Question

(a) When chlorine atoms react with atmospheric ozone, what are the products of the reaction? (b) Based on average bond enthalpies, would you expect a photon capable of dissociating a \(\mathrm{C}-\mathrm{Cl}\) bond to have sufficient energy to dissociate a \(\mathrm{C}-\mathrm{Br}\) bond? \((\mathbf{c})\) Would you expect the substance \(\mathrm{CFBr}_{3}\) to accelerate depletion of the ozone layer?

Short answer

The reaction between chlorine atoms and ozone produces chlorine monoxide (ClO) and molecular oxygen (O2). A photon with enough energy to dissociate a C-Cl bond, which has a higher bond enthalpy (339 kJ/mol) than the C-Br bond (280 kJ/mol), would also have sufficient energy to dissociate a C-Br bond. CFBr3, containing bromine, can potentially contribute to the acceleration of ozone layer depletion through ozone destruction reactions involving bromine species.

Step-by-step solution

(a) Reaction products of chlorine atoms and ozone

To find the reaction products when chlorine atoms react with atmospheric ozone, we need to start with the chemical reaction. The reaction of chlorine atoms with ozone can be represented as: Cl + O3 → ClO + O2 The reaction products are chlorine monoxide (ClO) and molecular oxygen (O2).

(b) Comparison of bond dissociation energy of C-Cl and C-Br

In this part of the exercise, we have to compare the energy required to dissociate a C-Cl bond with that of a C-Br bond. We will use the average bond enthalpies of C-Cl and C-Br bonds to make the comparison. The average bond enthalpy for C-Cl is approximately 339 kJ/mol, whereas the average bond enthalpy for C-Br is around 280 kJ/mol. If a photon has enough energy to dissociate a C-Cl bond, which has a higher bond enthalpy, then it should also have enough energy to dissociate a C-Br bond, which has a lower bond enthalpy.

(c) CFBr3 and depletion of the ozone layer

Now we evaluate if the compound CFBr3 can accelerate the depletion of the ozone layer. To do that, we need to consider that ozone depletion occurs primarily by the release of radical halogen atoms, such as chlorine and bromine, into the atmosphere. The compound CFBr3 contains carbon, fluorine, and bromine atoms. Although fluorine is not known to participate in ozone depletion reactions, bromine is known to be a potent catalyst for ozone destruction. Bromine reacts with ozone (O3) to produce bromine monoxide (BrO) and molecular oxygen: Br + O3 → BrO + O2 And then, BrO can further react with a monatomic oxygen (O) to produce bromine (Br) and O2, allowing the cycle to repeat: BrO + O → Br + O2 Given that CFBr3 contains bromine, and considering the ozone depletion mechanism involving bromine species, it can be concluded that CFBr3 can potentially contribute to the acceleration of ozone layer depletion. In summary, the reaction between chlorine atoms and ozone produces chlorine monoxide (ClO) and molecular oxygen (O2). A photon capable of dissociating a C-Cl bond is likely to possess enough energy to dissociate a C-Br bond due to the lower bond enthalpy of the C-Br bond. Lastly, CFBr3 containing bromine may accelerate the depletion of the ozone layer by participating in ozone destruction reactions.

Key concepts

Chlorine Reaction with Ozone

When chlorine atoms react with ozone molecules in the atmosphere, a significant chemical process unfolds. Chlorine, represented as Cl, reacts with ozone (O3) to form chlorine monoxide (ClO) and molecular oxygen (O2). This reaction can be expressed by the equation: Cl + O3 → ClO + O2.
This process is a crucial component of ozone layer depletion. Chlorine atoms, often released from man-made compounds like chlorofluorocarbons (CFCs), act as catalysts in this reaction. This means that a single chlorine atom can repeatedly engage in reactions, breaking down many ozone molecules over time.
It's important to note that while the reaction itself remains unchanged, the presence of chlorine in the stratosphere is a driving force in reducing the concentration of ozone, a substance that shields the Earth from harmful ultraviolet (UV) radiation.

Bond Enthalpy

Bond enthalpy, sometimes referred to as bond dissociation energy, is a measure of the strength of a chemical bond. It indicates the amount of energy required to break a mole of bonds in gaseous substances. Understanding bond enthalpy is key to addressing part of our exercise that involves comparing the C-Cl and C-Br bonds.

• The C-Cl bond has an average enthalpy of 339 kJ/mol.
• The C-Br bond has an average bond enthalpy of 280 kJ/mol.

Because the bond enthalpy of a C-Cl bond is higher than that of a C-Br bond, it implies that more energy is needed to break a C-Cl bond compared to a C-Br bond. Therefore, a photon with sufficient energy to dissociate a C-Cl bond will also have enough energy to break a C-Br bond due to its lower bond strength. This distinction is important in understanding how various bonds can be broken in atmospheric chemistry and helps in predicting which molecules might pose greater risks for environmental and atmospheric reactions.

Halogen Chemistry in Atmosphere

The chemistry of halogens, particularly in the atmosphere, is vital in comprehending how they impact our environment, especially in the context of ozone layer depletion. Halogens, such as chlorine and bromine, are known for their potent oxidative properties. These elements are often released from man-made substances, including CFCs and halons, that migrate to the stratosphere.
Bromine, although less abundant than chlorine, can be significantly more destructive to the ozone layer. In reactions similar to chlorine, bromine radicals catalyze the depletion of ozone: Br + O3 → BrO + O2.
The cycle continues with the regeneration of bromine from reactions with other atmospheric molecules, perpetuating its ozone-depleting impact. Given that CFBr3 contains bromine, it has the potential to contribute to this cycle. While fluorine is not typically involved in these reactions, the presence and reactivity of bromine make it a significant concern for ozone depletion. Understanding this chemistry highlights the importance of regulating substances that release halogens into the atmosphere to preserve our protective ozone layer.