Question

One of the concerns about the use of Freons is that they will migrate to the upper atmosphere, where chlorine atoms can be generated by the following reaction: $${\mathrm{CCl}}_2{\mathrm{F}}_2(g)\stackrel{hv}{⟶}{\mathrm{CF}}_2\mathrm{Cl}(g)+\mathrm{Cl}(g)$$ Chlorine atoms can act as a catalyst for the destruction of ozone. The activation energy for the reaction $$\mathrm{Cl}(g)+{\mathrm{O}}_3(g)⟶\mathrm{ClO}(g)+{\mathrm{O}}_2(g)$$is $2.1\mathrm{kJ}/\mathrm{mol}.$ Which is the more effective catalyst for the destruction of ozone, Cl or NO? (See Exercise 75.)

Short answer

Comparing the activation energies of the two reactions, it is observed that the reaction involving NO as a catalyst has a lower activation energy ($1.7\mathrm{kJ}/\mathrm{mol}$) than the reaction involving Cl as a catalyst ($2.1\mathrm{kJ}/\mathrm{mol}$). As a result, the more effective catalyst for the destruction of ozone is NO.

Step-by-step solution

Conclusion

Comparing the activation energies of the two reactions, it is observed that the reaction involving NO as a catalyst has a lower activation energy ($1.7\mathrm{kJ}/\mathrm{mol}$) than the reaction involving Cl as a catalyst ($2.1\mathrm{kJ}/\mathrm{mol}$). As a result, the more effective catalyst for the destruction of ozone is NO.

Key concepts

Freons

Freons, also known as chlorofluorocarbons (CFCs), are compounds primarily used as refrigerants, propellants, and solvents. Popular in the mid-20th century due to their inertness and non-toxicity, freons were initially considered safe for environmental use. However, scientific studies later discovered that freons could reach the upper atmosphere and contribute to ozone layer depletion.

Freons are stable at lower atmospheric levels, but when they ascend to the stratosphere, they are broken down by ultraviolet (UV) radiation, releasing chlorine atoms. This process initiates a chain reaction leading to the destruction of ozone molecules. Although freons are no longer in widespread use due to their environmental impact, understanding their chemistry remains crucial in grasping how human-made substances can impact Earth's atmospheric systems.

Chlorine Atoms

Chlorine atoms, once freed from freons due to UV radiation in the stratosphere, play a significant role in ozone depletion. Each chlorine atom can destroy thousands of ozone molecules through catalytic reactions. This happens because chlorine acts as a catalyst, perpetually engaging in the reaction cycle without being consumed.

In the atmosphere, a chlorine atom reacts with an ozone molecule (O extsubscript{3}), forming chlorine monoxide (ClO) and molecular oxygen (O extsubscript{2}). A typical set of reactions might look like this:

• Cl + O extsubscript{3} → ClO + O extsubscript{2}
• ClO + O → Cl + O extsubscript{2}

As these reactions repeat, a single chlorine atom can destroy many ozone molecules over time, severely impacting the ozone layer.

Catalysis

Catalysis is a process where a substance, known as a catalyst, speeds up a chemical reaction without being consumed. This principle is critical in understanding how small amounts of substances can have a severe impact on ongoing chemical processes, as seen with chlorine's effect on ozone.

Catalysts lower the activation energy needed for reactions, hence increasing their rate. In atmospheric chemistry, both chlorine and nitrogen oxides (NO) serve as catalysts for ozone destruction, but through different mechanisms. Chlorine acts in a simple cycle, while NO involves more complex interactions. Despite this, both effectively reduce ozone levels, emphasizing the importance of catalysis in environmental chemistry.

Activation Energy

Activation energy is the minimum amount of energy required for a chemical reaction to occur. It determines the reaction rate and, ultimately, its feasibility under given conditions. In the context of ozone layer depletion, the activation energy for reactions involving ozone destruction is crucial.

The reaction of chlorine with ozone has an activation energy of 2.1 kJ/mol, while the reaction involving NO has a lower activation energy of 1.7 kJ/mol. Reactions with lower activation energies proceed more quickly and are more effective under similar conditions. Therefore, though both chlorine and NO can catalyze ozone destruction, NO is more efficient due to its lower activation energy, which aligns with faster reaction rates and more rapid ozone depletion.