Burning Fossil Fuels: what is happening?When we are burning fossil fuels, carbon dioxide is emitted. We know this; we are told so almost every day. But how does this happen? This page will demonstrate what happens to the fossil fuel molecules as they burn in the air to produce Carbon Dioxide. This knowledge is important since it helps us to see what is happening around us as we use fossil fuels. Part 1: The chemicals involved.Below is a picture of methane and oxygen, ready to react and supply us with power. Methane has one Carbon atom bound to four Hydrogen atoms and Oxygen gas is two oxygen atoms bound together. See The Chemistry for more details if you're not sure what these pictures of molecules represent.
You may be wondering why methane and oxygen only react sometimes - like when the gas stove is already alight. That is because the molecules require an ACTIVATION ENERGY. What that means is they need to be moving fast enough so that when they smash into each other they actually recombine. I like to think of these molecules as dodgem cars. At a low speed, like in the fair ground ride, the cars are moving slowly. They bump into each other, but there is no damage done. They end up looking the same at the end of the collision as before. That is what is happening to these molecules at low temperature. They are moving, colliding, but there is no change in the molecules; they just bounce off. The rubber bumpers of the dodgem cars and the molecules is doing the job of keeping them in shape. By the way, methane and oxygen are both gases at room temperature. As gases, the molecules are constantly on the move. For the rest of this discussion, the LENGTH of the arrow indicates the SPEED of the molecule; the longer the arrow, the FASTER it is moving. Gases are also elastic; they can bounce off the walls of their container without loss of speed. So let's take another look at these methane and oxygen molecules again at room temperature, this time showing their movement with arrows:
What we have now is an extremely simplified version of methane burning; there is one methane molecule and two oxygen molecules in a sealed box and they are moving. They will bounce off the walls, bounce off each other, and continue like this forever and a day. They are not moving fast enough to react; this is true of burning fossil fuels of all kinds. Why there are two oxygen molecules and only one methane molecule will become clear later. At this stage, the second image is EXACTLY THE SAME as the first image. Our experience tells us that holding a lit match to the gas supply causes it to burst into flames; the stove is lit. Let's take that back to the analogy of the dodgem cars. We all know that at low speeds the cars are undamaged by collisions. Well then, let's speed them up. Now I want to fire two dodgem cars straight at each other at a speed of 1000 kilometres per hour each. Let's say that after much expense and elaborate setting up of equipment, I manage to do this. What will happen to the dodgem cars then? Will they just bounce off each other? No. They will disintegrate; some parts will go flying off in all directions and some will become smashed together. The original dodgem cars, however, will certainly cease to exist. By adding HEAT to the box, we speed up the molecules. The more heat we add, the faster they move. Let's assume I've added a lot of heat. The molecules would now look like this:
So now the molecules have enough speed to smash into each other and change structure. This speed, which is the same thing as heat and energy, is called the activation energy required for the particles to crash with enough force. When I light my gas stove, the match provides that extra energy needed to start the reaction. Without it the gases will not react, I will not be burning fossil fuels. Part 2: The Collision.When the methane and oxygen collide they break apart completely. Remember that each bond is TWO electrons; when the bonds are smashed each atom involved in the bond gets ONE of those electrons:
Those red dots are the key. Each is a SINGLE electron. This will not do. Electrons need to be in PAIRS to be stable, so these newly liberated atoms will recombine to pair up their electrons. Part 3: The End Products.There are many possibilities here. If anything went, below are the possible structures we could get. Here I am just reconnecting the atoms to pair up the red dots, so that each atom has 8 electrons in its outer shell.
And there are many, many more you could make, especially if you have an endless supply of each atom type. BUT only some of these are MORE STABLE than the original molecules, and since the chemicals involved are LOSING HEAT by giving it to the surrounds (to heat the milk pot for my breakfast, say), only those whose bonds contain LESS ENERGY than the methane and oxygen will form. And there are only two out of the whole bunch, and this is true of burning fossil fuels of all kinds: 
Which is why we get carbon dioxide and water when burning fossil fuels such as methane in air. Knowing that we will get these two products, it is easy to recombine the atoms from earlier to get this:
Thus we get two water molecules and one carbon dioxide molecule from one methane and two oxygens. The same is true for all hydrocarbons like oil and gas. The only difference between these and the methane example is that the molecules are bigger, with one extra carbon atoms in the chain for successive molecules. Obviously, for these molecules more oxygen will be required, and there will be more carbon dioxide and water produced from each. Burning fossil fuels implies these products. Burning Fossil Fuels: Carbon Dioxide cannot be avoided
Return from Burning Fossil Fuels to Fossil Future OR return to the Green Planet home page for more Solar Power Facts.
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