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Ch.18 - Chemistry of the Environment
All textbooksBrown 15th EditionCh.18 - Chemistry of the EnvironmentProblem 25
Chapter 18, Problem 25

The average bond enthalpies of the C ¬ F and C ¬ Cl bonds are 485 kJ/mol and 328 kJ/mol, respectively. Given the fact that O2, N2, and O in the upper atmosphere absorb most of the light with wavelengths shorter than 240 nm, would you expect the photodissociation of C ¬ F bonds to be significant in the lower atmosphere?

Verified step by step guidance
1
Step 1: Understand the concept of bond enthalpy, which is the energy required to break one mole of a bond in a gaseous molecule. Here, we are given the bond enthalpies for C-F and C-Cl bonds.
Step 2: Convert the given wavelength (240 nm) to energy using the equation E = \frac{hc}{\lambda}, where h is Planck's constant (6.626 \times 10^{-34} \text{ J s}), c is the speed of light (3.00 \times 10^8 \text{ m/s}), and \lambda is the wavelength in meters.
Step 3: Calculate the energy in kJ/mol by converting the energy from joules to kilojoules and then multiplying by Avogadro's number (6.022 \times 10^{23} \text{ mol}^{-1}).
Step 4: Compare the calculated energy from Step 3 with the bond enthalpy of the C-F bond (485 kJ/mol). If the energy is greater than or equal to the bond enthalpy, photodissociation is likely to occur.
Step 5: Conclude whether the photodissociation of C-F bonds is significant in the lower atmosphere based on the comparison in Step 4.

Key Concepts

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

Bond Enthalpy

Bond enthalpy, or bond dissociation energy, is the amount of energy required to break a bond in a molecule, measured in kJ/mol. It reflects the strength of a bond; higher values indicate stronger bonds. In this question, the bond enthalpies of C-F and C-Cl are compared to assess the likelihood of photodissociation, which is the breaking of bonds due to light absorption.
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Enthalpy of Formation

Photodissociation

Photodissociation is the process by which a chemical bond is broken due to the absorption of light, typically ultraviolet (UV) radiation. The energy from the absorbed light must be sufficient to overcome the bond enthalpy. In the context of the question, the significance of C-F bond photodissociation in the lower atmosphere is evaluated based on the available UV light and the bond strength.

Atmospheric Chemistry

Atmospheric chemistry studies the chemical composition and reactions occurring in the atmosphere. The presence of specific gases, such as O2 and N2, and their ability to absorb UV light influences the photodissociation processes. Understanding the atmospheric conditions and the wavelengths of light absorbed by different molecules is crucial for predicting the behavior of compounds like C-F in the lower atmosphere.
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Related Practice
Textbook Question

Which of the following reactions in the stratosphere cause an increase in temperature there? (a) O(g) + O2(g) → O3+(g) (b) O3*(g) + M(g) → O3(g) + M*(g) (c) O2(g) + hv → 2 O(g) (d) O(g) + N2(g) → NO(g) + N(g) (e) All of the above

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Textbook Question

(a) What is the difference between chlorofluorocarbons and hydrofluorocarbons?

Textbook Question

(a) When chlorine atoms react with atmospheric ozone, what are the products of the reaction?

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Textbook Question

(b) Based on average bond enthalpies, would you expect a photon capable of dissociating a C-Cl bond to have sufficient energy to dissociate a C-Br bond?