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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 54b
Chapter 4, Problem 54b

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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1
Start by understanding the key features of a reaction coordinate diagram. The x-axis represents the reaction coordinate (progress of the reaction), and the y-axis represents the potential energy of the system.
Since the reaction is exothermic, the energy of the products (P) will be lower than the energy of the reactants (R). Place the reactants at a higher energy level and the products at a lower energy level on the diagram.
The reaction is two-step, so there will be an intermediate (I) between the reactants and products. Draw a valley between two peaks to represent the intermediate. The first peak corresponds to the transition state (‡) of the first step, and the second peak corresponds to the transition state of the second step.
Label the activation energy (Eₐ) for each step. The activation energy is the energy difference between the reactants and the first transition state for the first step, and between the intermediate and the second transition state for the second step. Since the second step is rate-determining, its activation energy will be higher than that of the first step.
Finally, label the overall free energy change (∆G°) as the difference in energy between the reactants and products. Since the reaction is exothermic, ∆G° will be negative. Ensure all components (R, P, I, ‡, Eₐ, and ∆G°) are clearly labeled on the diagram.

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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 visually represents the energy changes during a chemical reaction. The x-axis typically shows the progress of the reaction, while the y-axis indicates the energy level. Key features include the energy of reactants, products, intermediates, and transition states, which help illustrate the energy barriers and overall energy change of the reaction.
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Exothermic Reaction

An exothermic reaction is a type of chemical reaction that releases energy, usually in the form of heat, to the surroundings. In a reaction coordinate diagram, this is depicted by the products being at a lower energy level than the reactants. The difference in energy between the reactants and products is represented as a negative change in Gibbs free energy (∆G°), indicating that the reaction is thermodynamically favorable.
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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 two-step exothermic reaction, the second step being rate-determining means that it has the highest activation energy (Eₐ) and thus controls how quickly the reaction proceeds. This step is represented by the highest transition state on the reaction coordinate diagram, highlighting its significance in the reaction pathway.
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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

For each of the reaction coordinate diagrams shown, (i) indicate the number of steps in the reaction, (ii) label the intermediates, (iii) identify the rate-determining step, and (iv) tell whether Keq is greater than, less than, or equal to zero.

(a)

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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

Predict the product(s) of the following halogenation reactions. Only one equivalent of the halogen is used in each case. If the reaction proceeds selectively, indicate this by only drawing the major product. If the reaction is not selective, draw all possible products.

(f)

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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.

(d) a slightly exothermic, three-step reaction where the third step is rate-determining.

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