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Ch.15 - Chemical Kinetics
Chapter 15, Problem 95d

Consider this energy diagram:
d. Is the overall reaction endothermic or exothermic?

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Identify the energy levels of the reactants and products on the energy diagram.
Determine the relative positions of the reactants and products: if the products are at a lower energy level than the reactants, the reaction is exothermic; if higher, it is endothermic.
Look for the energy difference between the reactants and products, which is the enthalpy change (\( \Delta H \)).
If \( \Delta H \) is negative, the reaction releases energy and is exothermic. If \( \Delta H \) is positive, the reaction absorbs energy and is endothermic.
Conclude whether the reaction is endothermic or exothermic based on the sign of \( \Delta H \).

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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 decrease in temperature of the environment. Conversely, exothermic reactions release energy, often in the form of heat, leading to an increase in the temperature of the surroundings. Understanding whether a reaction is endothermic or exothermic is crucial for interpreting energy diagrams.
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Energy Diagrams

Energy diagrams visually represent the energy changes during a chemical reaction. They typically show the energy of reactants and products, along with the activation energy required for the reaction to proceed. The position of the products relative to the reactants indicates whether the reaction is endothermic (products higher in energy) or exothermic (products lower in energy).
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Activation Energy

Activation energy is the minimum energy required for a reaction to occur. It represents the energy barrier that must be overcome for reactants to transform into products. In energy diagrams, this is depicted as the peak of the curve, and understanding this concept helps in analyzing the overall energy changes and the nature of the reaction.
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Related Practice
Textbook Question

Dinitrogen pentoxide decomposes in the gas phase to form nitrogen dioxide and oxygen gas. The reaction is first order in dinitrogen pentoxide and has a half-life of 2.81 h at 25 °C. If a 1.5-L reaction vessel initially contains 745 torr of N2O5 at 25 °C, what partial pressure of O2 is present in the vessel after 215 minutes?

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Textbook Question

Iodine atoms combine to form I2 in liquid hexane solvent with a rate constant of 1.5⨉1010 L/mols. The reaction is second order in I. Since the reaction occurs so quickly, the only way to study the reaction is to create iodine atoms almost instantaneously, usually by photochemical decomposition of I2. Suppose a flash of light creates an initial [I] concentration of 0.0100 M. How long will it take for 95% of the newly created iodine atoms to recombine to form I2?

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Textbook Question

Consider this energy diagram:

a. How many elementary steps are involved in this reaction?

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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.

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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

b. What is the expected order of the reaction based on the proposed mechanism?

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Textbook Question

The desorption (leaving of the surface) of a single molecular layer of n-butane from a single crystal of aluminum oxide is found to be first order with a rate constant of 0.128/s at 150 K. a. What is the half-life of the desorption reaction?-

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