Problem 88 - 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 energies differ, but the energy evolved (net energy change) is the same in both reactions.

Step 1: Understand the Exothermic Reaction

An exothermic reaction releases energy to the surroundings. On a reaction coordinate diagram, the products lie at a lower energy level than the reactants, indicating that energy is released during the reaction.

Step 2: Draw the Reaction Coordinate Diagram for Uncatalyzed Reaction

Draw a graph with the 'Reaction Coordinate' on the x-axis and 'Energy' on the y-axis. 1. Start with reactants at a higher energy level. 2. Rise to a peak representing the transition state - this peak is the activation energy (Ea). 3. Descend to a lower energy level where the products are, indicating the net energy release.

Step 3: Identify Energies in the Uncatalyzed Diagram

Mark the energy difference between reactants and the peak as the activation energy (Ea). The difference between reactants and products shows the net energy change, which should be negative, indicating an exothermic reaction.

Step 4: Draw the Reaction Coordinate Diagram for Catalyzed Reaction

Draw a similar coordinate graph. 1. Reactants should start at the same energy level as before. 2. The peak will be lower than the previous graph, indicating a reduced activation energy due to the catalyst. 3. Products remain at the same energy level as before to show the same net energy change.

Step 5: Identify Energies in the Catalyzed Diagram

Mark the new, lower activation energy (Ea') between reactants and the peak. The energy difference between reactants and products remains the same, showing the net energy change is still the same as in the uncatalyzed reaction.

Key Concepts

Activation Energy

Activation energy is a crucial concept in understanding how chemical reactions proceed. It refers to the minimum amount of energy that reacting molecules need to successfully collide and form products. Imagine a ball sitting at the bottom of a hill; the reaction cannot start unless you provide it with enough push to climb to the top. In a reaction coordinate diagram, this is depicted as a peak. This peak represents the transition state, which is the high-energy, unstable state that occurs between the transformation of reactants into products.

• A higher peak means more activation energy is needed.
• A lower peak means less activation energy is required.

By understanding activation energy, you can determine how fast a reaction might occur. Lower activation energies usually mean faster reactions as reactants need less energy input to reach their transition state.

Exothermic Reaction

Exothermic reactions are reactions that release energy to their surroundings, often in the form of heat. You might experience this as warmth released by a chemical heat pack. On a reaction coordinate diagram, this type of reaction is represented by products that sit at a lower energy level than the original reactants.
In simple terms:

• The products have less stored energy than the reactants, signifying energy release.
• The overall energy change is negative, indicating energy output.

A classic example of an exothermic reaction is combustion, like burning wood or fossil fuels. During these reactions, energy is released, which is why they feel hot and are used to produce heat and power.

Catalyst Effect

Catalysts are substances that speed up the rate of a reaction without being consumed or permanently altered themselves. They achieve this by lowering the activation energy required for the reaction to proceed. Imagine a smaller hill or path that the ball can use to get to the other side; less energy is needed to push it along this route.

• With a catalyst, fewer molecules need to achieve high energy to react.
• The reaction proceeds faster because of this lowered energy barrier.

In the reaction coordinate diagram, a catalyst would appear as a lower peak compared to the uncatalyzed reaction. This doesn't change the position of the reactants and products, ensuring the overall energy released or consumed by the reaction remains unchanged.

Energy Change in Reactions

Energy change in reactions is what determines whether a reaction is exothermic or endothermic. In the context of a reaction coordinate diagram, it is the difference between the energy levels of the reactants and the products.

• If products are at a lower energy level, the reaction is exothermic.
• If products are at a higher energy level, the reaction is endothermic.

Understanding the energy change is crucial for predicting the behavior of chemical reactions. In industrial or laboratory settings, knowing whether a reaction releases or absorbs energy helps in designing reactors and choosing appropriate conditions to safely and efficiently conduct reactions. This concept also ties into everyday phenomena, from burning fuel to digesting food.