Drawing particulate models of reaction mixtures | Chemical reactions | AP Chemistry | Khan Academy

In a previous video, we used a particulate model like this to understand a reaction—not just understand the reaction, but to balance the chemical reaction as well. When I hand drew these particles, the atoms in this particulate model here, I tried to draw it pretty close to their actual relative sizes. Carbon atoms are a little bit bigger than oxygen atoms, and they're both a lot bigger than hydrogen atoms.

What we're going to do in this video is extend our understanding using a particulate model to start to visualize what actually might go on in a mixture of some of these reactant molecules. So, what I have here on the left-hand side are the various molecules. I have two methane molecules here, and I have three water molecules. What I want to do with you is draw what we would expect to see after the reaction. I encourage you, like always, to pause this video and see if you can have a go at that—maybe with a pencil and paper—at least just try to imagine it in your head before I do this with you.

All right, now let's do this together. Now we know that for every methane and every water, we're going to produce one carbon monoxide and three molecular hydrogens. Each of those molecules of hydrogen has two hydrogens in them. So let's just say that this one and this one react; they're going to produce one carbon monoxide. I'm going to try to draw the relative sizes roughly right, so one carbon monoxide, and then they're going to produce six hydrogen atoms that are going to be in three hydrogen molecules. So let’s do that: that’s two and four, and then I’ll just do one here, and then six.

All right, so I took care of this one and this one, and now we can imagine that maybe this water molecule reacts with this methane molecule, and so that would produce another carbon monoxide. Let me draw that roughly at the right size—another carbon monoxide molecule and three more hydrogen molecules or, for a total of six more hydrogens. So that's one and two, and three.

And now we have this water right over here that had no one to react with in this situation—had no partner—and so that's just going to be a leftover reactant molecule. So let me just draw it right over here. So that water could be right over here, and so this was a useful way of starting to visualize what might be going on. Remember, this is happening at a very high temperature; they’re all bouncing around, etc. And then, when they react, you might get this. But then this water molecule has no one to react to, so it is—you could view it as a leftover after the reaction.