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Energy Diagrams

What are Energy Diagrams?

• Energy diagrams show the amount of relative potential energy in each step of a reaction.

• Energy diagrams typically include: the reactants, transition states, and products. Depending on the reaction, it can also include intermediates and activation energy.

• Energy diagrams are also referred to as reaction coordinate diagrams!

The Different Parts of an Energy Diagram:

Exothermic and Endothermic Reaction Energy Diagrams

• One of the most common uses of energy diagrams is to show whether a reaction is exothermic or endothermic.

• Energy diagrams between exothermic and endothermic reactions are slightly different, and it is important to know how to properly draw each diagram correctly!

Endothermic Reaction Energy Diagrams:

• Endothermic reactions gain energy as heat. So, when drawing the energy diagram, you want the reactants on the graph to be lower than the products. You're visually showing that the reaction is gaining energy!

• Endothermic reactions have a greater activation energy than exothermic reactions. So, the activation energy on endothermic energy diagrams is larger than on exothermic energy diagrams!

Exothermic Reaction Energy Diagrams:

• Exothermic reactions release energy during the reaction. So, when drawing the energy diagram, you want the reactants on the graph to be higher than the products. The products will end at a higher energy than the reactants? Why? We put energy into the system.

• Exothermic reactions have a lower activation energy than endothermic reactions. Thus, the activation energy on the exothermic energy diagram is smaller than in endothermic energy diagrams!

Helpful Steps to Create an Energy Diagram

1. Label the axes. The x-axis is labeled as the reaction coordinate, and the y-axis is labeled as energy.

2. Draw a line at the beginning of the graph for the reactants and a line at the end for the products. The products will be higher or lower depending on if the reaction is endothermic or exothermic.

3. Add a hump that connects the reactant and product lines. Remember to add multiple humps if the reaction has multiple transition steps.

4. Label the activation energy, transition step, and enthalpy!

Visualization of the steps:

From the start, chemistry has been about energy. Even in the 1700s, chemists were observing gases and measuring their ability to do work. In the modern era, we began to apply energy to electrons. Bohr's model considered electron orbits based on energy quanta. As quantum mechanics developed, we thought about electrons in terms of orbital energy levels. Eventually, we started to model covalent bonding. When covalent bonds break and reform, it's all about energy.

Today we'll look at energy diagrams. They show how energy changes during a chemical reaction. This written explanation will describe each part of an energy diagram. But, if you need a visual, look at our image description too.

Energy Diagram Metaphor

Unfortunately, running up a hill is an excellent metaphor for chemical reactions. I hate running! I know some people love it. But, I think it's boring. I'll gladly run if I'm playing a sport. But, pure running, no. When I was in college, I took a distance running class. It confirmed that I despise running. Sometimes we learn the hard way. I remember we had to run up a huge hill. It was so long and steep. At the top, you were gasping for breath. But, the top was a nice transition. The next part was downhill. It was easy! It's oddly similar to chemical reactions!

Parts of an Energy Diagram

The x-axis:

The energy diagram's x-axis is called the reaction coordinate. The reaction coordinate gauges the reaction's progress. But, it's not time. The reaction coordinate is abstract! It changes for different chemical reactions. We might measure bond length, bond angle, or the number of bonds formed. It depends on the chemical reaction! At our level, we typically write reaction coordinate. We know it's some measure of the reaction's progress. As you advance, your teacher might be more specific.

The y-axis:

The y-axis is energy. More specifically, it's potential energy. Remember, potential energy comes from electrostatic interactions. Molecules store it in their bonds. So, as the bonds break and reform, the graph's energy changes.

The Products and Reactants:

At the graph's beginning and end, you'll see flat lines. The beginning line is the reactant's starting energy. The ending line is the final products' energy. But why are they flattened? At the products, our reaction's finished. Its energy doesn't change. So, the line is flat. The reactants line is a little trickier. It's a convention. The moment we mix our reactants, the reaction begins. So, our curve should start immediately. The reactant's line shows their energy before mixing.

An energy diagram also gives thermodynamic information. We compare the reactants' and products' positions. If the reactants are higher up the y-axis, the reaction is exothermic. The reactants released heat to form the products. If the products are above the reactants, it's endothermic. The reactants absorbed heat. The energy difference between the two lines equals the enthalpy change ($\Delta H$). If we were doing the reaction experimentally, we would notice a temperature change.

The Curve:

Between the reactants and products is a hill-shaped curve. The hill's peak is the transition state. The energy change between the reactants and the transition state is the activation energy. Remember, it's the minimum energy required for the reaction. If the system has less, it simply won't go.

Multistep Reactions

We've studied reaction mechanisms. So, we know that reactions happen in many elementary steps. But, how does that change the energy diagram? Each elementary step has its own curve. We connect the curves into one graph. So, our overall reaction produces a chart with multiple hills and valleys. The reactants and products are still at the very start and very end.

But, what are the peaks and valleys? Let's start with the valleys. They're the intermediates! Remember, intermediates are the products for one elementary reaction. But, they're the reactants for the next step. So, each hill follows the pattern. The reactants are at the start. The products are at the end.

What about the peaks? They're the transition states. Intermediates act as reactants. So, they have to go through a transition state. Don't confuse the two! Intermediates are whole molecules or ions. Their bonds have finished forming. But, they are still reactive. So, they become the reactants for the following elementary step. But, transition states are halfway points. They have partially broken and formed bonds. The in-between state has very high energy. So, the transition state is the peak.

The tallest peak is the rate-limiting step. Why? Remember collision theory. We need the molecules to collide with the right orientation and energy. The tallest peak is the step that has the largest energy barrier. So, it determines the reaction rate. What about adding a catalyst? A catalyst lowers the activation energy. So, we would shrink the peak.

Super Advanced Chemistry: Energy Surfaces

If you take physical chemistry, you'll encounter energy surfaces. They are energy diagrams in three dimensions. The y-axis is still energy. But, the x and z axes are different reaction coordinates. An energy diagram is a slice of an energy surface. You don't need to know how they work. But you should know they exist.

(Video) Potential Energy Diagrams - Chemistry - Catalyst, Endothermic & Exothermic Reactions

by The Organic Chemistry Tutor

In this video, The Organic Chemistry Tutor explains energy diagrams. He starts by illustrating exothermic and endothermic reactions. You'll learn each part of the diagram and their differences. Once you've mastered those, he shows an example problem. You'll see how to calculate the various energy values. Finally, he shows you an energy diagram for multistep reactions. He also does an example problem for multistep reactions. This video can help you conquer energy diagrams!