Chemical Reactions Producing Light and Heat

[FlyingPanda]

Hi there,

I was just wondering about this point, why do some chemical reactions produce bright light (and even sound) whilst others do not?

For example: in the rapid oxidation of Magnesium in air after given the necessary activation energy, a bright white light is emitted. However, in many other chemical reactions, no light at all is produced.

What causes the light?

Also one other question: Why does heating up a coil of a filament lamp produce such a bright light? Why do some materials produce output different ratios of heat energy and light energy when current is passed through it?

Thanks.

 

[JaredJames]

This is caused by a collision on the atomic scale. In the case of a light bulb, the extremely high resistance of the filament wire increase the chance of a collision between the charged particles moving through the wire and the particles in the filament.

When the collision occurs, there is a change in energy state (can't remember if it happens once or twice) and it is the excess energy being shed that is a photon being emitted.

Can't really give much more than that as I'm not entirely sure myself and don't want to mislead if I'm incorrect.

 

[Drakkith]

Any chemical reaction will result in a release of heat, which will cause whatever material to emit electromagnetic radiation (Which is what light is) at various frequencies depending on the temperature. Look up Black Body Radiation.

In general, chemical reactions that release more energy or react quicker will produce more light than those that are less energetic or slower reacting.

As for a light bulb, the filament has a high resistance to an electrical current. This causes it to heat up as a current is passed through it. Just like above, when something gets hot it emits electromagnetic radiation, AKA light. I believe that the light output solely depends on temperature, not the material. However, certain lights work a bit differently than a standard incadescant light bulb.

 

[FlyingPanda]

Ok, so in any case that light is emitted, it is also a case in which heat is being emitted?

Is it possible to know the temperature of a compound based on the wavelength/intensity of the light?

 

[K^2]

Most chemical reactions produce light because of heat. But not all. If you look at glow sticks, for example, they glow without being nearly hot enough to do so. Similar thing happens with fireflies. They wouldn't be able to survive the heat necessary to produce this light. LEDs are in the same category. There is no actual chemical reaction, but what happens to electrons in an LED is similar to what causes light emission in reactions with cold glow.

Anyways, the physics. Electrons in an atom occupy different energy levels. The actual energies depend on the nucleus of the atom, the number of electrons, and to a slightly lesser degree, on neighboring atoms. Electrons try to occupy the lowest energy states available, but there can only be two per state due to Pauli Exclusion. What's interesting is that there are ways to get electrons to higher energy states. One is to heat the material. Another is to hit the atom with something, like a free electron. Finally, there are some chemical reactions resulting in excited electrons.

When an electron at high energy level drops down to a lower energy level, it emits a photon that carries away the difference. That's the light you see from pretty much any light source.

The real trick is to have the energy needed to produce the visible light without allowing any significant amount of that energy to go into motion of the nuclei. Most reactions result in the two reacting molecules or nuclei to snap at each other like a pair of magnets that you let go of. That results in a lot of vibration and production of heat. Getting them to "snap" together gently, yet have enough energy to fling an electron to a high energy level is not a trivial task.