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Tips on Deciphering and Interpreting Delta H - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Let’s talk about what delta H is. Delta H is enthalpy, don’t get that confused with entropy. Enthalpy has an H in it and so does delta H. This is the measurement of energy and the amount of heat, a reaction either absorbs or releases. And that’s not necessarily to be a reaction, it could be a process and we’ll talk about that in a second. It actually that absorbs or releases.

So in your class you’re probably seeing a bunch of delta H’s, with all this little like subscripts at the bottom. Delta Hrxn for like this one, delta Hcmb, what the heck does that mean? And are they the same thing, and what is going on here?

So first of all, let’s just make sure we just understand what delta H is, before we actually look through and decide for what all the different types of delta H’s there are. If we noticed the energy diagrams we have up here, and I’m sure you’ve seen this in class before, where the amount of reactants is compared to the amount of products in terms of their amount of energy. So energy is in the y-axis. Notice in the first graph, we have the difference of energy between the reactants and products is measured in this space. This is my delta H. Notice the energy is going down, we’re releasing energies, so it’s coming in negative delta H, it’s exothermic.

In this energy diagram to our right, we have reactants with pretty low energy, pretty stable, and then they’re reacting from products that are pretty high in energy, pretty unstable. So here is a difference between that reactants and products and this is our delta H. And since we’re going we’re up, we’re absorbing energy. We’re going to have this a positive number, indicate a positive number. And we’re saying this is endothermic. Just to note, looking at the energy diagram to our left, the exothermic, it is a negative number. That does not mean it has a negative amount of energy. Just the negative to show you that we’re releasing energy. Energy is going from something high to something lowj the difference in energy is negative.

So the formula for delta Hj always is basicallyj the energy of the final product or the products, minus the energy of the reactants or the initial. I’m going to write initial. In other words you might say instead of final, you might see the word products and instead of initial, you might see the word reactants. It really depends on whatever process or reaction, what type of process or reaction we are talking about. But this is exactly, this delta H, what I’ve just explained to you, is the same thing in every single one of these processes.

Let’s just take a closer look. We have this reaction 4 Irons plus 3Oxygens yields 2Fe2O3, which is rust. We now see delta H reaction rxn equals -1625kJ. Now this negative tells me it’s releasing energy it’s exothermic. This kilojoules tells me that the measurement that I’m measuring it in, the reaction saying this overall reaction, is releasing -1625kJ. So delta Hrxn, is a pretty much an umbrella term, talking about any type of reaction. And notice, that this is not kilojoules because we’re not talking about per moles or anything, we’re talking about this overall reaction.

Looking at the next one, it’s C6H12O6 plus 6O2 yields 6CO2 plus 6H2O. This is a combustion reaction, therefore I’m going to specify my delta H and tell you this is a combustion reaction, delta H comb. That comb is short for combustion, nothing different from the top and the bottom at all, from the first reaction and the second reaction. I’m just classifying what exactly the type of reaction I’m doing. Instead of writing rxn, I’m writing it’s a combustion reaction. Now agreeing that as the only type of reaction, it has its own delta H, because they’re extremely exothermic and we talk about them a lot in thermodynamics.

So we don’t have like delta H synthesis or delta H decomposition. We simply have delta H combustion, because it is very common place in thermodynamics. But notice, that it's this kilojoules per mole, which is not which is not the same thing as delta H of reaction. If we’re combusting something, we’re talking of delta H of combustion, since it is so common place, and we talk about that, we’re going to actually talk about each hydro carbon and the amount of energy released when it is combusted. So we need to make sure we have 1 mole of whatever we’re combusting. And we’re combusting C6H12O6.

Remember that, combustion always reacts with O2. So whatever what the other reactant is, is what we’re combusting and we need 1mole of it. If that means we end up with fractional coefficients and other things, that’s totally fine. In thermodynamics, fractional coefficients are actually okay. But we need to make sure we have 1 mole of C6H12O6, whatever we are combusting, because this is kilojoules per mole. Make sure you understand that.

Looking at the next one delta H, what is that little circle above it? That means at standard conditions. So at standard conditions, this is going to be in, I think it's in solid. This is also going to be a gas. Sulfur reacts with 3/2 Oxygen to yield SO3. This f means formation. We’re forming SO3.

Now I want to know that this is kJ/mol, so that means we must form one Mol. So this is KJ/mol. So is the formation reaction, so we’re forming SO3 from its elements. It must be from its elements, it cannot be from anything else. And fractional coefficients are okay because I want make to sure I have 1 Mol product.

Now I train my kids up all the time, because I ask them in a multiple choice question, to write a formation reaction, or it might be multiple choice but I need to make sure, they know how to write a information reaction. So the formation reaction and make sure we have one mole of whatever we’re forming, and in the reactants we have the elements that form it. Notice SO3 comes from Sulfur and Oxygen, don’t forget oxygen it’s a diatomic. It has to be bonded to itself. So it’s Sulfur plus 3/2 Oxygen gas yields SO3. Notice it's also exothermic, and hopefully you understand this delta H, our formation is at standard condition.

Now here we go further and we talk about the processes. The processes is going from liquid to gas. This delta H not, again its inner conditions is that it’s saying I’m vaporising. Vaporizing it takes as much energy as condensing, except condensing is exothermic vaporization is endothermic. This is the process and notice it’s KJ/mol, so you need to have 1 Mol. H to a liquid, to H to a solid we release the energy, so it’s a-6.01 KJ/mol this is solidifying. This is vaporizing, condensing, and solidify, and fusing. This is fusion, that’s going from solid to liquid.

So delta H can happen in processes too. But nonetheless, delta H always means the same exact thing over and over again. Just because they talk about different types of reactions, because there are many types of reactions in this world, delta H is delta H. Hopefully you understand that this little subscripts are just trying to give you more information about what type of delta H it is, what kind of reaction it is talking about.

So hopefully that helped you understand between the many different types of delta H’s.