Question

The average bond enthalpies of the \(\mathrm{C}-\mathrm{Cl}\) and \(\mathrm{C}-\mathrm{F}\) bonds are 340 and \(485 \mathrm{~kJ} / \mathrm{mol}\), respectively. Based on this information, explain why the \(\mathrm{C}-\mathrm{Cl}\) bond in a \(\mathrm{CFC}\) molecule is preferentially broken by solar radiation at \(250 \mathrm{nm}\)

Short answer

The C-Cl bond is broken because it requires less energy than the photon energy provided at 250 nm.

Step-by-step solution

Understanding Bond Enthalpy

Bond enthalpy refers to the amount of energy needed to break one mole of a bond in a gaseous molecule. The higher the bond enthalpy, the more energy is required to break the bond.

Bond Enthalpies Given

The problem provides that the average bond enthalpy of the C-Cl bond is 340 kJ/mol, while the C-F bond is 485 kJ/mol.

Photon Energy Calculation

Calculate the energy of the photons from solar radiation with a wavelength of 250 nm using the formula: \(E = \frac{hc}{\lambda}\), 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 \(250 \times 10^{-9} \text{ m}\).

Photon Energy in kJ/mol

Convert the energy calculated from Joules per photon to kJ/mol by multiplying by Avogadro's number \(6.022 \times 10^{23}\). This gives the energy in kJ that the solar radiation can use to break bonds.

Comparing Energies

Compare the photon energy in kJ/mol with the bond enthalpies. If the photon energy is greater than a bond's enthalpy, it can break that bond.

Conclusion

Since the photon energy is higher than 340 kJ/mol but lower than 485 kJ/mol, the photons can break the C-Cl bond but not the C-F bond.

Key concepts

C-Cl bond

The \(\mathrm{C}-\mathrm{Cl}\)bond represents the carbon-chlorine connection in molecules. This bond is critical in organic chemistry and is commonly found in chlorofluorocarbons (CFCs).
### Bond EnthalpyBond enthalpy is the energy required to break one mole of bonds in gas phase molecules. For the \(\mathrm{C}-\mathrm{Cl}\)bond, this energy is 340 kJ/mol. This relatively lower bond enthalpy indicates that less energy is required to break the bond compared to stronger bonds, such as the \(\mathrm{C}-\mathrm{F}\)bond.
### Implications in Molecular ReactionsKnowing the bond enthalpy helps predict reaction behaviors. C-Cl bonds in CFCs are broken under solar radiation, influencing environmental processes.

C-F bond

The \(\mathrm{C}-\mathrm{F}\)bond is the carbon-fluorine bond found in many organic molecules, including Teflon and other fluorinated compounds. It's known for its strength and resistance to breaking.
### Strong Bond CharacteristicsThe C-F bond has an average bond enthalpy of 485 kJ/mol, which is notably higher than that of the C-Cl bond. This makes C-F bonds remarkably stable and less likely to break in chemical reactions or under solar radiation.
### Real-world ApplicationsDue to its high bond enthalpy, C-F bonds provide durability and resistance in materials. This is one reason why fluorinated compounds are used in non-stick coatings and high-performance electrical insulation.

photon energy

Photon energy is the energy carried by a single photon, calculated using the formula:
\[E = \frac{hc}{\lambda}\]
where:

• \(E\) is the energy
• \(h\) is Planck's constant \(6.626 \times 10^{-34} \text{ J s}\)
• \(c\) is the speed of light \(3.00 \times 10^8 \text{ m/s}\)
• \(\lambda\) is the wavelength in meters

### Understanding Photon EnergyTo determine if solar radiation can break a bond, we calculate its photon energy. For a wavelength of 250 nm, following the formula, convert energy from Joules per photon to kJ/mol. This involves multiplying by Avogadro's number.### Breaking Chemical BondsPhoton energy comparisons with bond enthalpies can indicate which bonds will be broken. Higher energy photons can break stronger bonds, but weaker bonds require less energy.

solar radiation

Solar radiation consists of electromagnetic waves from the sun, including ultraviolet (UV) light, visible light, and infrared radiation. This energy plays a vital role in various natural and industrial processes.
### Types of Solar Radiation - UV Radiation: High energy, capable of breaking chemical bonds. - Visible Light: Lower energy, generally not sufficient to break bonds. - Infrared: Primarily associated with heat.
### Impact on Chemical Bonds Solar radiation, particularly UV light at 250 nm, has enough energy to break certain chemical bonds like the C-Cl bond. However, for stronger bonds like C-F, this radiation is insufficient. Understanding how different wavelengths impact molecular structures assists in studying environmental effects and developing chemical compounds that resist solar radiation.