When healthy, Earth’s stratosphere contains a low concentratio...

Question

When healthy, Earth’s stratosphere contains a low concentration of ozone (O₃) that absorbs potentially harmful ultraviolet (UV) radiation by the cycle shown at right.

Chlorofluorocarbon refrigerants, such as Freon 12 (CF₂Cl₂), are stable in the lower atmosphere, but in the stratosphere they absorb high-energy UV radiation to generate chlorine radicals.

The presence of a small number of chlorine radicals appears to lower ozone concentrations dramatically. The following reactions are all known to be exothermic (except the one requiring light) and to have high rate constants. Propose two mechanisms to explain how a small number of chlorine radicals can destroy large numbers of ozone molecules. Which of the two mechanisms is more likely when the concentration of chlorine atoms is very small?

Accepted Answer

Alright. Hello everyone. So this question says when healthy the earth's stratosphere has a low concentration of ozone that's 03 which absorbs potentially harmful ultraviolet radiation. The presence of a few bromine radicals appears to significantly reduce ozone concentrations. Proposed two mechanisms for how a small number of bromine radicals can destroy large numbers of ozone molecules, which of the two mechanisms is more likely to occur when bromine atom concentrations are extremely low. All right. So first and foremost, recall that the reaction being involved in this type of process is free radical chain reactions. And this is because bromine being a halogen is very, very prone to the formation of radicals. This is because the bond between the two electron bromine atoms is remarkably weak. Now, a free radical chain reaction is said to have three distinct steps, right? If you recall the first one is initiation, which is the formation of the first few radicals from there, we have propagation. So basically in the propagation step, the radicals formed during the initiation step react with other starting materials which create new radicals now eventually, right? The propagation step repeats itself over and over again. That's why radical reactions are called chain reactions. Eventually. Though once all radicals have been exhausted, we reach the termination step. And so in the termination step, once all radicals have been exhausted, they're going to be destroyed, which eventually stops the reaction. All right. So let's start with mechanism one. Now, following this formula that we talked about earlier for radical chain reactions, the first step is initiation, right? And so our initiation step is going to involve the aforementioned bromine radical. So we have bromine here one atom of bromine with an unpaired electron. And so our bromine radical here is going to react with one equivalent of ozone that's 03. So what's happening here is that bromine, the radical is going to combine with one atom of oxygen to create bro and this is going to result in ozone being converted to molecular oxygen. So we go from 03 to 02 and bro and bro is a radical. So let's not forget the unpaired electron because this br radical is going to be involved in the second step, specifically two equivalent of it. So in the propagation step, right, two bro radicals are going to combine with each other to create this modified peroxide, right. So basically we have the two oxygens connecting to each other and both oxygens are connected to their respective bromides. This is because the unpaired electron is now on oxygen. So last but not least. So for our next propagation step our product from the previous step that is this modified peroxide, right is going to break and once it breaks, it's going to form oxygen as well as two bromine radicals to repeat this process over and over again. Now, for this question, we're not going to show the termination steps here because our bromine radicals have been regenerated. The prior steps are going to repeat themselves over and over again until all reagents are exhausted. So with the mechanism, we have written, though, we can acknowledge one major limitation in the mechanism itself. And that is the second step, right? Because the product of the second step is not only incredibly unstable, it's very unlikely to happen because the concentration of bromine atoms is so low at all times the amount of bro that's going to realistically form the amount of bro radicals it's going to realistically form is very low at all times, not only is the concentration of this radical very low, but once again, the product which is this modified peroxide looking compound is incredibly unstable. So it's not likely to even form in the first place. So let's go ahead and proceed with the second mechanism. Now, before I go ahead and start with the second mechanism, I just want to point out here that the third step of the first mechanism requires the presence of light. So let me just go ahead and add that. All right. So now let's start with mechanism two So in mechanism two, we actually start with ozone itself. So we have 03 and in the presence of light, right 03 is going to break down into elemental oxygen and lone oxygen atoms. So we have 02 and O. So in the second step, we introduce or bromine radical. So we have our grooming radical reacting with another equivalent of ozone to produce the bro radical that we talked about before and elemental oxygen. So this is how we proceeded to the most important step of the second mechanism. Now, as mentioned before, the concentration of bro is inherently very low. And so because we have a low concentration of bro, we're going to have a high concentration of oxygen atoms, lone oxygen atoms. So here we can take a bro radical, have it react with a lone oxygen atom and have it produce molecular oxygen as well as regenerating the bromine radical that repeats this process over and over again. So for the sake of visibility, now that we have both mechanisms, let me go ahead and move the first so that its to the left of the second just so that we can see it on the screen at the same time. So four step two or sorry mechanism two, the fact that there's such a high concentration of low oxygen atoms means that mechanism too is the primary mechanism responsible for ozone depletion. So here to answer the last part of the question we would say that mechanism too is more likely to occur when oops misspelled, that when bromine atom concentrations are extremely low. And there you go. Right. So just to go ahead and re summarize why mechanism one is not likely to happen because of the low chances that the second step is actually going to happen. And so there you have it with that being said, if you watch this video all the way through, thank you so very much for doing so. And I hope you found this helpful.

Key Concepts

Ozone Depletion Mechanism

Exothermic Reactions

Chain Reactions

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