Problem 94 - Chemistry and Chemical Reactivity

Question

Draw a reaction coordinate diagram for an exothermic reaction that occurs in a single step. Mark the activation energy, and identify 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. 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

With a catalyst, activation energy is lower, but energy release is unchanged.

Step 1: Understand Reaction Coordinate Diagrams

A reaction coordinate diagram illustrates the progress of a reaction over time, showing the energy of reactants and products on the y-axis and the reaction progress on the x-axis. An exothermic reaction has products with lower energy than reactants.

Step 2: Sketching the Exothermic Reaction Diagram

Draw a curve starting from a higher energy level (reactants) going over a peak (transition state) and ending at a lower energy level (products). The "peak" represents the energy of the transition state.

Step 3: Identify and Mark Activation Energy

On the diagram, the difference from the reactant energy level to the peak of the curve is the activation energy (Ea). Mark this distance on the diagram.

Step 4: Identify and Mark Net Energy Change

The net energy change is the difference between the energy of the reactants and the energy of the products. This reflects the energy released in the exothermic reaction. Mark this distance on the diagram as well.

Step 5: Draw Diagram with Catalyst

Draw a second reaction coordinate diagram underneath the first. The catalyzed reaction curve should have a lower peak than the uncatalyzed reaction, indicating a reduced activation energy due to the catalyst.

Step 6: Mark Changes on Catalyzed Diagram

The activation energy with the catalyst (Eac) is lower and should be marked on the second diagram. The net energy change between reactants and products remains the same as in the uncatalyzed reaction.

Step 7: Compare Activation Energies and Energy Changes

With the catalyst, the activation energy is lower (Eac < Ea). However, the net energy change (difference between reactants and products) stays the same, as the catalyst does not alter the starting or ending energy levels.

Key Concepts

Exothermic Reaction

In an exothermic reaction, energy is released to the surroundings. This means that the energy of the products is lower than the energy of the reactants. Imagine starting at the top of a hill (reactants) and ending in a valley (products). The drop in energy is what we observe in exothermic reactions.

• Energy flows out of the system, causing the temperature to rise.
• Examples include combustion reactions like burning wood.

In a reaction coordinate diagram, this is depicted as a downward slope from reactants to products. Such a diagram shows how energy decreases over the course of the reaction, highlighting the energy release in exothermic processes.

Activation Energy

Activation Energy (Ea) is the energy barrier that must be overcome for a reaction to proceed. Think of it as the bump on a hill that must be climbed before rolling down into a valley.

• Even in exothermic reactions, this energy input is necessary to initiate a reaction.
• It is depicted as the peak in the reaction coordinate diagram.

To find the activation energy on the diagram, look for the difference in height between the energy of the reactants and the highest point of the curve. This peak represents the transition state, where bonds are broken and new ones are formed.

Catalyst Effect

A catalyst is a substance that speeds up a chemical reaction without being consumed. It lowers the activation energy required for the reaction.

• The catalyzed pathway appears as a lower peak on the reaction coordinate diagram.
• It allows the reaction to proceed at a faster rate without additional energy input.

Importantly, a catalyst does not change the net energy change of the reaction. It only provides an easier path for reactants to become products by offering a lower energy route. This distinction is clear in the reaction coordinate diagram where the starting and ending points remain the same whether a catalyst is present or not.

Net Energy Change

The net energy change in a reaction refers to the difference in energy between the reactants and the products. In exothermic reactions, this value is negative, indicating a release of energy to the surroundings.

• On a reaction coordinate diagram, the net energy change is depicted by the difference in energy levels between reactants and products.
• This difference highlights the amount of energy released during the reaction.

Even when a catalyst is used, the net energy change does not vary. The diagram shows that irrespective of the path (catalyzed or uncatalyzed), the energy difference from start (reactants) to finish (products) remains constant.