I never used to like chemistry. It always seemed like this vast forest that a student could wander into one sunny morning and emerge from four years later speaking a language that nobody else could understand. (Actually, I’m pretty sure that describes every university degree, but that’s beside the point.) Lately though, I’ve started to see its elegance. From its rules, patterns, and chemical interactions emerge many of the processes that govern not only humans but the Earth as we know it.
I want to talk a little bit about how things burn. How do fossil fuels burn? Why is carbon dioxide produced? Where do biofuels come in? I do want to stress rather adamantly that I am not a chemist. Explaining chemistry at an accessible level is one thing, but applying it at a technical level is quite another. Hopefully, at the very least, I can convince you to some degree that chemistry isn’t so scary after all.
Reduction-oxidation (redox) is a broad family of reactions that includes combusion (burning), corrosion (rusting), and cellular respiration (cells in your body break down glucose and releasing CO2). In all redox reactions, the oxidation state of a given atom is changed through the addition or removal of electrons. In very general terms, the oxidation state reflects how much energy an atom is able to release (although a chemist might shoot me for that definition). The lower this number (it can be negative), the more energy is available.
Let’s look at carbon. In methane (CH4), carbon is bonded to four hydrogen atoms and has an oxidation state of -4. -4 is the lowest possible oxidation state for carbon, and thus methane stores a lot of energy. On the other extreme, in carbon dioxide (CO2), the carbon is bonded to two oxygen atoms and has an oxidation state of +4. This is the highest possible oxidation state for carbon. CO2 is one of carbon’s most stable forms, since there is no energy left to give away. Not only does carbon dioxide not burn, but we use CO2 in fire extinguishers to stop other things from burning too.
The reaction from CH4 to CO2 is called oxidation. Electrons are released, energy is given off, and the oxidation state increases. The reverse reaction is called reduction and requires energy input to proceed. Let’s take a quick trip through the combustion of a single methane atom from start to finish. As anyone who has ever tried to start a campfire will know, there are two key ingredients – fuel and oxygen. Without wood, paper, tinder, coal, propane or gasoline, there’s nothing to burn. But why does a flame need oxygen? During oxidation, carbon releases electrons to the ever-hungry oxygen molecules that surround it. If enough oxygen is present, any carbon fuel will eventually be fully oxidized to form CO2. Limit the oxygen by putting a glass over a candle or spraying a flame with a fire extinguisher, and the reaction stops.
Once you understand oxidation and reduction, the rest of the details click nicely into place. Carbon monoxide, for one. Carbon monoxide (CO) forms when there isn’t enough oxygen present for carbon to burn completely. Leave a car running in a closed garage, and the engine will eventually be starved for oxygen, such that some carbon atoms will only oxidize partially. On that note, where does gasoline fit into our spectrum? Liquid hydrocarbons are generally harder to place than CH4, since they are made up of a mixture of different molecules. On average, carbon in oil has an oxidation state of -2. Carbon in coal has even less energy, with an oxidation state of 0. When you hear about coal being the most carbon intensive fossil fuel, this means that to get the same amount of energy as, for example, burning methane, you need more carbon molecules, releasing more CO2 in the process.
Let’s wrap up with a bit about reduction. If there’s one thing we’ve learned, it’s that oxidizing carbon releases energy, while reducing carbon requires energy. When you hear about cool ways to turn CO2 back into a fuel, you’re looking at different forms of reduction. The reaction that immediately springs to mind is photosynthesis. During photosynthesis, plants (and algae) use energy from sunlight to make sugar (reduced carbon) out of CO2. And what do you think they produce as a byproduct? Oxygen! While your campfire needs oxygen to burn fuel, plants do exactly the opposite. Biofuels from algae or ethanol take the products from natural reduction processes and transform them into fuel.
Algae grown for biofuel production in a lab experiment at UC San Diego
And so concludes your chemistry lesson for the day. It’s been nice knowing you all – in all likelihood, by the time I publish this article, the Columbia Chemistry Department will have a warrant out for my arrest.
Until next time, be warned: you may have just sustained a lethal dose of mostly harmless science.
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Cover photo courtesy of Gregory Jordan, Flickr Creative Commons. Other images from Wikimedia Commons.