Question

Draw a potential energy diagram for an exothermic reaction and indicate on the diagram the location of the transition state.

Short answer

In an exothermic reaction potential energy diagram, the transition state is indicated by the highest peak between the reactants, which start higher on the y-axis, and the products, which appear lower due to energy release.

Step-by-step solution

Starting the diagram with reactants

Begin by drawing the y-axis to represent the potential energy and the x-axis to represent the reaction progress. Mark a point on the left side of the y-axis to indicate the energy level of the reactants. This should be a single point since we consider reactants as one species before the reaction occurs.

Indicating the products

Mark a point on the right side of the y-axis, lower than the reactants, to represent the energy level of the products in an exothermic reaction. The energy level of the products is lower than that of the reactants because energy is released during the reaction.

Drawing the transition state

Draw a peak between the reactants and products. The highest point on this peak represents the transition state, which is a high-energy, unstable state that occurs during the transformation from reactants to products.

Completing the diagram

Connect the point representing the reactants to the peak of the transition state with an upward curve, then connect the peak down to the products with a downward curve. This shows the overall energy decrease from reactants to products. The peak's height above the reactants represents the activation energy required to reach the transition state.

Key concepts

Transition State

The transition state refers to a high-energy, unstable condition that molecules pass through during a chemical reaction. Think of it as the top of a mountain that reactants must climb before they can roll down to become products. At the transition state, old bonds are partially broken and new ones are partially formed, it's neither the reactants nor the products but a fleeting, intermediate structure.

It's crucial for students to grasp that the transition state is the pinnacle of the potential energy barrier that separates reactants from products. Its exact position on a potential energy diagram is depicted at the highest point along the reaction path. To go beyond the textbook, one should consider the transition state as a quantum mechanical concept, involving complex interactions that can only be partially visualized in a simple diagram.

Activation Energy

Activation energy is the 'push' needed to start a reaction. On a potential energy diagram, it's the energy difference between the reactants and the transition state. More scientifically, it's known as the minimum energy required to initiate a chemical reaction. This concept is vital because it explains why certain reactions don't occur spontaneously and need an external energy source, like heat or light.

One way to visualize this is to imagine pushing a ball up a hill – the effort to push the ball to the top represents the activation energy. It is vital for students to understand that a higher activation energy means a slower reaction rate, as fewer reactant molecules have sufficient energy to overcome this energy barrier.

Reaction Progress

The term 'reaction progress' on a potential energy diagram is another way of illustrating the 'journey' from reactants to products. On the x-axis of the diagram, reaction progress shows time passing and molecular transformations occurring. It's the storyline of the chemical reaction.

As you follow the curve from left to right, you journey from the energy of the reactants, ascend to the transition state's peak, and then descend to the lower energy level of the products. Importantly, the diagram doesn't just convey the sequence of events, but helps predict the behavior of a reaction. For example, a long reaction progress might suggest a complex mechanism, whereas a short one might indicate a single-step reaction.

Exothermic Reaction Energy Changes

In an exothermic reaction, energy is released into the surroundings, which is often felt as warmth. On the potential energy diagram, this release is shown by having the products at a lower energy level than the reactants. The difference in height between these points reflects the amount of energy given off.

Understanding this energy change is critical, as it influences the spontaneity and feasibility of reactions. For instance, exothermic reactions tend to be favorable and self-sustaining once they've started because they release energy, making it easier for the subsequent reactions to reach the activation energy without external input. This concept is essential in fields such as thermodynamics and biochemistry, where energy release is harnessed for work.