Energy Diagrams - Online Tutor, Practice Problems & Exam Prep

An energy diagram visually represents the energy changes during a chemical reaction, illustrating reactants, products, and the transition state. The transition state, or activated complex, is the highest energy structure along the reaction coordinate, indicating a peak in energy that reactants must overcome to form products. Activation energy (Ea) is the minimum energy required to initiate the reaction, measured from reactants to the transition state. A higher Ea suggests fewer molecules can convert into products, resulting in a slower reaction, while a lower Ea indicates a faster reaction. Stability of a reaction is determined by the overall energy change:

$\Delta E=\mathrm{products}-\mathrm{reactants}$

with negative

$\Delta E$

(or

$\Delta H$

for thermal energy) signifying an exothermic process, where energy is released, and products are more stable. Conversely, a positive

$\Delta E$

indicates an endothermic process, where energy is absorbed, leading to less stable, higher-energy products. Lower energy correlates with greater stability in chemical reactions.

Energy Diagrams Concept 1

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 going to label as R, are found on the left in the beginning and products, which are 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, 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 the transition state is referred to as the activated complex. So you may hear either term within lecture.

Your reaction coordinate is just the progress of a reaction pathway that lies along the X axis. So here our reaction coordinate will 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 EA. This is just the minimum energy required for a reaction to occur.

Here it would be here EA. So EA 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.

Energy Diagrams Example 1

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 products lie.

If we look here, energy is on the Y axis and here it resides on this line which is 140 kilojoules. So here we'd say that our products possess an energy of 140 kilojoules. Here the question doesn't ask for but if they wanted R is remember at the start here or reside on 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 on some type of energy line or energy value for any given energy diagram.

Energy Diagrams Concept 2

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 EA equals your transition state. So abbreviate as TS minus you'll reactant line. So we're going to say here, if we're to look at these two 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 reactant line looks like it touches 20, so this would be 70 kilojoules for the activation energy. So remember, it's the difference between the two. 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 the difference between the two which represents our activation energy.

Now what's important here, we're going to say that the higher the activation energy, then fewer reactive molecules have enough energy to convert into products. So we're going to 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 two 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 arc 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.

Energy Diagrams Example 2

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 kJ, reaction B is 80 kJ and reaction C is 215 kJ. 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.

Energy Diagrams Concept 3

Now looking at an energy diagram, we can also talk about stability of the chemical reaction itself. Here we're going to see 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 ΔE, 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 have seen my videos on thermochemistry, we've heard of this before. When dealing with enthalpy, which is ΔH, this is when the overall energy is classified as thermal energy. So basically instead of Justice 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 ΔH in this case, we'd be saying that we're talking about specifically about thermal energy. So you could have ΔE here or ΔH here. Same thing.

Now here we set 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 reactant line looks like it's around 30 kilojoules. Subtracting them gives us -20 kilojoules. O here my difference in energy is 20 kilojoules. A negative sign here for ΔE would also mean a negative sign for ΔH. So since we can interchange them, remember, a negative ΔH 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 two 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 ΔH. 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 representing product line at the end, OK.

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 that are lower in energy because remember in chemistry, lower energy means greater stability. So this chart, this first chart which has a negative ΔE value or you can say as an exothermic process, would be more stable overall than the other graph which shows an increase in energy and an endothermic process. So keep that in mind when looking at stability and energy diagrams.

Here’s what students ask on this topic:

What is an energy flow diagram?

An energy flow diagram, also known as an energy flow chart or energy flowchart, is a visual representation that illustrates how energy is transferred from one form to another within a system. These diagrams are used to depict the energy inputs, outputs, and losses within any given process or system, such as an ecosystem, an engine, or an entire economy.

In the context of an ecosystem, for example, an energy flow diagram might show the energy captured by plants through photosynthesis, how that energy is passed on to herbivores when they consume the plants, and further on to carnivores. It will also illustrate energy losses at each trophic level, primarily through metabolic processes and heat loss.

Energy flow diagrams are important tools in understanding the efficiency of energy conversion and the balance of energy within systems. They help identify where energy is being used effectively and where improvements can be made to reduce energy waste. These diagrams are not only crucial in environmental and biological studies but also in engineering, physics, and economics to analyze resource use and sustainability.

What does a reaction energy diagram represent?

A reaction energy diagram, also known as a potential energy diagram, graphically represents the energy changes that occur during a chemical reaction. It shows the energy levels of reactants and products, as well as the energy required to initiate the reaction, which is known as the activation energy.

On the diagram, the vertical axis typically represents the potential energy, while the horizontal axis often corresponds to the reaction progress or the pathway from reactants to products. The starting point indicates the energy of the reactants, and the ending point shows the energy of the products. A peak in the diagram represents the transition state, where the activation energy is at its highest. The difference in energy between the reactants and the peak is the activation energy, and the difference between the reactants and products shows whether the reaction is exothermic (releases energy) or endothermic (absorbs energy). An exothermic reaction will have products with lower energy than reactants, while an endothermic reaction will have products with higher energy.