Problem 90 - Chemistry The Molecular Science

Question

Draw a reaction energy diagram for an endothermic process. Mark the positions of reactants, products, and activated complex. Indicate the activation energies of the forward and reverse processes and explain how \(\Delta_{r} E\) for the reaction can be calculated from the diagram.

Step-by-Step Solution

Answer

An endothermic reaction diagram shows products higher in energy than reactants, with activation energies marked. \(\Delta_{r} E\) is calculated as the energy difference between products and reactants.

Step 1: Identify the components needed

In an endothermic reaction, the products are higher in energy than the reactants. We need to identify the positions of reactants, products, and the activated complex (transition state) on the energy diagram.

Step 2: Draw the diagram axes

Draw the reaction coordinate on the x-axis and energy on the y-axis. This sets up the framework for plotting the reaction.

Step 3: Plot the reactants and products

Place the reactants on the left at a certain energy level and the products on the right at a higher energy level. This reflects the endothermic nature of the reaction.

Step 4: Mark the activated complex

At the peak of the energy curve, label this point as the activated complex or transition state. This point represents the highest energy point along the reaction path.

Step 5: Indicate activation energies

Draw vertical arrows from the reactant level to the peak of the curve for the forward activation energy \(E_a^{\text{forward}}\), and from the product level to the peak for the reverse activation energy \(E_a^{\text{reverse}}\).

Step 6: Calculate \(\Delta_{r} E\)

The change in reaction energy \(\Delta_{r} E\) is the difference between the energy of products and reactants: \(\Delta_{r} E = E_{\text{products}} - E_{\text{reactants}}\). Since this is endothermic, \(\Delta_{r} E > 0\).

Key Concepts

Endothermic Reaction

In an endothermic reaction, energy is absorbed from the surroundings, resulting in products that are higher in energy than the reactants. This absorption of energy leads to a positive change in reaction energy, denoted as \( \Delta_{r} E \). Such reactions can be seen in processes like photosynthesis, where plants absorb sunlight to convert carbon dioxide and water into glucose and oxygen.

Key characteristics of endothermic reactions include:

• Energy is taken in, causing a temperature drop in the surroundings.
• Products have greater potential energy than reactants.
• They often feel cold to the touch because of the heat absorption.

Understanding these aspects helps in visualizing how an energy diagram for such reactions is constructed. The energy level of products is placed higher than that of the reactants, indicating the absorption of energy.

Activated Complex

The activated complex, also known as the transition state, represents the highest energy state in a chemical reaction. It is a temporary arrangement of atoms that forms at the peak of the energy curve. This complex is crucial because it illustrates the point where old bonds are breaking and new bonds are forming.

Some key attributes of the activated complex are:

• It is highly unstable and exists only for a fleeting moment.


• It cannot be isolated as a stable compound.


• Once reached, the reaction will proceed toward forming either products or reverting to reactants.

The position of the activated complex on an energy diagram is marked at the top of the curve. Its energy level indicates the amount of energy required to transform reactants into products.

Activation Energy

Activation energy is the minimum energy required for a reaction to occur. It is the energy barrier that must be overcome for reactants to be transformed into products through the activated complex. On a reaction energy diagram, activation energy is represented by the height between the reactant energy level and the peak (activated complex) for the forward process.

Activation energy can be divided into:

• Forward Activation Energy \( (E_a^{\text{forward}}) \): The energy needed to go from the reactants to the activated complex.


• Reverse Activation Energy \( (E_a^{\text{reverse}}) \): The energy needed from the products back to the activated complex.

Higher activation energy means the reaction is slower, as more energy is needed to initiate it. Understanding activation energy helps predict the kinetics and feasibility of a reaction.

Energy Levels

Energy levels in a reaction energy diagram depict the relative potential energies of the reactants, products, and the activated complex. Placing these levels accurately helps in understanding the thermodynamics of the reaction.

Here's what the energy levels indicate:

• Reactants: Positioned at a lower energy level in an endothermic reaction.
• Products: Positioned at a higher energy level, signifying energy absorption.
• Activated Complex: Represents the highest energy level, showing the energy barrier.

The difference in energy levels between reactants and products gives us \( \Delta_{r} E \), which is positive for endothermic reactions. Recognizing these positions allows one to calculate and assess the energy changes occurring throughout the reaction process.