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

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

Question?? Common explanation ： Zn (s) + 2H 2 O (l) → Zn 2 ＋ (aq) + 2OH － (aq) + H 2(g) Zn 2 ＋ (aq) + Cu (s) → Zn (s) + Cu 2 ＋ (aq) But the second reaction is non-spontaneous, because Zn 2 ＋ (aq) + Cu (s) → Zn (s) + Cu 2 ＋ (aq) △ E 0 = -1.56V ＜ 0

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

Result 1 w 0 ： original weight w t ： final weight Δ w ： w t -w 0 No any difference on Cu slice ZnSO 4 ZnCl 2 Zn(COOH) 2 Zn(NO) 2 Cu w 0 (g) Cu w t (g) Δ w (g) 0000 heating time (min)20 Temperature ( ℃ )

Result 1 Zn 2+ Cu slice

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

Result 2 w 0 ： original weight w t ： final weight Δ w ： w t -w 0 Zn deposits on the Cu surface even without heating CuCuNo.1No.2 w 0 (g) w t (g) Δ w(g)0.02

Conclusions Reaction Zn 2 ＋ (aq) + Cu (s) → Zn (s) + Cu 2 ＋ (aq) is non- spontaneous even with heating, because △ E 0 = -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??

Analysis of Zn 2 ＋ 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 [Zn 2 ＋ ] by ICP-MS in Instrumentation Center at National Tsing Hua University. SpotSpot 1Spot 2Spot 3 Zn 2+ (ppm) Spot 2 Spot 3 Spot 1

Then, we do more research based on our theory.

I. The correlation between the Cu area and the variation of weight There are positive correlation between the Cu area and variation of weight Zn powder NaOH (aq) The correlation between the Cu area and the variation of weight The Cu area (cm 2 ) The variation of weight (g)

II. The influence of surface area of Zn Experiment ： replace the Zn 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. Zinc powderZinc grain No.1ΔW (g) No.2ΔW (g) Avg.ΔW (g) In zinc powder In zinc grain Origin copper slice

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 ： 150ml NaOH (aq), 6.5g Zn powder, at 70~80 ℃.

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

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.

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

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.

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 ： 150ml NaOH (aq), 6.5g Zn powder, at 70~80 ℃.

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

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.

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.

Thanks for your listening!