When covering thermochemistry, it’s also important to discuss the topics of endothermic solutions versus exothermic solutions. If we take a look at exothermic solutions, connected to these ideas are two variables - q and delta H.
q is referred to as heat and delta
H represents enthalpy.
We need to remember that under constant pressure, we can say that q equals delta H.
If we’re saying exothermic here, that means that q would be negative in terms of its sign and delta H would also be negative. If we’re looking at any basic type of chemical reaction, we have AB + CD reactant that break down to form AD and BC as products.
For an exothermic process or solution, we’re going to say that the bonds between the reactants are weak. That way they can break apart very easily and reform products that contain stronger bonds. For an exothermic process, reactants have weak bonds but products have stronger bonds. If we think about this in terms of this container here and let’s pretend it’s in solution. Let’s say we have energetic molecules that are bouncing all over the place. In an exothermic process, they’re going to release their heat. If they’re releasing their heat, they’re releasing energy. Eventually, they start to slow down because they’ve released a lot of energy. If they keep doing this, they release so much that they’ll slow down even more and connect together and form bonds.
In an exothermic process, we release heat or energy to help form bonds. If these molecules are releasing heat, the water will pick up that heat. If I were to touch this container - this exothermic solution, the water has become warmer so when I touch it, it feels warm to the touch. So an exothermic process releases heat and my hand absorbs it. With exothermic processes, we have phase changes. We have gas, liquid and solid. A gas to a liquid is just condensation. A liquid to a solid is freezing. And then we can skip the liquid phase altogether and go straight from gas to solid which is deposition.
On the opposite side, we have endothermic processes. Here, the signs would be positive for q and delta H. We have our reactants and we have our products. Right now, the reactant bonds will be strong which means that energy needs to be used in order to break them down so that they can reform our products here. But here’s the thing. When we form those products, their bonds will be weak.
Let’s think about this in terms of everyday life. We have ice. We have an ice cube. The water molecules are tightly packed. What happens if I start to heat up that ice cube? It’s going to melt. How does that happen? The water molecules inside the ice will absorb that thermal energy and use it to increase their kinetic energy enough so that they can push themselves apart and melt. We’re going to say an endothermic reaction absorbs heat in order to break its bonds. If I were to touch this endothermic process, it’s absorbing heat. It’s going to absorb warmth from my hand. An endothermic process feels cold to the touch.
Here with their phase changes we have solid to liquid which is melting or fancy term, fusion. Liquid to gas is vaporization. Then solid all the way to gas, that is just simply sublimation.
Before we end, we need to realize the word fusion is tricky. With thermal chemistry, fusion just means melting. But when we start talking later on about nuclear chemistry, nuclear chemistry defines fusion as two basic radioactive particles combining together. It takes on an entirely different meaning. For thermochemistry, which we’re doing right now, fusion means melting. Later on with nuclear chemistry, it means the combining of different nuclei together. Just remember, it has two definitions. I know chemistry can be cruel but just remember that distinction.
Again, with thermal chemistry we have the idea of exothermic processes versus endothermic processes which are basically opposite processes.