This screencast uses the PhET simulation for atomic interactions, which can be found at this link. This is a graph of the two atoms found beneath it. The x-axis shows the distance between the two atomic nuclei, and the y-axis shows the chemical potential energy of the system.
For the sake of this simulation, the neon atom on the left is stuck where it is and cannot move, but the neon atom on the right can be moved to be closer to or farther away from the one on the left. We can move the atoms so that they are farther apart from each other. Let's see what happens when we move the atoms apart and then allow them to move freely. As you can see, the atoms will move toward each other slowly, speed up, and then seem to bounce off each other.
Let's take a closer look at what's going on. The simulation allows us to view the forces at play when these atoms interact. The green arrows represent the total forces present in this situation.
When the atoms are far apart from each other, do these arrows represent an attractive or repulsive force? Take a second to answer this question on your worksheet. Now we'll take a look at what happens when the two atoms are very close to each other and are overlapping.
Looking at the green arrows, is there now an attractive or repulsive force? Take a second to answer the question on your worksheet. Now let's find a distance where the total force between the atoms is zero.
As you can see, this corresponds to the bottom of the well on the graph above. The implication here is that anywhere else on the graph is higher on the y-axis. Does this mean that you have to add or remove energy in order to move the atoms from this position? Now that we've looked at the total force, let's see what individual forces are at play. The orange arrows represent attractive forces and the pink arrows represent repulsive forces.
When the atoms are apart, the attractive forces between them are larger than the repulsive forces, resulting in an overall attractive force. As you can see, the orange arrow is larger than the pink arrows, which indicates that the attractive force is stronger. When they are very close to each other, the opposite is true.
The repulsive forces are larger, resulting in a net repulsive force. If we move the atoms back to the bottom of the well, the forces are equal and opposite, resulting in no net force overall. Take a few minutes to pause the video and answer the questions about these forces on your worksheet.
Now that we've taken a look at two neon atoms, we'll take a look at two oxygen atoms. Since the oxygen graph actually exceeds the scale of the graph we had before, we'll zoom out on it so the whole picture can be seen. What differences do you notice between the graphs of the two different atoms? As you can see, something markedly different occurs between the two oxygen atoms.
Let's take a look at what's going on. If we pull the two atoms away from each other and release them, You can see that they attract together, but instead of bouncing off each other, they stay close together. Take a moment to pause the video and make note of some other differences that can be seen on the graph of two oxygen atoms when compared to two neon atoms.
Why might these differences make sense? Now that we've looked at the system with two atoms, let's take a look at an entire chemical reaction. This chemical reaction is the combustion of methane, which is when one molecule of methane reacts with two molecules of oxygen to give us a molecule of carbon dioxide and two molecules of water.
In order for this reaction to give us the products that it does, we first need to break the chemical bonds in the reactants, and then we'll get the products as those bonds form anew. We can see that we get our products and they're different for our reactants because the bonds form in a different way after they've been broken initially. So let's take a look at the energy that's going on in this reaction.
So I've added an axis over on the left. that shows that things closer to the top of the screen have a higher total energy than things toward the bottom of the screen. So if we take a look at what happens when we break the bonds in our methane and our oxygen, you can see that they move upwards on the screen. Which means that the total energy state of the system is higher, meaning that in order to get from the lower state to that higher state, we must have added some energy.
As the bonds form again, we move back to the bottom of the screen. Which means that we had to remove some energy to get from the higher state to the lower state, which means that some energy is released. If we look at the entire reaction overall, we can see why the combustion of methane, which is just the burning of natural gas, is an exothermic reaction and releases energy. We can see that starting with our reactions over here, they're in a higher energy state than our products, which means that overall... Energy will be released because we're getting, going from a higher energy state to a lower energy state as the reaction proceeds.
Having completed this webcast, take some time now to finish the argon-argon interactions and the remainder of the questions. The simulation can be found at this link.