning, scorching.... This is as much as I can tell you about "where heat starts". Heat (thermal energy transfer) of a flame is all: radiative, diffusive and convective. It's just that convection carries away the most under normal circumstances.

I hope this helps. Write again if you have more questions.
Cheers,
Daniel