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 dia- gram depicting the energy of the reaction as it progresses. Label the position of the reactants and products and indicate the activation ener- gy and enthalpy of reaction.

Short answer

In the diagram, the reactants' baseline is at the bottom, leading up to a peak representing the activation energy, which is twice as high as the vertical distance to the higher final energy level of the products, indicating the endothermic reaction and the enthalpy change.

Step-by-step solution

Draw a Baseline for Reactants

Begin with a horizontal line near the bottom of the diagram to represent the energy baseline of the reactants before the reaction starts.

Draw the Activation Energy

From the baseline, draw a curve that peaks upwards to represent the activation energy required for the reaction. Since the reaction is endothermic, make sure that the peak of the curve is higher than the starting point to reflect the input of energy.

Draw the Energy of Products

From the peak of the activation energy curve, draw the curve downward but ensure it ends at a point higher than the starting baseline because the reaction is endothermic, meaning the products have more energy than the reactants.

Indicate the Enthalpy Change

Label the vertical distance between the reactants' baseline and the products' final energy level as the enthalpy change (\(\triangle H\)).

Label the Activation Energy

Mark the vertical distance between the reactants' baseline and the peak of the activation energy curve as the activation energy (\(E_a\)). Indicate that the activation energy is twice the value of the enthalpy change.

Finalize the Diagram

Label the 'Reactants' on the left side of the baseline and the 'Products' on the right side of the final energy level. Add directional arrows to show the progress of the reaction and the energy flow.

Key concepts

Endothermic Reaction

When we talk about an endothermic reaction, we describe a process where energy is absorbed from the surroundings into the system as the reaction proceeds. In simpler terms, this means that the reactants need to draw in heat to turn into products, typically resulting in a temperature drop in the external environment. For students, an everyday example might be the experience of cold packs used to treat sports injuries, which become cool due to such a reaction taking place.

Endothermic reactions are characterized by a positive enthalpy change, implying that the energy content of the products is higher than that of the reactants. This is important in our energy diagram as it will show the final energy level of the products above that of the reactants. Understanding this concept is crucial as it lays the foundation for why endothermic reactions require heat and how that is represented on energy diagrams.

Activation Energy

Activation energy, often symbolized as E_a, is a fundamental concept when discussing chemical reactions. It represents the minimum amount of energy needed for the reactants to transform into products. You can think of it as the initial push necessary to start a reaction, much like the effort needed to push a boulder up a hill before it can roll down the other side.

In the energy diagram, this is depicted as a peak that must be overcome before the reaction can proceed towards product formation. For the endothermic reaction described in our exercise, the activation energy is illustrated as being twice the value of the enthalpy change. This visual portrayal on a potential energy diagram helps students grasp that despite the energy absorbed in the reaction, a higher initial energy investment is necessary to initiate the process.

Enthalpy Change

Enthalpy change, denoted as \(\Delta H\), is a term used to describe the heat exchange in a reaction at constant pressure. It is a measure of the total energy change in a system. When \(\Delta H\) is positive, it indicates an endothermic reaction, where the system gains heat and the energy of the products is greater than that of the reactants. Conversely, a negative \(\Delta H\) signifies an exothermic reaction where the system releases heat.

In our problem, the enthalpy change is represented on the energy diagram as the difference in height between the reactants' and products' energy levels. By explicitly marking this on the diagram, students can visually associate the terms 'endothermic' and 'positive enthalpy change' with the higher energy level of the products compared to the reactants.

Potential Energy Diagram

A potential energy diagram provides a graphical representation of the energy changes during a chemical reaction. It indicates how the potential energy of a system changes from reactants to products, showing the activation energy peak and the overall energy change. These diagrams are powerful tools for visual learners as they can literally see the 'hills' and 'valleys' that represent the energy barriers and changes.

In an endothermic reaction diagram, the products are placed at a higher energy level than the reactants, indicating that energy is absorbed. The peak, representing the point of highest energy, reflects the necessary activation energy. To improve students' understanding, indicating the specific values for activation energy and enthalpy change, their relativity, and the direction of the reaction can be useful. Clear labels and arrows aid in depicting the flow of the reaction and the energy dynamics involved.