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Conversion of Cu into Silver (Zn)

Conversion of Cu into Silver (Zn). Speakers ： Hsieh, P.C. & Hsu, T.W. Motive. Cu slic e. An interesting experiment ： put Cu slice in the Zn powder and NaOH solution. Result ： the appearance of Cu is converted into silver. NaOH (aq). Zn powder. Why ?. Question??.

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Conversion of Cu into Silver (Zn)

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  1. Conversion of Cu into Silver (Zn) Speakers：Hsieh, P.C. & Hsu, T.W.

  2. Motive Cu slice An interesting experiment：put Cu slice in the Zn powder and NaOH solution. Result：the appearance of Cu is converted into silver NaOH(aq) Zn powder Why ?

  3. Question?? • Common explanation： Zn(s)+ 2H2O(l)→ Zn2＋(aq) + 2OH－(aq) + H2(g) Zn2＋(aq)+ Cu(s) → Zn(s) + Cu2＋(aq) • But the second reaction is non-spontaneous, because Zn2＋(aq)+ Cu(s) → Zn(s) + Cu2＋(aq) △E0 = -1.56V＜0

  4. Experiment 1 Cu slice Procedure ： put Cu slice in the zinc salt solution with heating for a certain time → observe the difference of Cu slice Zn 2＋ Does Zn deposit on Cu slice?

  5. Result 1 • w0：original weight • wt：final weight • Δw：wt-w0 No any difference on Cu slice

  6. Result 1 Cu slice Zn2+ Zn2+ Zn2+ Zn2+

  7. Experiment 2 Cu slice Procedure ： put Cu slice in the zinc powder and NaOH solution without heating → stand for one week → observe the difference of Cu slice NaOH(aq) Zn powder Does Zn deposit on Cu slice?

  8. Result 2 w0：original weight wt：final weight Δw：wt-w0 Zn deposits on the Cu surface even without heating

  9. Conclusions • Reaction Zn2＋(aq)+ Cu(s) → Zn(s) + Cu2＋(aq)is non-spontaneous even with heating, because △E0 = -1.56V＜0 • In the Zn(s) and NaOH solution, Zn can deposit on the Cu slice even without heating. So what happened in fact??

  10. Analysis of Zn2＋ Spot 3 • Experiment： • put 1M NaOH(aq)、6.5g Zn powder and Cu slide in a beaker, and keep the reaction temperature at 75~85℃ for a certain time • Withdraw clear solution at different spots in the beaker, and analyze the [Zn2＋] by ICP-MS in Instrumentation Center at  National Tsing Hua University. Spot 2 Spot 1

  11. Then, we do more research based on our theory.

  12. I. The correlation between the Cu area and the variation of weight NaOH(aq) Zn powder The correlation between the Cu area and the variation of weight 1 The variation of weight (g) 2 3 4 The Cu area (cm2) • There are positive correlation between the Cu area and variation of weight.

  13. II. The influence of surface area of Zn Origin copper slice • Experiment：replace theZn powder by Zn grain to repeat the same experiment, and compare the results. • Result： There are the positive correlation between the surface area of Zn and variation of Zn weight. In zinc grain In zinc powder

  14. III. The influence of [NaOH] • The Zn can be dissolved in strong base. So, does the concentration of NaOH influence the variation of Zn depositing weight? • We change the concentration of NaOH with 0.1M、0.5M、1.0M、3.0M、6.0M separately. Experiment factors：150mlNaOH (aq), 6.5g Zn powder, at 70~80 ℃.

  15. Result [NaOH] – the variation of Zn the variation of weight When [NaOH] is more than 1.0M, the depositing of Zn keeps contant. When [NaOH] is less than 0.1M, Zn can’t deposit on the Cu surface. [NaOH] (M) When [NaOH] isbetween 0.5M and 1.0M, there is positive correlation.

  16. Discussion: The influence of [NaOH] • Zn can’t deposit when [NaOH] is less than 0.1M，because of the lower concentration of the zinc ion. • The depositing of Zn on the Cu surface keeps almost constant when [NaOH] is between 1.0M~6.0M, so 1.0M is the optimal concentration of NaOH.

  17. IV. The variation of Zn weight deposited with time The variation of Zn - Time NaOH(aq) The variation of Zn (g) Zn powder Day

  18. Discussion: • When the Zn deposits on the Cu surface, the dissolution of Zn by base happens too. • At first, depositing rate>dissolving rate：the weight of Cu slice increased. • When depositing rate=dissolving rate：the weight of Cu slice is unchanged. • Finally, depositing  rate<dissolving rate：weight of Cu slice decreased.

  19. V. Can it happen in acid? • Zn is an amphoteric element, it can be dissolved in both acid and base. So can this reaction also be happened in acid? • Experiment：use HCl(aq) with 0.1M, 0.5M, 1.0M, 3.0M, 6.0M separately to replace the NaOH(aq). Experiment factors：150mlNaOH (aq), 6.5g Zn powder, at 70~80 ℃.

  20. Result: Variation of Zn - [HCl] • The reaction also is happened in acid. Variation of Zn (g) [HCl] (M) When [HCl] is more than 0.5M, the depositing of Zn decreases obviously. When [HCl] is 0.1M, Zn can deposit on the surface of Cu. When [HCl] is 0.5M, the depositing of Zn gets up to the maximum.

  21. Conclusion 1. There is positive correlation between the surface area of Cu slice and the depositing weight of Zn. The higher the surface area of Zn is, the higher the depositing weight of Zn. 2. There are the positive correlation between the surface area of Zn and variation of Zn weight.

  22. Conclusion 3. If [NaOH] is 0.1M, there will be no Zn deposited on Cu slice. when [NaOH] is 1.0M~6.0M, the depositing weight of Zn is almost the same. So we know the “optimal concentration” of [NaOH] is 1.0M. 4. The depositing weight of Zn wouldn’t always increase, but depends on depositing rate and dissolving rate. 5. In acid, Zn also can deposit on Cu slice. The reason is almost the same as in base.

  23. Thanks for your listening!

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