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My question involves the evaporation of silicate material (e.g., rocks, minerals, geologic material), specifically I am trying to work the number of electrons (n) that are involved in the evaporation reactions described by Sossi et al. (2019) and Sossi and Fegley (2018).

As an example, I will discuss the evaporation of zinc oxide (ZnO), which is shown in multiple sources (see footnotes) as:

enter image description here

Sossi et al. (2019) state that this reaction involves two electrons (n=+2), however they do not indicate how they came to this value, so I have been trying to work it out myself. I should also note that they describe these reactions as producing a metal bearing gas.

I have two lines of thinking for determining n for this reaction:

First line of reasoning

ZnO(l) has two components with charges Zn = +2 and O = -2

Zn(g) evaporates as a metal, so the charge is Zn = 0

and

1/2O2 is just oxygen gas with charge 1/2O2 = 0.5*(-2*2) = -2

This line of reasoning implies that there was a change in the charge on Zn of -2. In other words, two electrons were lost, so two electrons were involved in the reaction. It involves the production of gaseous zinc metal, which fits nicely into the description of a metal bearing gas, at least in my mind. However, this leaves the reaction unbalanced because the products have a -2 charge, which leads me to my second line of reasoning.

Second line of reasoning

ZnO(l) his a stable compound, so ZnO = 0 charge

Zn(g) evaporates as a cation, so the charge is Zn = +2

and

1/2O2 is just oxygen gas with charge 1/2O2 = 0.5*(-2*2) = -2

Now the reaction is charge balanced, but I am not clear on how two electrons were involved in this case, unless I can consider the two electrons that "went with" Zn after the evaporation of ZnO as the two electrons that are involved in the reaction. However, I don't know if Zn++(g) counts as a metal bearing gas, which the authors say is present.

I have been tying myself in knots trying to work this out, so any guidance in the right direction would be helpful.

Footnotes

Other sources that discuss the evaporation of ZnO include Anthrop and Searcy (1964) and Kodera et al. (1968)

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1 Answer 1

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Your following sentence is wrong (I copy it in italics without change)

1/2O2 is just oxygen gas with charge 1/2O2 = 0.5(-22) = -2

No! There is no charge $-2$ on the oxygen atom in the molecule $\ce{O2}$, and no charge at all in the molecule $\ce{O2}$ There is a charge $-2$ on the oxygen atom in the initial oxide $\ce{ZnO}$. But during the decomposition of $\ce{ZnO}$, the two electrons available on the Oxygen ion are transferred to the zinc ion $\ce{Zn^{2+}}$ which becomes a non-charged atom $\ce{Zn}$.

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  • $\begingroup$ Thank you for catching that mistake! That means that Zn is evaporating as Zn metal (Zn+0), correct? $\endgroup$
    – eyemarrs
    Jun 18 at 22:02
  • $\begingroup$ Boiling point of Zn is 907°C, deep below the melting point 1974°C of ZnO, at which it is said being already decomposing. $\endgroup$
    – Poutnik
    Jun 18 at 23:05
  • $\begingroup$ @Poutnik I understand what you're saying, but I don't follow why that's relevant. Are you saying that my comment is incorrect? $\endgroup$
    – eyemarrs
    Jun 19 at 12:24
  • $\begingroup$ @eyemarrs It is just a side info. $\endgroup$
    – Poutnik
    Jun 19 at 19:41
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    $\begingroup$ Yes, Zn is evaporating as Zn metal, mixed with neutral oxygen atoms. But when this gas is cooled down, it synthesizes back zinc oxide. $\endgroup$
    – Maurice
    Jun 19 at 20:31

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