Textbook Question
Consider this energy diagram:
a. How many elementary steps are involved in this reaction?
Ch.14 - Chemical Kinetics
Chapter 14, Problem 95c
Consider this energy diagram:
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c. Which step is rate limiting?
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Identify the steps in the reaction mechanism from the energy diagram. Each step corresponds to a peak and valley in the diagram, representing the transition state and intermediate, respectively.
Determine the activation energy for each step. The activation energy is the energy difference between the reactants and the transition state (peak) for that step.
Compare the activation energies of all the steps. The step with the highest activation energy is typically the rate-limiting step, as it requires the most energy to proceed and thus occurs the slowest.
Consider the overall reaction rate. The rate-limiting step controls the speed of the entire reaction because it is the slowest step in the sequence.
Conclude which step is rate-limiting based on the highest activation energy identified in the previous steps.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Energy Diagrams
Energy diagrams visually represent the energy changes during a chemical reaction. They typically show the energy of reactants, products, and the transition states, allowing us to identify the activation energy required for the reaction to proceed. The peaks in the diagram correspond to transition states, while the valleys represent stable states.
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Energy Diagrams
Rate Limiting Step
The rate limiting step in a reaction mechanism is the slowest step that determines the overall rate of the reaction. It is often associated with the highest energy transition state in the energy diagram. Identifying this step is crucial for understanding how the reaction proceeds and can help in optimizing reaction conditions.
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Activation Energy
Activation energy is the minimum energy required for reactants to undergo a chemical reaction. It is represented by the difference in energy between the reactants and the transition state in an energy diagram. A higher activation energy indicates a slower reaction rate, as fewer molecules will have sufficient energy to overcome this barrier.
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Related Practice
Textbook Question
Consider this energy diagram:
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b. Label the reactants, products, and intermediates.
Textbook Question
Consider this energy diagram:
d. Is the overall reaction endothermic or exothermic?
Textbook Question
Consider the reaction in which HCl adds across the double bond of ethene: HCl + H2C=CH2 → H3C-CH2Cl The following mechanism, with the accompanying energy diagram, has been suggested for this reaction:
Step 1 HCl + H2C=CH2 → H3C=CH2+ + Cl-
Step 2 H3C=CH2+ + Cl- → H3C-CH2Cl
a. Based on the energy diagram, determine which step is rate limiting.