Ozone depletion with the OH radical

Question

$\ce{OH*}$ radicals are known to deplete the ozone layer. Fill out the gaps below to complete the propagation stage: \begin{align} \ce{? + O3 &-> ? + O2}\\ \ce{? + O3 &-> ? + O2} \end{align}

So I am an A-Level chemistry student in the UK. We have covered ozone depletion in the context of free radical substitution with examples as follows: \begin{align} \ce{ClCH3 &->[\text{U.V.}] Cl* + *CH3}\\ \ce{Cl* + O3 &-> ClO* + O2}\\ \ce{ClO* + O &-> Cl* + O2} \end{align}

I know that we need the overall equation $\ce{O3 + O -> 2O2}$. However, all the examples covered in class have been of the form: \begin{align} \ce{X + O3 &-> Y + O2}\\ \ce{Y + O &-> X + O2} \end{align}

Clearly, the question provided at the top does not follow this form. We have never seen depletion reactions of the form $\ce{? + O3, ? + O3}$. Some internet research has lead to the idea that:

$$\ce{HO2, HO3, H2O3}$$

Are all chemicals that can occur in these atmospheric reactions. We have not studied any of them, but my best guess for an answer to the question is:

\begin{align} \ce{*OH + O3 &-> HO2* + O2}\\ \ce{HO2* + O3 &-> HO3* + O2} \end{align}

Is this right?

Yes, you are pretty right. This is $\ce{HO_x}$ chain reaction. — Nilay Ghosh, Jan 08 '22 at 02:19
@NilayGhosh Thank you! Are there are other radicals which induce reactions not of the form ... + O$_3$, .... + O, do you know? It would be good to be prepared when something weird like this comes up — FShrike, Jan 08 '22 at 09:51
@NilayGhosh Actually, more importantly: what is the overall equation? In the in-class examples, we've ignored the radicals since they appear at the start and end, i.e. they have an overall spectator effect, but in this one the HO turns into HO3, so would it be: $$\ce{HO + 2O_3}\to\ce{HO3 + 2O_2}$$ — FShrike, Jan 08 '22 at 09:57
https://chemistry.stackexchange.com/q/9517/4945 seems to be relevant. Also the Nobel lecture should be freely accessible and have quite a bit discussion on this. — Martin - マーチン, Jan 08 '22 at 19:41
@Martin-マーチン I was just confused with the hydroxide radical in particular - I am not familiar with $\ce{HO_x}$ radicals and I had to guess the solution. This seemed like a good place to find someone to confirm or deny, as Nilay Ghosh so kindly did. — FShrike, Jan 08 '22 at 19:57
I understand that, and the question is clear, and needs a good answer. In any case I recommend reading the Nobel lecture, is here now https://www.nobelprize.org/prizes/chemistry/1995/rowland/lecture/ and I should update the other post. — Martin - マーチン, Jan 08 '22 at 20:05

AJKOER · Answer 1 · 2022-01-09T01:00:55.217

Another likely correct answer that obeys the constraints of the equation:

$\ce{X + O3 → Y + O2}$

and forms likely stable radicals (as opposed to your suggested answer noting this reference "On the stability of the elusive HO3 radical") for two members of the chain associated with an ozone depletion cycle which results in the net reaction:

$\ce{2 O3 → 3 O2}$

is namely, the HOx ozone depletion cycle, where I claim that x may assume, at least functionally, the values of 0 and 1 as to oxygen presence. That is, possible acceptable answers could be $\ce{.H}$ and $\ce{.HO}$.

Support, per a source citing the HOx chain with x=1, namely: $\ce{.HO}$ discussed briefly here, presents the reaction chain for $\ce{.OH}$ as:

$\ce{.HO + O3 → .HO2 + O2}$

$\ce{.HO2 + O → .HO + O2}$

And, by extension for x=0 (no oxygen, just hydrogen):

$\ce{.H + O3 → .HO + O2}$

Source: "Theoretical study of the H + O3 ↔ OH + O2 ↔ O + HO2 system" where to quote:

The key features of the H+O3 potential energy surface are expected to be transferable to the X+O3 systems where X=Cl, OH, NO, and NH2.

And the associated next step reaction:

$\ce{.HO + O → .H + O2}$

Source: "O + OH → O2 + H : A key reaction for interstellar chemistry. New theoretical results and comparison with experiment".

which completes the cycling of $\ce{.H}$ as required.

Ozone depletion with the OH radical

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