Tuesday, September 27, 2011

The flame of life and death: My favorite (insufferable) chemical reaction

For me, the most astounding thing about science has always been the almost unimaginably far-reaching and profound influence that the most trite truths about the universe can have on our existence. We may think that we are in charge of our lives through our seemingly sure control of things like food, water, energy and material substances and we pride the ability of our species to stave off the worst ravages of the natural environment such as disease, starvation and environmental catastrophe. We have done such a good job of sequestering ourselves from the raw power of nature that it's all too easy to take our apparent triumph over the elements for granted. But the truth is that we are all without exception critically and pitifully beholden to a few numbers and a few laws of physics.

And a few simple chemical reactions. Which brings me to my favorite reaction for this month's blog carnival. It's a reaction so elementary that it will occupy barely a tenth of the space on a napkin or t-shirt and which could (and should) be productively explained to every human being on the planet. And it's a reaction so important that it both sustains life and very much has the potential to end it.

By now you might have guessed it. It's the humble combination of hydrocarbons with oxygen, known to all of us as combustion.

First the reaction itself which is bleedingly simple:

CnH2n+2 + (3n+1)/2 O2 → (n+1) H2O + n CO2 + Energy

That's all there is to it. There, in one line, is a statement about our world that packs at least as much information into itself as all of humanity's accumulated wisdom and follies. A hydrocarbon with a general formula CnH2n+2 reacts with oxygen to produce carbon dioxide, water and energy. That's it. You want a pithy, multifaceted (or two-faced, take your pick) take on the human condition, there you have it. While serving as the fundamental energy source for life and all the glory of evolution, it's also one that drives wars, makes enemies out of friends, divides and builds ties between nations and will without a doubt be responsible for the rise, fall and future of human civilization. Faust himself could have appeared in Goethe's dream and begged him to use this reaction in his great work.

First, the hydrocarbon itself. Humanity launched itself onto a momentous trajectory when it learnt how to dig carbon out of the ground and use it as fuel. Since then we have been biding our time for better or worse. The laws of quantum mechanics could not have supplied us with a more appropriate substance. Carbon in stable hydrocarbons is in its most reduced state, which means that you can get a bigger bang out of your buck by oxidizing it compared to almost any other substance. What billions of controlled experiments over the years in oil and natural gas refineries and coal plants have proven is that you really can't do better than carbon when it comes to balancing energy density against availability, cost, ease of handling and transportation and safety. In its solid form you can burn it to stay warm and to produce electricity, in its liquid form you can pump it into an incredibly efficient and compact gas tank. For better or worse we are probably going to be stuck with carbon as a fuel (although the energy source can wildly differ).

The second component of the chemical equation is oxygen. Carbon is very fortunate in not requiring a pure source of oxygen to burn; if it burned, say, only in an environment with 70% or more oxygen that would have been the end of modern civilization as we know it. Air is good enough for combusting carbon. In fact the element can burn under a wide range of oxygen concentrations, which is a blessing because it means that we can safely burn it in a very controlled manner. Varying the amount of oxygen can also lead to different products and can minimize the amount of soot and toxic byproducts. The marriage of carbon and oxygen is a wonderfully tolerant and productive one and we have gained enormously from this union

The right side of the combustion equation is where our troubles begin. First off, water. It may seem like a trivial, harmless byproduct of the reaction but it's precisely its benign nature that allows us to use combustion so widely. Just imagine if the combustion of carbon had produced some godforsaken toxic substance in addition to carbon dioxide as a byproduct. Making energy from combustion would then have turned into a woefully expensive activity, with special facilities required to sequester the poisonous waste. This would likely have radically altered the global production and distribution of energy and human development would have been decidedly hampered. We may then have been forced to pick alternative sources of energy early on in our history, and the face of politics, economics and technology would consequently have been very different.

Moving on we come to what's almost ubiquitously regarded as a villain these days- carbon dioxide. If carbon dioxide were as harmless as water we would live in a very different world. Sadly it's not and its properties again underscore the profound influence that a few elementary facts of physics and chemistry can have on our fate. The one property of CO2 that causes us so much agony is the fact that it's opaque to long-wavelength infrared radiation and absorbs it, thus warming the surroundings. This is not a post to discuss global warming but it's obvious to anyone not living in a cave that the issue has divided the world like no other. We still don't know for sure what it will do, either by itself or because of the actions taken by human beings from merely perceiving its effects. But whatever it is, it will profoundly alter the landscape of human civilization for better or worse. We can all collectively curse the day that the laws of physics and chemistry decided to produce carbon dioxide as a product of combustion.

Finally we come to the piece de resistance. None of this would have mattered if it weren't for the most important thing combustion produces- energy (in fact we wouldn't have been around to give a fig). In this context combustion is exactly like nuclear fission; twentieth-century history would have been very different if all uranium did was break up into two pieces. Energy production from combustion is what drives life and human greed. We stay alive by eating carbon-rich compounds - especially glucose - which are then burned in a spectacularly controlled manner to provide us with energy. The energy liberated cannot be used directly for our actions and thoughts. Instead it is used to construct devilishly clever chemical packages of ATP (adenosine triphosphate) which then serves as the energy currency.

Our bodies (and those of other creatures) are staggeringly efficient at squeezing oxidation-derived energy out of compounds like glucose; for instance in the aerobic oxidation of glucose, a single glucose molecule can generate 32 molecules of ATP. Put another way, the oxidation of a gram of glucose yields about 4 kilocalories of energy. This may not seem like a lot until we realize that the detonation of a gram of TNT yields only about 1 kilocalorie (the reason the latter seems so violent is because all the energy is liberated almost instantaneously). Clearly it is the all-important energy term in the combustion equation that has made life on earth possible. We are generously contributing to this term these days by virtue of quarter pounders and supersizing but our abuse does not diminish its importance.

The same term of course is responsible for our energy triumphs and problems. Fossil fuel plants are nowhere as efficient in extracting energy from carbon-rich hydrocarbons as our bodies, but what matters is whether they are cheap enough. It's primarily the cost of digging, transporting, storing and burning carbon that has dictated the calculus of energy. Whatever climate change does, of one thing we can be sure; we will continue to pay the cheapest price for our fuel. Considering the many advantages of carbon, it doesn't seem like anything is going to substitute its extraordinarily fortuitous properties anytime soon. We will simply have to find some way to work around, over or through its abundance and advantages.

If we think about it then, the implications of combustion for our little planet and its denizens are overwhelming and sometimes it's hard to take it all in. At such times we only need to take a deep breath and remember the last words spoken by Kevin Spacey's character from "American Beauty":

"Sometimes I feel like I'm seeing it all at once, and it's too much, my heart fills up like a balloon that's about to burst... And then I remember to relax, and stop trying to hold on to it, and then it flows through me like rain and I can't feel anything but gratitude for every single moment of my stupid little life..."

That's right. Let's have it flow through us like rain. And watch it burn.

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