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Understanding the Difference between Delta H and Delta S - 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.

Tips on understanding the difference between delta H and delta S. Let's decide what delta H and delta S are. Delta H is enthalpy. And the reason I made my H capitalised, is because that's how I remember that delta H is enthalpy. It has an H in it. This measurement of heat or energy transfer. Now I'm just giving very simplified definitions, but you should have these in your heads.

Delta S is entropy. It's a measurement of randomness or disorder. Notice I have deltas in front of these. That's because we typically talk about changes, reactions or processes that actually happen in Chemistry. We can however talk about just straight up H and S, let's do that.

Well H is the measurement of heat or energy, but it's a measurement of the transfer of heat or energy. We cannot decipher how much heat or energy something has in it. We can only measure the change it undergoes through a chemical process. You cannot have this H by itself, we can't measure it. We don't have the means of measuring that.

We can however measure S, and this is how we do it. S is the measurement of this disorder and randomness and movement, within a particle or a process. So let's talk about what I mean by that. So we're going to have elements and compounds.

So if we have an element, elements have a delta H of zero, always. For compounds, when we're talking of the delta H, it's a formation of that. It actually is a process going from its elements reforming that. So that's where that delta H comes from. For elements it's always zero, even the o2 or the diatomics. The compounds it's a formation of that. So it's not actually what is in the heat that it actually contains just as compound, it's actually how the compound is made, how much energy it takes to make that compound. But since we don't make elements, the delta H for elements is zero. What about delta S? What the heck is that?

Well delta S or just S, we can measure. Let's say we're talking about O2. O2 looks like this; double-bonded, it's a lot of electrons around it, that's too many. So it moves in space, so S is the movement, measurement of randomness, disorder, movement. So oxygen can be together, it can be stretched apart, these bonds are mobile. They can squeeze together, they can stretch apart. They can turn, if they want to turn. They can turn this way, if they want to turn this way. They are constant moving, so they do have mass measurement. However oxygen isn't made, therefore it doesn't have the energy, it doesn't need energy to create because it is naturally created.

So there is no delta H but there is an S, and S is measuring the movement, or the disorder or the chaos in this oxygen has. Every substance has measurement of disorder or chaos and movement. They do have some sort of amount of energy and movement within it.

What about a process? Well process we use half delta H's. Let's look at this process. Iron and oxygen combine together to make rust. This is an exothermic process. Our delta H equals -826. So this actually is a formation reaction, well not really, because it doesn't have one mole. Formation reactions you're only allowed to have one mole of a product. But this, it does have a delta H and I'm going to say this is the reaction. So it does require energy or releases energy when this reaction occurs. So iron and oxygen were high in energy, then it released the energy when it became rust or Fe2O3.

Delta S, we have to look at the states of matter. How much disorder or chaos is actually happening within the system. So we have, this is a solid state, this is a gaseous state, this is a solid state. Think about it. Solids are very tightly or very ordered. Gases are very chaotic, very disordered. So you have a high amount of disorder over here.

So over here, the products it's all solids. You have a low amount of disorder. So my delta S and then with a numerical value of it. My delta S is going from high to low, it's going to be negative. This is kilojoules. It is going to decrease. It's not necessarily negative amount. It's actually just saying my delta S is decreasing in value. So it's going from a high disorder to a low disorder. So delta S is the measure of randomness or chaos or movement, as in the particles or compounds. H is the measurement of how much energy it contains within it. And we can't measure H by itself. We must measure the change in energy or change in heat.

So that's the difference between delta H and delta S, and it helps understanding why elements have a zero delta H, but not a zero S. So hopeful that helped you understand the difference between entropy and enthalpy.