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Ch. 5 - Chemical Reaction Analysis: Thermodynamics and Kinetics
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
Mullins1st EditionOrganic Chemistry: A Learner Centered ApproachISBN: 9780137566471Not the one you use?Change textbook
All textbooksMullins 1st EditionCh. 5 - Chemical Reaction Analysis: Thermodynamics and KineticsProblem 54d
Chapter 4, Problem 54d

Draw a reaction coordinate diagram, making sure to label reactants (R), products (P), intermediates (I), transition states (‡), activation energies ( Ea) , and ∆G°, for each of the following.
(d) a slightly exothermic, three-step reaction where the third step is rate-determining.

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Start by understanding the reaction type: A slightly exothermic reaction means the overall free energy change (∆G°) is negative but small, indicating that the products are slightly more stable than the reactants. A three-step reaction implies there are three distinct stages, each with its own transition state and possibly intermediates.
Draw the x-axis labeled as 'Reaction Progress' and the y-axis labeled as 'Energy'. This will serve as the reaction coordinate diagram. The energy levels will represent the relative stability of reactants, intermediates, transition states, and products.
Place the reactants (R) at a higher energy level than the products (P) since the reaction is exothermic. Label the products (P) slightly lower than the reactants to reflect the small ∆G° value. Ensure the energy difference between R and P is small to indicate the slight exothermic nature.
Add three transition states (‡) along the reaction progress, one for each step. The third transition state should have the highest energy barrier (activation energy, Eₐ) because the third step is rate-determining. Label each transition state with ‡₁, ‡₂, and ‡₃, and ensure ‡₃ is the highest peak.
Include intermediates (I) between the transition states. Label them as I₁ and I₂, and place them at energy levels lower than the transition states but higher than the reactants and products. Ensure the diagram reflects the energy changes and activation energies (Eₐ) for each step, and clearly mark ∆G° as the difference between the reactants and products.

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Key Concepts

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

Reaction Coordinate Diagram

A reaction coordinate diagram is a graphical representation that illustrates the energy changes during a chemical reaction as it progresses from reactants to products. The x-axis typically represents the reaction progress, while the y-axis shows the energy of the system. Key features include labeled reactants, products, intermediates, and transition states, which help visualize the energy barriers and the overall energy change of the reaction.
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Activation Energy (Eₐ)

Activation energy (Eₐ) is the minimum energy required for a chemical reaction to occur. It represents the energy barrier that must be overcome for reactants to transform into products. In a reaction coordinate diagram, Eₐ is depicted as the difference in energy between the reactants and the highest point of the transition state. Understanding Eₐ is crucial for determining the rate of a reaction and the influence of temperature on reaction kinetics.
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Rate-Determining Step

The rate-determining step is the slowest step in a multi-step reaction mechanism, which dictates the overall reaction rate. In the context of a three-step reaction, identifying the rate-determining step is essential for understanding how the reaction proceeds and how intermediates are formed. This step typically has the highest activation energy and is represented in the reaction coordinate diagram as the highest transition state, influencing the kinetics and thermodynamics of the entire reaction.
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Related Practice
Textbook Question

Draw a reaction coordinate diagram, making sure to label reactants (R), products (P), intermediates (I), transition states (‡), activation energies ( Ea) , and ∆G°, for each of the following.

(a) an exothermic, one-step reaction

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Textbook Question

Calculate ∆H° for the following alkene addition reaction, one we discuss further in Chapter 7. Predict the sign of ∆S° . (The BDE for C―C π bond is approximately 65 kcal/mol.)

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Textbook Question

Draw a reaction coordinate diagram, making sure to label reactants (R), products (P), intermediates (I), transition states (‡), activation energies ( Ea) , and ∆G°, for each of the following.

(c) a slightly endothermic, three-step reaction where the first step is rate-determining

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Textbook Question

Draw a reaction coordinate diagram, making sure to label reactants (R), products (P), intermediates (I), transition states (‡), activation energies ( Ea) , and ∆G°, for each of the following.

(b) an exothermic, two-step reaction where the second step is rate-determining

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Textbook Question

(b) Mechanistically, the reaction occurs as shown below. Why is this reaction favored? Based on the stability of the anions, estimate Keq.

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Textbook Question

Reactions (a) and (b) are disfavored overall (∆G° > 0), yet they are favored based on ∆H°. Identify the bonds formed and broken for (a) and (b).

(b)

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