Consider a reaction that occurs by the following mechanism:
A + BC → AC + B
AC + D → A + CD
The potential energy profile for this reaction is as follows:
(d) Is the reaction endothermic or exothermic? Add labels to the diagram that show the values of the energy of reaction ΔE and the activation energy Ea for the overall reaction.

Name: Chemistry
Author: McMurry
ISBN: 9781292336145

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Identify the overall reaction by combining the two given elementary steps: A + BC + D → CD + B.

Determine the enthalpy change (ΔE) for the overall reaction by comparing the energy of the reactants and products. If the products have lower energy than the reactants, the reaction is exothermic; if higher, it is endothermic.

Locate the activation energy (Ea) on the potential energy diagram. This is the energy difference between the reactants and the highest point on the energy profile (the transition state).

Label the potential energy diagram with ΔE, showing the difference in energy between reactants and products, and Ea, showing the energy required to reach the transition state from the reactants.

Conclude whether the reaction is endothermic or exothermic based on the sign of ΔE: negative ΔE indicates exothermic, while positive ΔE indicates endothermic.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Endothermic and Exothermic Reactions

Endothermic reactions absorb energy from their surroundings, resulting in a net increase in the system's energy. Conversely, exothermic reactions release energy, leading to a net decrease in energy. The sign of the change in enthalpy (ΔH) indicates the type of reaction: positive for endothermic and negative for exothermic.

Activation Energy (Ea)

Activation energy is the minimum energy required for a chemical reaction to occur. It represents the energy barrier that reactants must overcome to transform into products. In a potential energy profile, Ea is depicted as the difference in energy between the reactants and the highest point of the energy barrier (transition state).

Energy of Reaction (ΔE)

The energy of reaction (ΔE) is the difference in energy between the reactants and the products. It indicates whether the overall reaction is endothermic or exothermic. A negative ΔE signifies that the products have lower energy than the reactants, indicating an exothermic reaction, while a positive ΔE indicates an endothermic reaction.

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