Energy Diagrams - Video Tutorials & Practice Problems
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Energy Diagram represents a curved plot illustrating the energies of reactants, products and transition states of a reaction.
Energy Diagrams
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Energy Diagrams Concept 1
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An energy diagram can be thought of as a curved plot on a graph that illustrates the energies of reactants, products, and transition state as a reaction occurs. Now here we have the basic layout for an energy diagram. And we're going to say here that reactants, which we're gonna label as r, are found on the left in the beginning, and products which you're going to label as P, are found to the right at the end. So if we take a look here at this energy diagram, at the very beginning we have our reactants here, and at the very end of the curve represents our products. Now here, our transition state, which we'll abbreviate as TS, this represents the maximum or max energy structure along a reaction coordinate between reactants and products. So here, the very top of this curve would be our transition state. Now sometimes their transition state is referred to as the activated complex. So you may hear either term within lecture. Your reaction coordinate, this is just the progress of a reaction pathway that lies along the x axis. So here our reaction coordinate would be here. It looks at the entire chemical reaction as we go from reactant up to transition state down towards our products. Now here we're going to say that the last term you should familiarize yourself with is activation energy, sometimes referred to as our energy barrier. This is abbreviated as e sub a. This is just the minimum energy required for reaction to occur. Here, it would be here, e a. So e a is basically the distance from your reactant starting point up to your transition state. So when we're dealing with any typical type of energy diagrams, these are the important terms you need to remember when trying to describe the layout of this particular energy diagram.
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Energy Diagrams Example 1
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What is the energy value for the product within the following energy diagram? So remember, here, this energy diagram is a curve. It looks a little bit different from what we might have saw earlier, but remember, at the end of the curve represents our products. So here is where our products lie. All we have to see is where in terms of our energy do do products lie. If we look here, energy is on the y axis and here it resides on this line which is a 140 kilojoules. So here we'd say that our products possess an energy of a 140 kilojoules. Here the question doesn't ask for, but if they wanted r is remember at the start, here it will reside on a 110 kilojoules. And remember up here is our transition state. It looks like it's a little bit over 180 kilojoules. So keep this in mind when you're looking at any particular energy diagram. We have our reactants, our transition state, and our products. Each of them are lie lie on some type of energy, line or energy value for any given energy diagram.
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Energy Diagrams Concept 2
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The speed of a chemical reaction is based on the height of the activation energy. Now here activation energy, which is your energy barrier and abbreviated as e sub a equals your transition state, so abbreviated as t s minus your reactant line. So we're gonna say here, if we're to look at these 2 energy diagrams, remember this is your transition state, this is your reactant. This is your transition state, this is your reactant. From this we could calculate the energy of activation for both of these curves. So if we were to do that, we'd say that the transition state up here looks like it's around 90, so this would be 90. And then the reactive line looks like it touches 20, so this would be 70 kilojoules for the activation energy. So remember it's the difference between the 2. And then in our other curve, the transition state looks like it's 60 and the reactant line still looks like it's around 20, so that's 40 kilojoules. So remember it's a difference between the 2 which represents our activation energy. Now what's important here, we're gonna say that the higher the activation energy, then fewer reactive molecules have enough energy to convert into products. So we're gonna say here a higher activation energy equates to a slower reaction, and a smaller activation energy equates to a faster reaction. Here we just calculated the activation energy for our 2 energy diagrams, this one being 70 kilojoules and this one being 40. So remember, the one that has the higher activation energy would represent our slower reaction and the one that has the smaller or lower activation energy would represent our faster reaction. So just remember, we can use activation energy to give us a good idea of how quickly or how slowly any typical chemical reaction will go. So keep this in mind when calculating activation energy.
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Energy Diagrams Example 2
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Which reaction will occur in the shortest amount of time? So shortest amount of time really means which chemical reaction is fastest. So remember, we talked earlier about speed and activation energy. If we want the fastest chemical reaction, well, that would mean that we want the lowest activation energy. If we take a look here, reaction a is 143 kilojoules, reaction b is 80 kilojoules, and reaction c is 215 kilojoules. The answer is b. Since it has the lowest activation energy, that would mean that it moves the fastest, and therefore would have the shortest amount of time necessary to go to completion.
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Energy Diagrams Concept 3
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Now looking at an energy diagram, we can also talk about stability of the chemical reaction itself. Here we're going to say that stability is the difference in overall energy between the reactants and products and can determine the favorability, so how likely is this chemical reaction to occur for any type of chemical reaction. Now here we're going to say that our overall energy, which is delta e equals products minus reactants. Now remember for any typical energy diagram, our reactants are at the beginning and our products are at the end. And we've kind of understood this idea of products minus reactants. For those of you who've seen my videos on thermal chemistry, we've heard of this before when dealing with enthalpy, which is delta h. This is when the overall energy is classified as thermal energy. So basically, instead of just writing the term energy where it's a broad idea of energy, where it could be thermal energy, cosmic energy, nuclear energy, you could be more specific and say delta h. In this case, we'd be saying that we're talking about specifically about thermal energy. So you could have delta e here or delta h here, same thing. Now here, we said it products minus reactants. If we take a look here, we can see that our product line here is at 10 kilojoules, and then our reacted line looks like it's around 30 kilojoules. Subtracting them gives us negative 20 kilojoules. So here my difference in energy is negative 20 kilojoules. A negative sign here for delta e would also mean a negative sign for delta h, so since we can interchange them. Remember, a negative delta h sign means that we're dealing with an exothermic process. Remember, exothermic means that our reaction releases energy. This is why our reactants are higher in energy and our products are lower in energy. Releasing the energy means that we're forming products that are lower in energy. On the other side, we have our product line looks like it's around 50, and then our reactant line looks like it's around, I'd say, 15 or so. So here when we subtract those 2 from each other that gives us 35, positive 35. Now we're going to say here that it's positive which means it's endothermic for delta h. Remember, endothermic processes mean we absorb energy, so our reactants absorb energy, and as a result, we form products that are higher in energy. So that's why the curve moves up to represent the product line at the end. Okay? So keep this in mind, we can look at overall energy to give us an idea of what's more favorable. And in terms of these two graphs, it's always better to have products o which has a negative delta e value, or you can say is an exothermic process, would be more stable overall than the other graph which shows an increase in energy and an endothermic process. K? So keep that in mind when looking at stability and energy diagrams.