Fire! 🔥

Fire - we all know it when we see it. Look! there's a fire over there... Let's have a campfire and roast marshmallows. Well, to a chemist, fire is combustion which is a very specific reaction.

🔥

A combustion is reaction with oxygen - specifically oxygen gas or O2. When we ask the question "does it burn?" we are asking does it react with oxygen and then self sustain the reaction.


Most of the stuff we burn, we burn it as a fuel with the purpose of extracting the energy from the fuel and transferring it to something else. There are lots of things we can extract energy to. Sometimes we just want the heat from the fire directly - as in "I'm cold, let's have a fire to warm up.". Or we take the heat from the fuel and use it to cook with - as Alton Brown has said and written "Food + Heat = Cooking". We also use that energy to provide energy in the form of torque from a combustion engine to power a motor to propel a vehicle. And of course we take all that heat from combustion and convert it (at least some of it) into a distributable energy - electricity. So there is a multitude of ways in which we tap into the power of combustion.

Let's have a look at some rather prevalent combustion reactions which are listed below according to the fuel that is used. In general, the fuel is a hydrocarbon and there are literally 100s of possibilities for fuels. Some fuels also have some oxygen already in there as well, e.g. wood, alcohol.

natural gas is mostly methane

CH4 + 2O2 → CO2 + 2H2O

gasoline is a mixture of many hydrocarbons one is octane

C8H18 + 12½O2 → 8CO2 + 9H2O

kerosine is also a mixture of many hydrocarbons (10-16 carbons) - one is dodecane

C12H26 + 18½O2 → 12CO2 + 13H2O

wood is mostly cellulose which is a polymer of glucose - just glucose

C6H12O6 + 6O2 → 6CO2 + 6H2O

Then there are some fuels and fuel additives that can also have some nitrogen and sulfur as a component whether by chance or on purpose. If any nitrogen or sulfur is in the fuel, we will get NO2 and SO2 gases as products - which, as we know is not good because those are big time air polluters. The thing is that no matter what the fuel source, we can write out a fairly accurate chemical reaction for the combustion of that fuel. We can account for every bit of carbon that gets burned, and we know just how much CO2 is produced. This is just simple stoichiometry.

Energy is Stoichiometric Also

Not only can we do the accounting for CO2, but we also can do the accounting for all the energy that is released during combustion as well. Accounting for and quantifying the energy of chemical reactions is the science and practice of thermochemistry. Thermochemistry just adds one more layer on top of our stoichiometry prowess. If we can count moles of reaction, we can count joules of energy. That is the essence of thermochemistry... and here is a quick example: Propane has a heat of combustion of 2220 kJ/mol. Therefore the fully balanced reaction that also shows energy output is

C3H8 + 5O2 3CO2 + 4H2O + 2220 kJ

So this can be scaled up or down depending on how much actual propane you burn. That equation is for one mole of propane which is 44.1 grams of propane. So how much heat can we get if we burn 250 grams of propane? Just use the unit factor of 2220 kJ/44.1 g and you'll have your answer:

\(\require{cancel} \newcommand\ccancel[2][black]{\color{#1}{\bcancel{\color{black}{#2}}}} \left({250\,\ccancel[red]{\rm g}\over 1}\right) \left({2220\,{\rm kJ}\over 44.1\,\ccancel[red]{\rm g}}\right) = 12585\,{\rm kJ}\)

So there's your answer: 12585 kJ of heat is released when you burn 250 grams of propane. Heck, if you wanted to you could even change that somewhat clunky 2220 kJ/ 44.1 g into a much nicer (do the math) 50.34 kJ/g (kJ per gram of propane)... now you only have the one number to use for ANY gram amount. If you only burn 3 grams of propane, you'll release 151.0 kJ of heat. Burn 5 grams and get 251.7 kJ... and so on! Stoichiometry works for both amounts (moles and grams) AND energy (joules and kilojoules).

HANG ON! Hold up...

This is all great and we could just keep doing more stoichiometry problems and the like except for one rather big issue. We really haven't addressed the whole conceptual aspect of this energy transferal/dissipation and how the study of that phenomena is a really BIG part of chemistry and physics. That big issue is thermodynamics and we need a little taste of it in order for us to move deeper into energy as a topic. So let's move on to learning some thermo speak if you will. Next section! →



back to top next

© 2019-2021 · mccord