Problem 100 - 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 energy released is the same in both reactions.

Step 1: Understand the Reaction Coordinate Diagram

A reaction coordinate diagram depicts the energy changes during a chemical reaction. It features the potential energy of the system on the y-axis and the reaction coordinate on the x-axis. An exothermic reaction shows a decrease in energy from reactants to products.

Step 2: Plot the Exothermic Reaction Diagram

Draw a curve starting higher on the left side (reactants) and ending lower on the right side (products). The peak of the curve represents the transition state, and the height of the peak above the reactants' energy is the activation energy. The difference in energy from reactants to products is the net energy change, showing a negative value for energy released.

Step 3: Label Activation Energy and Net Energy Change

Mark the activation energy as the vertical distance from the reactants' energy level to the peak of the curve. Indicate the net energy change as the vertical distance from reactants to products, which should be downward, signifying the release of energy.

Step 4: Draw the Catalyzed Reaction Diagram

Draw a second curve starting and ending at the same points as the first to show the same reactants and products. This curve should have a lower peak between them, indicating a lower activation energy due to the catalyst.

Step 5: Compare Activation Energies and Energy Changes

The activation energy for the catalyzed reaction is lower than for the uncatalyzed reaction, shown by a lower peak in the diagram. However, the net energy change from reactants to products remains the same for both reactions as the catalyst does not alter the overall energy released.

Key Concepts

Exothermic Reaction

An exothermic reaction is a chemical process that releases energy, usually in the form of heat, to its surroundings. This can often be seen in reactions like combustion, where substances burn and release heat and light. In terms of energy on a reaction coordinate diagram, exothermic reactions show a distinct pattern: the energy level of the products is lower than that of the reactants.

This means there is a net release of energy as the reaction progresses. In the diagram, you will notice a downward slope as you move from reactants to products, indicating that the products have less potential energy. This is why exothermic reactions can often feel warm to the touch.

Activation Energy

Activation energy is the minimum amount of energy required for a chemical reaction to occur. It represents the energy barrier that must be overcome for reactants to transform into products. On a reaction coordinate diagram, the activation energy is depicted as the peak of the curve that separates the reactants from the products.

Think of the activation energy as a hill that reactants need to climb in order to start forming products. Without enough energy to get over this hill, the reaction simply won't happen. However, once over the hill, the exothermic nature of the reaction helps in driving the process forward, releasing energy to the surroundings.

Catalyst

A catalyst is a substance that speeds up a chemical reaction without being consumed itself. It works by providing an alternative reaction pathway with a lower activation energy. When you see a reaction coordinate diagram with a catalyst, the peak is lower than in the uncatalyzed reaction.

This lower peak indicates that the energy required to start the reaction is reduced. Catalysts are particularly useful because they make it easier and faster for a reaction to occur by reducing the energy barrier. Despite the catalyst's presence, the total energy change from reactants to products remains unchanged. Thus, while the catalyst gets the reaction going quicker, it doesn't alter the net energy released by an exothermic reaction.

Energy Change

Energy change in a reaction refers to the difference in energy between reactants and products. For an exothermic reaction, this change is negative, reflecting the release of energy as the reaction proceeds.

On a reaction coordinate diagram, the energy change is shown as the vertical drop from the reactants' energy level to the products' energy level. This drop helps visualize how much energy is actually released during the reaction.

Importantly, whether or not a catalyst is present, the net energy change from reactants to products remains consistent. A catalyst doesn't change the amount of energy released, it just lowers the activation energy needed to start the reaction. This constancy in energy change ensures that the reaction yields the same amount of energy, contributing to its classification as an exothermic reaction.