Combustion Engines: Anatomy, Benefits & Energy Source

[fysik]

let's take the combustion of an hydrocarbon
CH4 + O2 = CO2 + H20

the products of this reaction is:
1) large heat
2) large amount of gases
3) high kinetic velocity of gases
4) heat of gases

what is the exact thing we take advantage in internal combustion engines?
the large amount of gases produced? (given the equation, it doesn't seem there are much more gases produced)
the heat? (I think it is a by product that is waste)
the high kinetic energy of the gases? (so it's the kinetic energy of the gases and not their amount?)
the high heat of the gases (so basically the high heat produced makes the gases have high kinetic energy?)

also, where exactly the energy comes from in atomic level?

thanks!

 

[Dr. Courtney]

Combustion engines take advantage of the expansion of the products, which can be contributed to by both the heat and by the increased number of moles of gas after the reaction. If you balanced the equation above, you would see more moles in the products. Combusting larger hydrocarbons produces even more moles of products per mole of reactants.

The heat comes from the chemical bonds. Breaking chemical bonds tends to release energy. The best chemical reactions for combustion engines are the ones that release a lot of energy from the chemical bonds so that the products are in a lower energy state than the reactants. Making longer hydrocarbons from water and carbon dioxide requires inputs of energy to make the new chemical bonds. Combustion, that is burning hydrocarbons and getting the water and carbon dioxide releases this energy.