Problem 101 - Chemistry and Chemical Reactivity

Question

Draw a reaction coordinate diagram for an exothermic reaction that occurs in a single step. Identify the activation energy and the net energy change for the reaction on this diagram. Draw a second diagram that represents the same reaction in the presence of a catalyst, assuming a single-step reaction is involved here also. Identify the activation energy of this reaction and the energy change. Is the activation energy in the two drawings different? Does the energy evolved in the two reactions differ?

Step-by-Step Solution

Answer

The activation energy is lower with a catalyst, but the net energy change remains the same in both reactions.

Step 1: Understanding the Reaction Coordinate Diagram

A reaction coordinate diagram plots the progress of a reaction on the x-axis and the energy on the y-axis. For an exothermic reaction, the energy of the products is lower than the energy of the reactants, showing a net release of energy.

Step 2: Plotting the Single-Step Reaction

Draw a curve on the graph starting from the reactants' energy level. It rises to a peak, which represents the transition state, then descends to a lower energy level than the initial elevation representing the products' energy level.

Step 3: Identifying Activation Energy and Net Energy Change

Mark the height difference between the reactants' energy and the peak of the curve as the activation energy. The net energy change is the difference in energy level between the reactants and products, showing that energy is released.

Step 4: Drawing the Catalyzed Reaction Diagram

Draw a similar curve but with a lower peak on another graph to represent the catalyzed reaction. This indicates that the catalyst lowers the activation energy.

Step 5: Comparing Activation Energy and Energy Change

Identify and mark the new activation energy on the catalyzed diagram. Compare it to the original; it should be lower. The net energy change (difference between reactants and products) remains unchanged in both diagrams.

Key Concepts

Activation Energy

Activation energy is a crucial concept in understanding how reactions occur. It refers to the minimum energy required to initiate a chemical reaction. This energy is necessary to allow the reactants to reach the transition state, which is the point at the top of the energy barrier in the reaction coordinate diagram.
In the context of an exothermic reaction, the activation energy can be visualized as the peak in the graph that the reactants must overcome to transform into products. Even though the overall reaction releases energy, a certain input of energy is initially needed to start the process.
The height of this energy barrier determines the speed of the reaction. A higher activation energy means that fewer molecules will have the energy needed to react, resulting in a slower reaction.

Exothermic Reaction

An exothermic reaction is a type of chemical reaction that releases energy, usually in the form of heat. This occurs when the energy required to break the bonds in the reactants is less than the energy released when new bonds are formed in the products.
In a reaction coordinate diagram, an exothermic reaction is depicted with the energy level of the products lower than that of the reactants. This downward slope indicates the net release of energy as the reaction proceeds. It's important to note that the energy change, usually indicated by \( \Delta E \), is negative, reflecting the decreased energy in the products compared to the reactants.
Exothermic reactions are common in everyday life, such as combustion and some oxidation reactions, providing the warmth we often cherish in our daily environment.

Catalyst Effect

A catalyst is a substance that speeds up a chemical reaction without undergoing permanent changes itself. It achieves this by providing an alternate pathway for the reaction with a lower activation energy.
In a reaction coordinate diagram, the presence of a catalyst is visualized by a lower peak than the one in the uncatalyzed reaction. This reduction in the energy barrier allows more reactant molecules to have enough energy to reach the transition state, thus increasing the reaction rate.
Even though catalysts reduce the activation energy, they do not alter the net energy change of the reaction. The \( \Delta E \) between the reactants and products remains constant. Hence, catalysts are incredibly valuable in both industrial and biological processes, enabling more efficient energy transformations.

Energy Change

Energy change, or \( \Delta E \), is a measure of the difference in energy between the reactants and products in a chemical reaction. For exothermic reactions, this change is negative because the reaction releases energy to the surroundings.
In a reaction coordinate diagram, the energy change is depicted as the vertical difference between the energy levels of the reactants and the products. This difference illustrates the amount of energy released during the reaction.
Understanding energy change is vital because it helps predict whether a reaction will occur spontaneously. Exothermic reactions are often spontaneous because they increase the overall entropy of the system by releasing energy.