Energy Diagrams - Online Tutor, Practice Problems & Exam Prep

An energy diagram illustrates the energies of reactants, products, and the transition state during a chemical reaction. The activation energy (E_a) is the minimum energy required for the reaction, influencing the reaction speed; higher E_a results in slower reactions. The overall energy change (ΔE) between reactants and products determines reaction favorability, with negative ΔE indicating exothermic and favorable reactions. Enthalpy (ΔH) and Gibbs free energy (ΔG) are specific forms of overall energy, where negative values signify stability and product formation.

Energy Diagrams Concept 1

An energy diagram represents a curved plot on a graph that illustrates the energies of reactants, products, and transition state as a reaction occurs. We're going to say here that our reactants, abbreviated as "r", are found on the left in the beginning. So here, if we take a look at this energy diagram, at the very beginning of the curve is this box which represents our reactants. We're going to say our products, which we represent by "p", are found right at the end. So here at the very end of this curve, we have this last box here. This represents our products. Then we're going to say our transition state, which we're going to abbreviate as "ts". This equals the max energy structure along a reaction coordinate between reactants and products. So the top of the curve, that is our transition state, so "ts". Sometimes it's referred to as also the activated complex. Your transition state, activated complex are the same thing.

We're going to say our reaction coordinate is just the progress of a reaction that lies along the x-axis. So here our y-axis is looking at the change in energy starting out at 80 kilojoules and going up all the way up to 200, and this is on our y-axis. Our x-axis is here, and this is our reaction progression. So, this is our reaction coordinate. It's looking at the lifetime or lifespan of the reaction as we go from reactants in the beginning to products at the end, and everything in between.

Now we're going to say here that our activation energy, also referred to as our energy barrier, abbreviated as Ea. This is just the minimum energy required for a reaction to occur. So we're going to say here that the height, the difference in height to the reactive line, that represents our activation energy or energy barrier. So when we're looking at the very top of the hill to down here where the reactants are, that is our activation energy. Just keep in mind these are the most fundamental and most important parts of any energy diagram that you'll come face to face with.

Energy Diagrams Example 1

What is the energy value for the product within the following energy diagram? So here we have our curve. Remember, at the end of the curve, that's where our product resides. So here at the end, this is where our product resides on this part of the curve. All we have to do now is trace it back to the y-axis and see what value is connected to it. So here we'd say that the product line of our energy diagram resides at a 140 kilojoules in terms of energy. So the energy value for our products within this particular energy diagram is 140 kilojoules.

Energy Diagrams Concept 2

The speed of a chemical reaction is based on the height of the activation energy. We're going to say here that the activation energy, abbreviated as ea, equals your transition state minus your reactant line. So remember, when we're looking at any particular energy diagram, the top of the curve is our transition state. And where the curve starts, that's where our reactants are initially. We're going to say here that the higher the activation energy, then fewer reactive molecules have enough energy to convert into products. Remember that the difference in height between your transition state and your reactant line represents your activation energy. If we look at both of these, this would be our activation energy. So basically, we're going to say the higher your activation energy, then the slower the reaction. And the smaller your activation energy, then the faster the reaction.

If we take a look here at these two reactions, remember, we just said that activation energy equals transition state minus your reactant line. In this first curve, our transition state looks like it resides on this line here which is 90 minus your reactant line. Your reactant line here resides at 20. So the difference here is 70 kilojoules. That's the minimum energy required for our reactants to traverse this curve and hopefully become products. Over here, our transition state is 60 and our reactant line is still 20, so that's 40 kilojoules. Remember, we just said that the larger your activation energy, the slower your reaction. So we'd say that the first graph is our slower reaction. The second graph has a smaller activation energy, so this is our faster reaction. Right? So just keep in mind that your energy of activation or activation barrier basically determines how fast your reaction will go. The higher your ea is, the slower the reaction. The smaller your ea, the faster the reaction.

Energy Diagrams Example 2

Which reaction will occur in the shortest amount of time? We have reaction a, which has an activation energy of 143 kilojoules, reaction b, which has an activation energy of 80 kilojoules, and finally reaction c, which has an activation energy of 215 kilojoules. So we want the shortest amount of time. That means the one that occurs the fastest. Remember, we said that the smaller your activation energy, the faster the reaction will go. So we'd say that option b, reaction b, has the smallest activation energy, which would translate to the fastest chemical reaction, which would translate to the shortest amount of time. Okay? So option b is the correct answer.

Energy Diagrams Concept 3

The difference in overall energy between the reactants and products can determine the favorability of a reaction. We're going to say overall energy is represented by the symbol ΔE, and it equals products minus reactants. Connected to this idea of overall energy, we can talk about enthalpy and Gibbs free energy specifically. Enthalpy, which is ΔH, represents when the overall energy is classified as thermal energy. Because when we talk about overall energy, it could be nuclear, solar, cosmic, or any type of energy. When we refer to enthalpy, we are specifically talking about thermal energy, which deals with heat. Now, Gibbs free energy is ΔG. This is when the overall energy is connected to the favorability of product formation. If you're able to make products in your chemical reaction, that's a good thing. That means the chemical reaction is favorable. That's the whole point of chemical reactions, to change reactants into products.

