I've recently conducted an experiment where I reacted zinc with sodium hydroxide to form sodium zincate (Na2ZnO2). However, I'm having trouble comprehending why sodium zincate has the composition Na2ZnO2. Can someone explain the chemical reactions and principles that lead to this specific composition? Additionally, what factors influence the stoichiometry of this reaction, and how does it relate to the properties of sodium zincate?
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1$\begingroup$ The eventual closure can be lifted if the user addresses closure reasons and asks for reopening. Note that re-posting of an eventually closed question, instead of editing of the closed one, is frown upon and such a question would be closed too. ( See chemistry.stackexchange.com/q/176394/35806 ) $\endgroup$– PoutnikCommented Sep 29, 2023 at 7:15
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1$\begingroup$ Review the guide How to ask and Asking FAQs to prevent clarification requests, objections, down-voting or closure. $\endgroup$– PoutnikCommented Sep 29, 2023 at 7:35
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$\begingroup$ Why can't the people who poorly answered my question review their answers? $\endgroup$– uggupugguCommented Sep 29, 2023 at 7:51
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1$\begingroup$ Do yourself first what you require from others. Review your poorly formulated questions. What particularly you do not understand and what you did to get understanding before asking? $\endgroup$– PoutnikCommented Sep 29, 2023 at 8:16
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1$\begingroup$ It's not a singular specifies. This refers to series of zinc oxides or hydroxides depending on reaction condition and can be interlinked together. $\endgroup$– Nilay GhoshCommented Sep 29, 2023 at 13:36
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2 Answers
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In fact both hydroxyzincates and oxozincates are known. Neither is described by any single formula; $\ce{Na2Zn(OH)4}$ and $\ce{Na2ZnO2}$ are simplified representations. The former are obtained in aqueous solutions, while the latter are produced in high-temperaure nonaqueous reactions using sodium oxide as the basic component.
Wikipedia gives a summary of known hydroxyzincates, which can be crystallized with different cations and different coordination of the zinc:
Solutions of sodium zincate may be prepared by dissolving zinc, zinc hydroxide, or zinc oxide in an aqueous solution of sodium hydroxide.[1] Simplified equations for these complex processes are:
$\ce{ZnO + H2O + 2 NaOH -> Na2Zn(OH)4}$
$\ce{Zn + 2 H2O + 2 NaOH -> Na2Zn(OH)4 + H2}$
From such solutions, one can crystallize salts of containing the anions $\ce{Zn(OH)4^{2−}}$, $\ce{Zn2(OH)6^{2−}}$, and $\ce{Zn(OH)6^{4−}}$. $\ce{Na2Zn(OH)4}$ consists of tetrahedral zincate ion and octahedral sodium cations.[2] The salt $\ce{Sr2Zn(OH)6}$ features zinc in an octahedral coordination sphere.
Among oxozincates $\ce{Na2ZnO2}$[3], $\ce{Na2Zn2O3}$[4], and $\ce{Na_{10}Zn4O9}$[5] are reported in the Wikipedia article. Trinschek and Jansen, authors of all three of these references, report a solid-state synthesis in Ref. [3]:
By reacting $\ce{Na2O}$, which was produced in situ from $\ce{NaN3}$ and $\ce{NaNO2}$, with reactive $\ce{ZnO}$ in the solid state, the synthesis of $\ce{Na2ZnO2}$ has been achieved. $\ce{Na2ZnO2}$ is metastable up to about 750°C.
References
Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
R. Stahl , R. Niewa , H. Jacobs, "Synthese und Kristallstruktur von Na2Zn(OH)4", Zeitschrift für anorganische und allgemeine Chemie, 625, 48-50, doi:10.1002/(SICI)1521-3749(199901)625:1<48::AID-ZAAC48>3.0.CO;2-L
D. Trinschek, M. Jansen (1996). "Na2ZnO2, ein neues Natriumzinkat". Zeitschrift für Naturforschung B, 51(5), 711-714. doi:10.1515/znb-1996-0515
D. Trinschek, M. Jansen (1996). "Eine neue Modifikation von Na2Zn2O3", Z. Naturforschung 51b, 917-21.
D. Trinschek, M. Jansen (1996). "Ein neues Oxozinkat mit trigonal-planar koordiniertem Zink Na10Zn4O9", Zeitschrift für anorganische und allgemeine Chemie 622, 245-50.
answered Sep 29, 2023 at 12:48
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Apart from its initial values zero, the element zinc has only one known oxidation number. It is +$2$. In this oxidation number +$2$, zinc appears in general as an ion $\ce{Zn^{2+}}$. It may also occur as a complex of the ion $\ce{Zn^{2+}}$ with water $\ce{H2O}$, or with the ions $\ce{O^{2-}}$ or with the ions $\ce{OH-}$. In aqueous solution, there is an equilibrium between both ions $\ce{O^{2-}}$ and $\ce{OH-}$, according to : $$\ce{O^{2-} + H2O <=> 2 OH-}$$ If we consider the only case of the ions $\ce{O^{2-}}$, they may produce with zinc $\ce{Zn^{2+}}$ the following species with one or two ions $\ce{O^{2-}}$ : $\ce{ZnO}$ or $\ce{ZnO2^{2-}}$
If the amount of $\ce{OH-}$ ions is weak in solution, the following reaction happens between zinc and water : $$\ce{Zn + H2O -> ZnO + H2}$$ but this reaction does not last more than a couple of microseconds, because $\ce{ZnO}$ is insoluble in water and stops the contact metal - solution.
If the amount of $\ce{OH-}$ is relatively important in solution, the following reaction happens after the first one : $$\ce{ZnO + 2 OH- -> ZnO2^{2-} + H2O}$$ so that the total reaction of the two last equations is : $$\ce{Zn + 2 OH- -> ZnO2^{2-} + 2 H2}$$ Apparently this last equation is exactly what uggupuggu is looking for.
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$\begingroup$ Technically, zinc is known in the +1 oxidation state. But yes, in aqueous systems it will be +2. $\endgroup$ Commented Oct 3, 2023 at 13:46