Enthalpy

So we've been talking kind of at a very macro level about heat transfer by using an example of heat being transferred between our hand and a glass of water. So now let's go more of a micro level. Let's talk about heat transfer and chemical reactions. So when we're talking about heat transfer and chemical reactions we're talking about the term called enthalpy. Now it turns out that if we think about chemical reactions as our system, so go ahead and write that here, chemical reactions can either release or absorb energy that can change the temperature of its surroundings. Which we generally think of when we're talking about chemical reactions as the solution that the chemical reaction is taking place in, and of course if the chemical reaction is taking place in our body the surrounding is really just our body, which you can think of really as just a big bag of water. So a question might be on your mind. Why did chemists come up with this fancy term enthalpy to describe the heat transfer of chemical reactions? It turns out that enthalpy is a very useful quantity to calculate for many chemical reactions because not only does it tell us something about heat transfer, but it also is a component of Gibbs free energy, which is an important parameter that chemists use to determine whether or not a reaction will take place or not. Generally speaking, I think it's OK to conceptualize with some simplification that enthalpy is essentially just a fancy term to describe heat transfer for chemical reactions. And because we can never define an absolute quantity of heat, because remember heat is the amount of energy transferred, it's not an absolute term, enthalpy is always referred to as a change in enthalpy, and oftentimes it's written as delta H. And specifically this change in enthalpy describes the change in heat energy. That is whether heat was lost or gained from the perspective of the system. So even thought that there is this kind of important interplay, this conservation of energy between the system and the surroundings, this term enthalpy is really just telling us what's happening from the perspective of the system. And I think this will make more sense as I give you an example below, but before I do that I just want to note what the units of enthalpy are. And the units of enthalpy are joules per mole of reactant in the chemical reaction. And this of course makes sense because joules is a unit of energy, and we're talking about heat, which is a form of energy. And it's notable to know that having it per mole allows us to take the amount of whatever is reacting into account. This is something that we can't really take into account if we just, say, measure the change in temperature that occurred over the course of a reaction. All right, so now let's go ahead and take a look at an example. So I'm going to go ahead and scroll down. Let's go ahead and use an example that you will be fairly familiar with. So when our bodies get super hot to cool ourselves down our bodies essentially evaporate water from the surface of our body to help cool ourselves down. And this is really just a fancy way of saying that we start sweating. Right? Now the chemical reaction for sweating we can really think about as just essentially the evaporation of water. So that is to say taking water from the liquid phase and turning it into water vapor, or water in the gaseous phase. The change in enthalpy for this particular reaction, and note that I'm saying for this particular reaction, for our system, is defined as the enthalpy of our product, which is our water in its gaseous form, minus the enthalpy of our reactant, which is water in its liquid form. This of course is a way to conceptualize enthalpy, but remember it's really actually not possible to measure an absolute quantity of enthalpy for anything. We can only ever measure this change in enthalpy, which involves monitoring the change in temperature of the surroundings during the course of a reaction as well as some other considerations that we're not going to go into in this video. But just to note that it's really the change in enthalpy that we're measuring and not these absolute quantities, even though technically this is how we're conceptualizing enthalpy. So the key idea here about this process of evaporation is that it requires energy to occur. Just think about boiling water. In order to get water into its gaseous phase you need to heat it up. So the fact that we're adding energy to our system should tell you something about what this sign of this change in enthalpy should be. The products are essentially gaining more energy in the form of heat than the reactants. So we say that in a process that absorbs heat the change in enthalpy is greater than 0. In other words, the change in enthalpy is positive. And whenever a reaction has a positive change in enthalpy, which by definition means that heat is being absorbed, we call it an endothermic reaction. And the way that I like to remember this is that endo is I think a Latin prefix for within. So heat is going in to our system, which is our chemical reaction. So you're probably wondering at this point, well, if we're absorbing heat, how is this connected to us cooling off? Well, this is kind of the important point that I alluded to before. This change in enthalpy only tells us what's going on with our system, which of course is our chemical reaction, but our body in this case is our surrounding. So we can use this relationship above that tells us that the heat lost or gained by the system is equal and opposite to the heat lost or gained by the surroundings. So in this case we know that our system is absorbing heat, which means that our surroundings, our body, must be losing heat. And that's how we cool down. Now you can also imagine that we have chemical reactions in which the change in enthalpy is less than 0, and we call these types of reactions exothermic reactions. And the way I kind of think about these is that ex, this ex term is kind of a Latin prefix for out of. So heat essentially is going out of the system instead of being absorbed. One example of a very prevalent reaction in our bodies that is exothermic is the hydrolysis, or reaction with water, of ATP, which is of course the energy currency of our cells. So ATP reacts with water, and it loses a phosphate group to become ADP and a free phosphate group. And this reaction has a negative change in enthalpy, or in other words it releases heat. The fast that this reaction is exothermic is physiologically significant for many reasons. But one of those reasons is when we're talking about shivering. So we all know that we shiver when we're cold. And the reason we do that is because we want to warm ourselves up. And that heat energy is indirectly tied to this hydrolysis of ATP which releases heat and allows our muscles to contract to warm us up. So just to wrap things up here, I think the key takeaway is that enthalpy describes heat transfers for chemical reactions, and notably it's from the perspective of the chemical reaction, not the surroundings. And so chemical reactions can either lose heat, in which case they are classified as exothermic reactions and having a negative enthalpy, or they require an input of heat, and they're classified as endothermic, and have a positive change in enthalpy.