Looking at our energy diagrams, we see that ΔE, as earlier mentioned, is products minus reactants. The product line looks like it's around 15. The reactant line looks like it's around 40, which gives us -25. We could say kilojoules as the standard units. Remember, ΔE is the umbrella term, but this side here could easily say Gibbs free energy, it could say enthalpy, it doesn't matter. They are all types of energy. They all would have the same type of value. So this would also be -25. This would also be -25. When ΔH is negative, it is exothermic. Exothermic is energy releasing. When ΔG is negative, it's exergonic. This is favorable in product forming because your products have lower energy. Their energy is 15 kilojoules. In chemistry, lower energy means greater stability. That's a good thing.

Looking to the other side, products minus reactants. It appears that my product line is around 50, my reactant line is around 30, so that's a positive 20. This would also be positive if the energy were specifically thermal energy, and this would be positive 20 as well if this were Gibbs free energy. When it's positive it's endothermic for ΔH, which means energy absorbing. The reactants absorb energy and that's why my products at the end have more energy. Here, if ΔG is positive, it's called endergonic. This is not favorable. You don't want to end with products that have more energy than your reactants did. More energy is less stable. So keep this in mind when we talk about overall energy. That's an umbrella term, and then when we want to be more specific, we can go into enthalpy or Gibbs free energy.

Which of the following statements is true regarding the energy diagram provided?

i. The reaction is endothermic.
ii. The activation energy is +10 kJ.
iii. The reaction releases energy.
iv. The enthalpy of the reaction is –25 kJ.

• A. I only
• B. II and IV
• C. I and III
• D. II, III and IV

Which of the following reactions proceeds the slowest? 

              

              Reaction 1                                             Reaction 2                                       Reaction 3 

• A. Reaction 1
• B. Reaction 2
• C. Reaction 3

Here’s what students ask on this topic:

What is an energy diagram in chemistry?

An energy diagram in chemistry is a graphical representation that illustrates the energy changes during a chemical reaction. It shows the energies of reactants, products, and the transition state. The y-axis represents energy, while the x-axis represents the reaction coordinate, which is the progress of the reaction. Key components include the reactants (R) at the beginning, the products (P) at the end, and the transition state (TS) at the peak of the curve. The activation energy (Ea) is the energy difference between the reactants and the transition state, indicating the minimum energy required for the reaction to occur.

How does activation energy affect the speed of a chemical reaction?

Activation energy (Ea) significantly affects the speed of a chemical reaction. It is the minimum energy required for reactants to transform into products. A higher activation energy means that fewer reactant molecules have sufficient energy to overcome the energy barrier, resulting in a slower reaction. Conversely, a lower activation energy allows more reactant molecules to participate in the reaction, leading to a faster reaction. Mathematically, the activation energy can be expressed as:

$E_a=E_{\mathrm{TS}}-E_R$

What is the difference between exothermic and endothermic reactions in energy diagrams?

In energy diagrams, exothermic and endothermic reactions are distinguished by the overall energy change (ΔE) between reactants and products. For exothermic reactions, ΔE is negative, indicating that energy is released, and the products have lower energy than the reactants. This is represented by a downward slope from reactants to products. For endothermic reactions, ΔE is positive, meaning that energy is absorbed, and the products have higher energy than the reactants, shown by an upward slope. Mathematically, ΔE can be expressed as:

$\Delta E\; =\; E$_products - E_reactants

What is the role of the transition state in an energy diagram?

The transition state in an energy diagram represents the highest energy point along the reaction coordinate. It is the point where the reactants are transformed into products, and it is often referred to as the activated complex. The energy difference between the reactants and the transition state is the activation energy (Ea), which is the minimum energy required for the reaction to proceed. The transition state is crucial because it determines the activation energy and, consequently, the reaction rate. A lower transition state energy results in a lower activation energy and a faster reaction.

How do enthalpy (ΔH) and Gibbs free energy (ΔG) relate to energy diagrams?

Enthalpy (ΔH) and Gibbs free energy (ΔG) are specific forms of overall energy change (ΔE) in energy diagrams. Enthalpy (ΔH) refers to the thermal energy change and indicates whether a reaction is exothermic (ΔH < 0) or endothermic (ΔH > 0). Gibbs free energy (ΔG) relates to the favorability of a reaction, with negative ΔG indicating a spontaneous and favorable reaction (exergonic) and positive ΔG indicating a non-spontaneous reaction (endergonic). Both ΔH and ΔG can be derived from the overall energy change:

$\Delta E\; =\; E$_products - E_reactants