All chemical substances are made up of charged particles: protons and electrons, which are arranged into atoms, which can be further arranged into molecules, ions, lattices, etc. Because these particles have charges, they experience attractive and repulsive forces, and because of these forces they have a certain amount of potential energy.

When a chemical reaction takes place, the potential energy of the particles involved changes. The changes that occur can be represented in a reaction coordinate diagram, which tracks potential energy (y-axis) as the reaction proceeds (x-axis).

Note that we do not show specific units on a reaction coordinate diagram. "Reaction progress" is a pretty abstract idea and cannot be measured with units. "Potential energy" is a more concrete value, and can sometimes be measured quantitatively, but in these diagrams we will care less about the specific numbers than the overall rising and falling.

In all reaction coordinate diagrams, we start by plotting the potential energies of the reactants on the left. For example, this diagram might show the reaction below, in which case the reactants would be C₂H₂ and Br₂.

C₂H₂ + Br₂ → C₂H₂Br₂

As the atoms in the reactants begin rearranging themselves into products, we move right in our plot. The reaction progress is starting. In every reaction, the initial movement will involve a rise in potential energy. This is because, for a change to take place, some existing bonds have to be broken. This process raises the atoms into a higher potential energy state. This requires energy, which must be absorbed from the surrounding molecules as heat. We will talk more about the effects of heat on reactions later in the lesson.

As a result, you will never see a reaction coordinate diagram which starts with a drop in potential energy.

Eventually, as the reaction progress continues, the potential energy of the atoms reaches a maximum value and begins to drop. At this point, the reaction has been initiated by the breaking of bonds, and now new bonds are able to form, which begins to lower the potential energy (releasing heat). In our example reaction, the bond in Br₂ has to break first, after which the new carbon-bromine bonds can start to form.

This point of maximum potential energy is known as the transition state. The height of this point above the reactants is an important feature of the reaction coordinate diagram, which we will discuss in greater detail below.

Eventually, the atoms finish forming any newly made bonds and the reaction progress arrives at the products (in this example reaction, C₂H₂Br₂).

Thus, our reaction coordinate diagram shows a rise from the reactants to the transition state, then a drop down to the products. This will be true of all the RCDs you see in this class. In reality, reactions are usually more complicated than this, involving multiple "hills" between the reactants and products.

However, this simple picture will suffice for us to analyze two important features of any chemical reaction: its rate (how fast reactants are converted to products) and its thermicity (how much heat is released or absorbed by the reaction). We will discuss the release and absorption of heat in the next section.