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: $$ \mathrm{CCl}_{2} \mathrm{~F}_{2} \stackrel{\mathrm{hv}}{\longrightarrow} \mathrm{CF}_{2} \mathrm{Cl}+\mathrm{Cl} $$ Chlorine atoms can act as a catalyst for the destruction of ozone. The activation energy for the reaction $$ \mathrm{Cl}+\mathrm{O}_{3} \longrightarrow \mathrm{ClO}+\mathrm{O}_{2} $$ is \(2.1 \mathrm{~kJ} / \mathrm{mol}\). Which is the more effective catalyst for the destruction of ozone, \(\mathrm{Cl}\) or \(\mathrm{NO}\) ? (See Exercise \(69 .\) )

Short answer

The more effective catalyst for the destruction of ozone is NO, as its activation energy for the reaction with O3 is lower (1.1 kJ/mol) compared to the activation energy for the Cl-O3 reaction (2.1 kJ/mol).

Step-by-step solution

Recall the reaction of NO and O3 from Exercise 69

: Let us recall the reaction between NO and ozone (O3): $$ \mathrm{NO}+\mathrm{O}_{3} \longrightarrow \mathrm{NO}_{2}+\mathrm{O}_{2} $$ The activation energy for this reaction was previously given in Exercise 69 as \(1.1 \mathrm{~kJ} / \mathrm{mol}\).

Review the given reaction between Cl and O3

: The provided reaction between Cl and ozone (O3) is: $$ \mathrm{Cl}+\mathrm{O}_{3} \longrightarrow \mathrm{ClO}+\mathrm{O}_{2} $$ The activation energy for this reaction is given as \(2.1 \mathrm{~kJ} / \mathrm{mol}\).

Compare the activation energies of the two reactions

: We can determine the more effective catalyst by comparing the activation energies of the two given reactions. A lower activation energy indicates a more effective catalyst because less energy is required to initiate the reaction, which results in faster catalyst activity. For the NO-O3 reaction: Activation energy: \(1.1 \mathrm{~kJ} / \mathrm{mol}\) For the Cl-O3 reaction: Activation energy: \(2.1 \mathrm{~kJ} / \mathrm{mol}\)

Decide the more effective catalyst based on activation energy

: Since the activation energy of the NO-O3 reaction is lower than that of the Cl-O3 reaction, NO is the more effective catalyst for the destruction of ozone. Answer: The more effective catalyst for the destruction of ozone is NO.

Key concepts

Freons and the Atmosphere

When discussing ozone depletion, Freons often come to the spotlight, but what are Freons exactly? Freons are a group of halogenated compounds, also known as chlorofluorocarbons (CFCs), which were commonly used for refrigeration, air conditioning, and as propellants in aerosol sprays. Unfortunately, these compounds have the potential to travel to the upper atmosphere, where ultraviolet (UV) radiation can cause them to release chlorine atoms through photodissociation.

The reaction showcased in the exercise is a prime example of this process where Freons, specifically CFC-12, are broken down:\[ \mathrm{CCl}_{2} \mathrm{F}_{2} \stackrel{\mathrm{hv}}{\longrightarrow} \mathrm{CF}_{2} \mathrm{Cl} + \mathrm{Cl} \]
Here, UV radiation (hv) breaks CFC-12 into chlorodifluoromethane (CF2Cl) and a chlorine atom. These chlorine atoms are very stable and can last in the upper atmosphere for up to 100 years. The stability allows them to catalyze the destruction of an enormous amount of ozone-over their lifespan, making them significantly harmful.

Chlorine and Ozone Reaction

Ozone (O3) in the stratosphere plays a critical role in protecting life on Earth by absorbing most of the Sun's harmful ultraviolet radiation. The chlorine atom, once freed from its molecular bonds in Freons, can initiate a chain reaction that depletes this protective ozone layer. The chemistry behind this is profound yet alarming.

When a chlorine atom encounters an ozone molecule, it takes one oxygen atom, creating chlorine monoxide (ClO) and leaving a molecule of oxygen (O2), as seen in the equation from the exercise:\[ \mathrm{Cl} + \mathrm{O}_{3} \longrightarrow \mathrm{ClO} + \mathrm{O}_{2} \]
Furthermore, the 'catalytic' nature of this chemistry allows a single chlorine atom to destroy thousands of ozone molecules because the chlorine atom is regenerated after each cycle. This destructive sequence continues until the chlorine atom eventually forms a stable compound that removes it from the catalytic cycle. Understanding the ozone destruction chain provides key insights into why certain catalysts are more effective than others in this reaction.

Activation Energy

Activation energy is the minimum energy required to initiate a chemical reaction. In the context of ozone destruction, a catalyst is a substance that lowers the activation energy, making it easier for the reaction to proceed even under less energetic conditions. Lower activation energy corresponds to a more efficient catalytic process because it promotes a quicker and more spontaneous reaction.

The exercise mentions the activation energy for the reaction of chlorine with ozone is \(2.1 \mathrm{~kJ} / \mathrm{mol}\), higher than that of nitric oxide (NO) with ozone, which was \(1.1 \mathrm{~kJ} / \mathrm{mol}\). Consequently, NO as a catalyst leads to a faster reaction with ozone compared to chlorine, indicating its higher effectiveness. It's worth noting that although the comparison is based on activation energy, other factors, such as catalyst concentration and longevity in the atmosphere, contribute to the overall impact on ozone depletion.