Exothermic reactions involve releasing thermal energy by the system to the surroundings. Now we're going to say as molecules in our system release heat, they slow down and with enough energy lost they form bonds. So exothermic reactions are heat releasing, bond forming reactions.
We're going to say in terms of phase changes, let's think of it as energetic gas molecules. As we release heat those water molecules, they're going to come closer together. So think of liquid gaseous water bouncing all over the place in a container. They slowly start to release their heat and they slow down because they're losing energy. If they slow down enough, they condense into a liquid, so this is condensation.
If the liquid water that we've collected we put it in the freezer, they will continue to lose heat and they will solidify. So liquid to solid is freezing some substances. Under right conditions, they can skip the liquid phase altogether. One example is carbon dioxide. We call it dry ice. You could take it out of a very cold container where it's in its gashes phase and put it outside. It'll skip the liquid phase altogether.
So if you're going from a gas to if you're going from a gas to a solid though, where we're taking that gaseous carbon dioxide and putting it back into the container where it solidifies again, that's called deposition. Now let's think about it, Let's say a container has a liquid and that liquid is an exothermic reaction, its exothermic, so it will be releasing heat. So if I were to touch that container, I would feel the heat that it's releasing to my hand. So exothermic reactions feel warm to the touch.
Now if we were to think of it in terms of an energy diagram, in an energy diagram, our Y axis is our energy and then X axis is the progress of the reaction. Remember, the whole point of a chemical reaction is to go from reactants to products. And what we need to realize is that in an exothermic reaction, the reactants have more energy. They release their excess energy and they drop down to become products. Over time, their products have less energy because we're releasing energy to go from reacting to product. Our enthalpy ΔH would have a negative sign. So just remember, this is the way we depict an exothermic reaction in terms of an energy diagram.