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Ch.18 - Chemistry of the Environment
Chapter 18, Problem 28b

(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?

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1
Step 1: Understand the concept of bond enthalpy. Bond enthalpy is the energy required to break a bond between two atoms. The higher the bond enthalpy, the stronger the bond and the more energy is required to break it.
Step 2: Look up the average bond enthalpies for C-Cl and C-Br. The bond enthalpy for C-Cl is typically higher than that for C-Br.
Step 3: Understand the concept of photon energy. The energy of a photon is directly proportional to its frequency and inversely proportional to its wavelength. Therefore, a photon with higher energy has a higher frequency and a shorter wavelength.
Step 4: Apply the concepts. If a photon has enough energy to break a C-Cl bond (which has a higher bond enthalpy), it should also have enough energy to break a C-Br bond (which has a lower bond enthalpy).
Step 5: Conclude. Therefore, a photon capable of dissociating a C-Cl bond would have sufficient energy to dissociate a C-Br bond.

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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 specific bond in a molecule in the gas phase. It is a measure of bond strength; stronger bonds have higher bond enthalpies. Understanding bond enthalpy is crucial for predicting whether a photon can provide enough energy to break a particular bond, such as C-Cl or C-Br.
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Photon Energy

A photon is a quantum of electromagnetic radiation, and its energy is directly proportional to its frequency, as described by the equation E = hν, where E is energy, h is Planck's constant, and ν is frequency. To determine if a photon can dissociate a bond, one must compare the energy of the photon to the bond enthalpy of the bond in question. If the photon's energy is greater than or equal to the bond enthalpy, bond dissociation can occur.
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Comparative Bond Strengths

Comparative bond strengths refer to the differences in bond enthalpies between similar bonds, such as C-Cl and C-Br. Generally, C-Cl bonds are stronger than C-Br bonds due to the smaller size and greater electronegativity of chlorine compared to bromine. This concept is essential for predicting whether a photon that can dissociate a C-Cl bond can also dissociate a C-Br bond, as it involves understanding the relative energies required for breaking these bonds.
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