Question

Explain why, atom for atom, stratospheric bromine destroys more ozone than does chlorine.

Short answer

Bromine destroys more ozone atom for atom due to its higher reactivity and catalytic efficiency compared to chlorine.

Step-by-step solution

Understand Stratospheric Ozone Destruction

Ozone (O₃) destruction in the stratosphere occurs when ozone molecules react with halogen atoms, such as chlorine (Cl) and bromine (Br), resulting in the breakdown of ozone into oxygen molecules.

Explore Halogen Efficiency

The effectiveness of halogen atoms in destroying ozone is known as their ozone depletion potential (ODP). Bromine atoms are significantly more efficient at ozone destruction than chlorine atoms, with bromine being about 60 times more effective on a per-atom basis.

Examine the Reaction Mechanism

Both bromine and chlorine participate in catalytic cycles that break down ozone. However, bromine compounds are more reactive and less stable, allowing them to rapidly regenerate bromine atoms in the stratosphere, thus facilitating more ozone-destroying reactions.

Consider Atmospheric Concentrations

Although bromine is more efficient, chlorine is present in much higher concentrations in the atmosphere, which is why chlorine still has a significant impact on ozone depletion. But atom for atom, bromine is more destructive.

Summarize the Factors

In essence, bromine's higher reactivity and catalytic efficiency in the destruction of ozone molecules, despite its lower atmospheric concentration compared to chlorine, explain why it destroys more ozone atom for atom.

Key concepts

Halogen Atoms

Halogen atoms, specifically chlorine and bromine, play a crucial role in the destruction of stratospheric ozone. These atoms are part of a group of elements known as halogens, which also include fluorine, iodine, and astatine. However, when it comes to ozone depletion, chlorine and bromine are the main culprits. Upon their release into the stratosphere, often from human-made chemicals like chlorofluorocarbons (CFCs) and halons, they initiate chemical reactions that destroy ozone molecules.

Here's how it works: when CFCs reach the stratosphere, they are broken down by ultraviolet (UV) radiation, releasing chlorine atoms. Similarly, chemicals releasing bromine are broken down, releasing bromine atoms. These free halogen atoms engage in a series of reactions that result in the breakdown of ozone (O₃) into oxygen molecules (O₂).

• Halogen atoms act as catalysts, meaning they can facilitate the breakdown of a large number of ozone molecules without being consumed in the reaction.
• Bromine is particularly more reactive and efficient than chlorine, thus causing more extensive damage to ozone layer, atom for atom.

Ozone Depletion Potential (ODP)

Ozone Depletion Potential (ODP) is a measure of a substance’s capability to cause ozone layer destruction compared to a reference substance, usually CFC-11, which is assigned an ODP of 1. It allows scientists to compare the impacts of different chemicals on the ozone layer.

Bromine has a substantially higher ODP than chlorine. This is because bromine is far more efficient at breaking down ozone. While bromine's actual concentration in the atmosphere is smaller than chlorine, each atom of bromine can destroy much more ozone. To put it in perspective, bromine is about 60 times more effective at destroying ozone than chlorine on a per-atom basis.

• A high ODP indicates a greater potential for ozone layer damage.
• The high efficiency of bromine makes it a potent ozone-depleting element.
• Efforts to curb substances with high ODPs, such as bromine-releasing compounds, have been key to international environmental agreements like the Montreal Protocol.

Catalytic Cycles in Ozone Destruction

Catalytic cycles are chain reactions involving chlorine and bromine that lead to ozone depletion in the stratosphere. These cycles are not one-time events, but ongoing processes that substantially decrease the concentration of ozone.

Here's how catalytic cycles in ozone destruction generally work:

• First, a halogen atom like chlorine (Cl) or bromine (Br) reacts with an ozone molecule (O₃), converting it into oxygen (O₂) and forming a compound like chlorine monoxide (ClO) or bromine monoxide (BrO).
• Then, in subsequent reactions, these compounds further react to release the halogen atom back into the stratosphere, ready to destroy more ozone. For example, ClO might react with another molecule to release Cl again, thereby participating in further destruction cycles.
• The key feature of these cycles is that the halogen atoms are regenerated, thus a single halogen atom can participate in many rounds of ozone destruction.

The reason bromine is more effective lies in its ability to rapidly regenerate and participate in catalytic cycles more efficiently than chlorine. This makes bromine highly effective at depleting the ozone even though it is present in smaller amounts in the atmosphere compared to chlorine.