Question

A chemical reaction is endothermic and has an activation energy that is twice the value of the enthalpy change of the reaction. Draw a diagram depicting the energy of the reaction as it progresses. Label the position of the reactants and products and indicate the activation energy and enthalpy of reaction.

Short answer

An endothermic reaction diagram shows reactants at a lower energy level, a peak indicating activation energy, and products at a higher energy level. Activation energy is at a peak twice the height of the enthalpy change.

Step-by-step solution

Understanding Endothermic Reactions and Activation Energy

In an endothermic reaction, the energy required to initiate the reaction (activation energy) is greater than the energy released. This means that the overall energy of the products will be higher than that of the reactants. Since the activation energy is twice the value of the enthalpy change, it emphasizes that a significant energy barrier must be overcome for the reaction to proceed.

Sketching the Reaction Coordinate Diagram

Draw a horizontal line towards the bottom of your graph to represent the energy level of the reactants. Above this line, draw a peak to represent the activation energy — make sure this peak is significantly higher than the reactant line as it is twice the enthalpy change. From the peak, draw a line down that ends higher than the reactants' line, representing the energy level of the products in an endothermic reaction.

Labeling the Diagram

Label the starting line as 'Reactants' and the ending line as 'Products'. The peak should be labeled as 'Activation Energy'. The difference in height between the reactants and the products lines should be labeled as '\(\Delta H\)', the enthalpy change. Also label this as an endothermic reaction, indicating that the overall energy has increased from reactants to products.

Key concepts

Activation Energy

Understanding the concept of activation energy is crucial when studying chemical reactions. Imagine activation energy as the 'entrance fee' that reactants must pay to transform into products in a chemical reaction. It represents the minimum amount of energy required to start a reaction by breaking the bonds in the reactants.

In the context of an endothermic reaction, where the overall process absorbs energy, this 'fee' is even more important. The problem states that the activation energy is twice the enthalpy change (ΔH), highlighting a substantial energy barrier that must be overcome. With such a high activation energy, reactants need to gain a significant amount of energy to reach the transition state, where old bonds break and new bonds begin to form.

Key Points about Activation Energy:

• It is the threshold energy that initiates the reaction.
• Higher activation energy means reactants need to absorb more energy to react.
• In an endothermic reaction, the activation energy is particularly crucial as it dictates the energy input needed to drive the reaction forward.

Enthalpy Change

The enthalpy change (ΔH) represents the difference in energy between the products and reactants of a chemical reaction. For an endothermic reaction, ΔH is positive, indicating that the products have higher energy than the reactants because the reaction absorbs energy from the surroundings. This is the reason why you feel a cooling effect when an endothermic reaction takes place—it takes in heat, leaving the environment colder.

The problem points out that the activation energy is twice as much as the ΔH of the reaction. This shows that although the reaction requires substantial energy to proceed, the net energy absorbed (the change in enthalpy) is only half of that initial input.

Understanding Enthalpy Change:

• A positive ΔH signifies an endothermic process.
• It tells us how much energy is absorbed or released during the reaction, excluding work done by the system.
• In our exercise, the enthalpy change is essentially the 'net gain' of energy by the system.

Reaction Coordinate Diagram

A reaction coordinate diagram is a valuable tool for visualizing the energy changes that occur during a chemical reaction. It plots the progress of the reaction against the potential energy of the system, showing how energy changes from reactants to products.

In our endothermic reaction example, the diagram typically starts with a line representing the energy level of the reactants. A peak follows this, indicating the activation energy needed to reach the transition state. Finally, the diagram ends with a line higher than where it began, representing the energy level of the products.

Components of a Reaction Coordinate Diagram:

• The horizontal axis represents the progression of the reaction from reactants to products.
• The vertical axis shows the potential energy of the system at each stage.
• The peak represents the activation energy barrier.
• The difference in height between the end and start signifies the enthalpy change of the reaction.

By analyzing these diagrams, students can gain insights into the energetics of reactions and the role of activation energy and enthalpy change.