What is Fire? Combustion Reaction Tutorial | kinetic & potential energy, heat & light | Chemistry
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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.

27 thoughts on “What is Fire? Combustion Reaction Tutorial | kinetic & potential energy, heat & light | Chemistry

  1. Really nice explanation! The molecular explanation is very helpful. Could you comment on why the fuel needs to reach its ignition temperature for the oxidation reaction to occur? What does that additional heat do at the molecular level, without which the oxygen won't acquire the hydrogen from the methane? Thanks again!

  2. Thank you. I've been looking for a real deep explanation of what is fire.

    So if I understood right methane is a combustible on earth because the atmosphere down here is full of oxygen, but if we were on another planet with a different atmosphere, there gaz would more adequate for combustion?

  3. you forgot to draw the little hill which is to start a alkane combustion reaction in the form of a spark or a little flame. This is to create the radicals of which the electrons can oxidize to form water and carbon dioxide.

  4. May I ask why does potassium catch fire when it's thrown into water?
    I know that potassium is a fairly reactive metal and it does explode in water
    I would like to know how does the chemical energy (I think) convert to thermal energy so easily

  5. im still having trouble understanding the potential energy, how does bond energy actually convert to other energy?

  6. is there a scientific method to finding out which chemical reactions would release more energy. I know hydrocarbons are used in cars and our bodies use carbohydrates, but I would like to find a chemical reaction that has a high energy output without experimenting first.
    The reason I'm asking is because I have this idea of making an autonomous robot that recharges it's batteries using chemical reactions. Kind of like the robots in the cartoon known as Futurama. I know you'll find more energy in gasoline then in any battery of equal mass but I have a problem with the idea of having loud and smelly engines indoors. I just don't think that product would sell very well on the market.

  7. so the kinetic energy is the heat? is that because the molecules move rapidly through air and cause friction which then generates heat? or am i misunderstanding something

  8. hi,can you explain what light chemically is like we know our brain chemically makes images by the help of it, Electromagnetic radiation but my question is by itself what is LIGHT ? thank you.

  9. I've been looking for and wondering about how fire and combustion works. This was a simple and effective explanation – great job!

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