Understanding Hydrocarbon Combustion

Question

First ... I know almost nothing about chemistry.

So, for the past week or so, I've kind of been fascinated with this whole combustion thing (not in an evil way, of course). So, I've been reading, and youtubing etc. etc. What I have so far is below, some of which is also seen in this thread:

Mechanism of combustion of hydrocarbons

So I want to make sure the picture in my mind is right, so that I could reasonably explain it to ... say ... a 7th-grader. Let's take the (simple?) example of one molecule of methane CH4 and two oxygen O2 molecules.

So we'll put some energy "into" the methane and oxygen molecules, never mind where it came from. Now ... when we do that, the atoms vibrate (think of the bonds between the atoms as little springs). More energy ... stronger vibrations. If we put enough energy into them, the "springs" will "break" and the molecules come apart. Now we have 4 Hydrogens, 1 Carbon, and 4 Oxygens floating around. Think of this breaking-up happening to a small percentage of molecules among trillions and zillions of CH4 and O2 molecules.

Eventually the free atoms find each other and "fall" into deeper, stronger bonds of two H2O and a CO2. That's important: deeper, stronger bonds. Still the same number of atoms! Just rearranged. There are actually charts that show the energy levels, and for quantum reasons beyond my understanding and that aren't important at this level, there is a "well" of distances between atoms where the bonds are most stable. So remember when they fell into the new bonds? Well before they fell they had potential energy, that, after falling into each other, is now kinetic energy of those molecules vibrating. Eventually the new bonded molecules run into other molecules and transfer their energy to them, and the molecules that we just created settle down into the even deeper and stronger (more stable) energy states. The scales have to balance. Energy is never created or destroyed ... only changed.

Long story short: The energy given off by the atoms falling into each other to form the new H20 and CO2 molecules is greater than that needed to break apart CH4 and O2. That's why the reaction is exothermic, i.e. giving off heat. Eventually, this happens to enough molecules that they start bouncing around and vibrating faster and faster, that enough energy makes it to another set of CH4 and O2 molecules, and it happens again, and again, eventually consuming all the fuel.

Whew ... so did I get it right?

Answer 1

The whole story is pretty good, but there's one major thing I'd correct.

Molecules can undergo decomposition at higher temperatures (that was the springs stuff you were talking about) but that's not really what happens here. Think of it rather that heat is the energy of particles moving about, the hotter something is the faster the particles are moving. When CH₄ and O₂ are in a mixture they bump into each other a lot, however they're too stuck together for anything to happen unless they're moving a lot faster (like if you slowly place a lego creation on the floor it wont come apart, but if you hurl it against the wall it will). Now if there's enough heat (e.g. you put a match in a methane jet) and two O₂ molecules and a CH₄ molecule all collide then the atoms will all come apart, and if they're in the right orientation then instead of just reforming into their initial bonds they'll form into one CO₂ molecule and two H₂O molecule. Combustion has occurred.