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I am currently learning about electrochemistry using Zumdahl's Chemistry [1, pp. 709–710]. One of the critical thinking questions is

Why can $\ce{Zn}$ not be plated out from an aqueous solution of $\ce{Zn^2+}$ using the choices in Table 18.1?

TABLE 18.1 | Standard Reduction Potentials at 25 °C (298 K) for Many Common Half-Reactions

I feel like it would be plated out when with a half-reaction with a lower potential such as

$$\ce{Al^3+ + 3e- -> Al}$$

Combined,

$$\ce{3 Zn^2+(aq) + 2 Al(s) -> 3 Zn(s) + 2 Al^3+(aq)}$$

Also, looking at the reactivity series, it seems that there are plenty of options that should be able to plate out zinc. It would great if someone could explain to me what's wrong with my thinking.

References

  1. Zumdahl, S. S.; Zumdahl, S. A.; DeCoste, D. J. Chemistry, 10th ed.; Cengage: Boston, MA, 2017. ISBN 978-1-305-95740-4.
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    $\begingroup$ Hint: the crucial point is aqueous solution. $\endgroup$ – Karl Jul 21 at 20:59
  • $\begingroup$ Thanks for the hint. I think I get it now. Just to be sure, since the zinc is in aqueous solution, it is more electrically favored for aluminum to reduce the H+ in water to H2 than Zn2+ to Zn. But then H+ isn't reduced because it needs external potential to occur. Anything I left out? $\endgroup$ – Frank Jul 22 at 1:05
  • $\begingroup$ Please note that it's preferred to 1. use self-sufficient and non-clickbaity title (e.g. an equivalent to "click here to see more"); 2. add complete citations, including publisher and page number(s); 3. preserve quoted text as close to the source as possible. These tech details aside, this is a good question. I also took a liberty to re-upload the table for better readability. $\endgroup$ – andselisk Jul 22 at 4:18
  • $\begingroup$ Zinc can be purified via the electrowinning process: en.wikipedia.org/wiki/Zinc_smelting . From the article, Zn is deposited on Al cathodes, oxygen gas is evolved at the anode and the electrolyte is sulfuric acid (possibly with other stuff, but I don't know). This would be an interesting thing to test: a beaker with zinc sulfate and sulfuric acid, an Al cathode, a Cu anode and a couple of batteries or DC power supply. The standard reduction potentials do not take things like overpotential into account, and that apparently gives some opportunities. Good question (+1). $\endgroup$ – Ed V Jul 22 at 13:28
  • $\begingroup$ What external potential? H2 against Al, that's a difference of 1.7V in standard potentials. As long as you have a blank Al surface, you get hydrogen gas. I don't think the overpotential of H2 on zinc is larger than 1.7V. (de.wp says it's 0.77 de.wikipedia.org/wiki/%C3%9Cberspannung_(Elektrochemie)) $\endgroup$ – Karl Jul 23 at 16:06

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