Potential Energy Diagram

Exothermic and Endothermic Reaction

Before diving into chemical potential energy diagrams, we need to remember the difference between kinetic and potential energy. Kinetic energy is the energy produced by motion, whereas potential energy is the energy available as stored energy within an object, like a molecule, due to the configuration of its parts or atoms.

We also need to review the basic concept of exothermic and endothermic reactions. In order for a reaction to occur, the collision between reaction particles must be energetic, and this energy is known as activation energy.

During a chemical reaction, heat may be released or absorbed. If energy is released, we call it an exothermic reaction.

In exothermic reactions, the potential energy of the products is lower than the potential energy of the reactants. Because of this, the change in enthalpy (ΔH) is negative.

Endothermic reactions, on the other hand, are those chemical reactions that absorb energy. These reactions have a positive ΔH and the potential energy of the products tend to be higher than that of the reactants.

Potential Energy Diagram Labeled

When dealing with chemical reactions, chemists use potential energy diagrams (or reaction coordinate diagrams) to show the potential energy in a chemical reaction.

Figure 1 shows a simple example of a potential energy graph for an exothermic reaction. Here, the x-axis is labeled as the reaction coordinate or time, while the y-axis is labeled as the potential energy.

Figure 1. Potential energy diagram for an exothermic reaction, Isadora Santos - StudySmarter Originals.

However, sometimes we may encounter chemical reactions involving more than one elementary step. In this case, we would have a multi-step reaction coordinate diagram (figure 2). The figure below shows the potential energy diagram for a multi-step reaction mechanism. Notice that the first step in this chemical reaction is endothermic, whereas the second step is exothermic.

Figure 2. Multi-step reaction coordinate diagram, Isadora Santos - StudySmarter Originals.

Let's look at a problem!

Potential Energy Diagram for Endothermic Reaction

Now that we know how a potential energy diagram is labeled, let's look at the potential energy diagram for an endothermic reaction, which are reactions that absorb energy, and as a result, the temperature of the reaction mixtures in the solution decreases.

Figure 4 shows the potential energy diagram for the endothermic reaction for the decomposition of magnesium oxide (MgO). As expected for an endothermic reaction, notice that the products (2 Mg + O₂) possess more potential energy than the reactants (2 MgO).

$$ \text{2 MgO + energy} \to \text{2 Mg} \text{ + }\text{O}_{2} $$

Figure 4. Potential Energy Diagram for an endothermic reaction, Isadora Santos - StudySmarter Originals.

Another interesting example of an endothermic reaction is photosynthesis. Photosynthesis is the process of using sunlight energy to make food (glucose) from water and carbon dioxide. During photosynthesis, light energy gets converted to chemical energy.

$$ \text{6 CO}_{2} \text{ + 6 H}_{2}\text{O} \to \text{C}_{6}\text{H}_{12}\text{O}_{6}\text{ + O}_{2} $$

Figure 5. Potential Energy graph for Photosynthesis, Isadora Santos - StudySmarter Originals.

Potential Energy Diagram with a Catalyst

When catalysts are present, they create an alternate reaction pathway with lower activation energy (figure 6). This reduction in the activation energy also increases the rate of the reaction exponentially.

Figure 6. Potential energy diagram for a chemical reaction with and without the presence of a catalyst, Isadora Santos - StudySmarter Originals.

A common example of a catalyst is an enzyme. Enzymes are proteins that act as catalysts in biochemical reactions. For example, carbonic anhydrase is an enzyme that can catalyze the synthesis of carbonic acid, from carbon dioxide and water, when the CO₂ concentration in the body is too high. This regulation of CO₂ concentration by the enzyme helps keep the body's pH balanced.

Figure 7 shows the potential energy diagram for this reaction. Notice that, when the enzyme carbonic anhydrase is present, less activation energy is needed to initiate the reaction. However, it does not change the ΔH of the reaction as it does not get used up.

Figure 7. Potential energy diagram for the reaction between CO2 and H2O to form H2CO3, Isadora Santos - StudySmarter Originals.

Electric Potential Energy Diagram

Lastly, let's talk about electric potential energy. Electric potential energy is the potential energy ("stored" energy) a charge has when moved by an electrostatic force. We can use the formula below to calculate electric potential energy.

$$ U = k\frac{q_{1}\times q_{2}}{r} $$

Where,

• Unis the electric potential energy (J)
• k is the Coulombs constant
• q₁ is point charge 1
• q₂ is point charge 2
• r is the distance between the two point charges

Figure 8 Shows the diagram of electric potential. The x-axis shows the distance away from the charge, and the Y-axis shows the electric potential at a certain point.

Figure 8. Electric potential energy diagram, Isadora Santos, StudySmarter Originals.