A proposed two-step mechanism for the destruction of ozone...

A proposed two-step mechanism for the destruction of ozone in the upper atmosphere is a. What is the overall balanced equation for the ozone destruction reaction? b. Which species is a catalyst? c. Which species is an intermediate? d. What is the rate law derived from this mechanism if the first step in the mechanism is slow and the second step is fast? e. 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 reaction "$$\underset{\text { Freon }-12}{\mathrm{CCl}_{2} \mathrm{F}_{2}} \stackrel{h v}{\longrightarrow} \mathrm{CF}_{2} \mathrm{Cl}+\mathrm{Cl} $$,Chlorine atoms also can act as a catalyst for the destruction of ozone. The first step of a proposed mechanism for chlorinecatalyzed ozone destruction is $$\mathrm{Cl}(g)+\mathrm{O}_{3}(g) \longrightarrow \mathrm{ClO}(g)+\mathrm{O}_{2}(g) \quad \text { Slow }$$. Assuming a two-step mechanism, propose the second step in the mechanism and give the overall balanced equation.

A proposed two-step mechanism for the destruction of ozone in the upper atmosphere is a. What is the overall balanced equation for the ozone destruction reaction?
b. Which species is a catalyst?
c. Which species is an intermediate?
d. What is the rate law derived from this mechanism if the first step in the mechanism is slow and the second step is fast?
e. 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 reaction
"$$\underset{\text { Freon }-12}{\mathrm{CCl}_{2} \mathrm{F}_{2}} \stackrel{h v}{\longrightarrow} \mathrm{CF}_{2} \mathrm{Cl}+\mathrm{Cl}
$$,Chlorine atoms also can act as a catalyst for the destruction of ozone. The first step of a proposed mechanism for chlorinecatalyzed ozone destruction is
$$\mathrm{Cl}(g)+\mathrm{O}_{3}(g) \longrightarrow \mathrm{ClO}(g)+\mathrm{O}_{2}(g) \quad \text { Slow }$$.
Assuming a two-step mechanism, propose the second step in the mechanism and give the overall balanced equation.

Key Concepts

Overall Reaction Equation

The overall reaction equation is the net result when the individual steps of a mechanism are summed, with all intermediates canceled out. It shows only the starting materials (reactants) and the final substances produced (products), ensuring that mass and atoms are conserved throughout the reaction.

Catalyst

A catalyst is a substance that increases the rate of a chemical reaction by lowering the activation energy, yet it is not consumed in the process. In a multi-step mechanism, a catalyst appears in one or more steps but is regenerated at the end, so it does not appear in the overall balanced equation.

Intermediate

An intermediate is a short-lived species formed in one elementary step of a reaction mechanism and consumed in a subsequent step. Because it is generated and then used up during the mechanism, it does not appear in the overall balanced chemical equation.

Rate Law Derivation

Rate law derivation involves expressing the rate of a reaction in terms of the concentrations of the reactants present in the rate-determining (slow) step. In a mechanism with a slow first step and a fast subsequent step, the overall rate is typically governed by the slow step, and if intermediates are present, their concentrations might be expressed in terms of reactants using a steady-state or pre-equilibrium approximation.

Reaction Mechanisms

Reaction mechanisms provide a detailed, step-by-step explanation of how reactants transform into products. They help in understanding the roles of catalysts and intermediates, as well as in determining the rate law, by identifying which steps are slow (rate-determining) and which are fast. A clear mechanism is essential for relating experimental kinetic data to the underlying molecular events in a reaction.

Transcript

00:04 Okay, so in your book, you are given this figure of two reaction steps, and you're told that this is related to the destruction of ozone in the upper atmosphere.

00:17 So this is a proposed two -step mechanism for the destruction.

00:21 So part a, we'll start at the start.

00:24 Why not? it wants us to write an overall balanced equation.

00:28 So let's go ahead and convert this image into a chemical equation.

00:34 So each of the red spheres are oxygen and the blue spheres are nitrogen.

00:40 So this first part, 03 plus, so this is o3, this is no.

00:51 I'm actually going to erase that so it doesn't get messy.

00:57 N -o gives us n -2 plus o2.

01:08 But that's just the first part of this reaction.

01:12 The second part has n -o -2 reacting with one solo oxygen to produce an o plus o2.

01:31 Okay, so overall, we have to combine these equations in order to get the overall reaction of the destruction of ozone.

01:45 So i'll write everything out before i draw it with the canceled species.

01:52 Actually, let's start with ozone, because this whole problem is about ozone.

01:57 O3 plus no plus n o2 plus o.

02:07 We'll yield no2 plus no plus 2o.

02:22 But actually it's not necessarily asking for a net equation.

02:26 So let's check to make sure everything is balanced.

02:30 So for nitrogen, we have one, two on the left, and we have one, two on the right.

02:39 For oxygen, we have three, four, five, six, seven on the left, and one, two, three, four, five, six, seven on the right.

02:49 So this equation currently is balanced.

02:55 Now, it wants to know about which species is a catalyst in this reaction.

03:03 Button thing that we know about catalysts is that they are used in a reaction and they are regenerated in the same reaction.

03:13 So here we employed no and we ended up producing no as one of the products.

03:23 So catalyst is in oh wow, i'm having a lot of trouble here.

03:32 Let's try again.

03:44 Okay.

03:45 So that is b.

03:47 This is a.

03:52 It asks which species is an intermediate? we know that intermediates are produced, and then they disappear by the end of the reaction.

04:04 If you look, we have no2 produced, but then it is consumed in the next step.

04:11 So our intermediate is no2.

04:24 It then asks us about deriving a rate law for this mechanism, if the first step is slow.

04:35 So we can say this is slow.

04:40 And the second step is fast.

04:44 So if we have two steps, one is going slow, one is going fast.

04:48 The one we're going to have to wait on is the slow one.

04:52 Okay? this is going to be limiting the reaction.

04:55 And so our rate law is mainly going to focus on the slow step.

05:03 And i'm worried about there being confusion.

05:07 So i'm just going to, for the overall reaction, cancel these out, okay? so make it simpler.

05:20 2 .2.

05:23 Not necessarily important for the questions they're asking, but i'm doing it nonetheless.

05:29 Now, back to d.

05:31 D is showing that this is the slow reaction.

05:34 So the rate law will depend on this part.

05:38 We have rate is equal to k.

05:46 We're going to say that the rate of this reaction is k1...

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