Welcome to a mad scientist production of …. Fire!
We’re going to use the combustion of methane, CH₄, to:
Fire is a chemical change and in chemical changes nature favors a decrease in potential energy.
This is why fire happens.
Our reactants, CH₄ or methane and oxygen have a high potential energy while the products,
carbon dioxide and water, have a low potential energy.
But there’s a bit of an obstacle here.
We have that arrow telling us that the stuff on the left magically turns into the stuff on the right.
But how does that happen?
If we take a look at what that arrow means, what happens to make the methane and oxygen become carbon dioxide and water,
then we can begin to understand what fire is.
So let’s take a look at that.
We’re going to start with one CH₄ and one O₂ molecule, and those are both gases which move very quickly.
This is very slow motion. They’re going to collide and atoms rearrange.
Water forms here, so what happens when water forms?
Remember that:
How is that possible you ask?
Two things occur that make that possible:
1) When water forms it gains a lot of kinetic energy which we see here as high speed and rotation.
AND 2) when water forms, water’s electrons go to a lower energy state which (back)emits visible and infrared light!
So the loss in potential energy equals the amount of kinetic energy and light energy generated.
In our balanced equation we had another oxygen molecule, so let’s add another one now and see what happens as we watch the reaction proceed.
The collisions produce carbon dioxide. What’s going to happen next? Two things.
When the carbon dioxide forms it gains a lot of kinetic energy and it emits light.
And then water again forms at high kinetic energy and emits light.
So where light is emitted is where the reaction occurs.
The flame, the emitted light, is where oxygen and methane are rearranging into carbon dioxide and water.
The heat we feel is the high kinetic energy CO₂ and H₂O coming out of the flame.
Our nervous system gets upset when those high speed particles hit our skin, transferring their kinetic energy to the molecules in our skin.
Before moving on let’s get a quick summary of what’s going on here.
We have high potential energy CH₄ and O₂, producing low potential energy CO₂ and H₂O.
That difference in potential energy is transformed into kinetic energy and light energy.
The total energy does not change;
our reaction obeys the law of conservation of energy.
Let’s expand the idea of heat a little bit more.
Heat is carried by the high kinetic energy water and carbon dioxide.
The heat transfers to the air and eventually dissipates.
Why does that happen?
Everything we’ve been talking about here can be applied in general to just about any fire you encounter.
CH₄ is what’s called a hydrocarbon and hydrocarbons provide the fuel for almost any fire you might see.
Hydrocarbons are any compounds made out of carbon and hydrogen.
There are many hydrocarbons that we encounter such as natural gas (and methane is the largest component of natural gas so that’s what you see burning on your stove),
as well as waxes, and petroleum products such as gasoline, and when you burn them, they produce carbon dioxide, water, and light.
Hydrocarbons can also be somewhat oxidized, including carbohydrates, and lignin, which incidentally are the two major components of wood,
and there are also fats and alcohols, and when you burn them you produce carbon dioxide, water, and light.
And THAT is the story of fire.