The work of Dr. Mahmoud Marsafy

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REMEMBER: you must do your own experiment risk assessment, ideas are illustrated but health and safety details are NOT included

24. The Displacement of Copper from Copper(II) Sulphate Solution using Zinc

The CHART A above introduces an attempt to investigate the stoichiometry of the important cationic spontaneous redox replacement reaction for Zn(s) immersed in Cu²⁺(SO₄^2-) solution. It introduces the utilisation of a remarkable pocket size sensitive balance (100 gram load & precision of 10 mg). In my opinion, such type of balance could well substitute all the expensive sensitive balances being utilised now in schools.  The balance is very affordable, price-wise. I bought it in Cairo for about 20$. Also a small electric hot plate of very low wattage is used for evaporation to dryness of solution supernatants, coupled with accurate weighing of the residue. Further the reaction has been accomplished in 1 ml solution containing ONE MILLI-MOLE of Cu²⁺.
 
In one chart the quantitative loss of weight of the zinc strip is documented and a comparison with conventional technique is presented together with a presentation of the mechanism of the transfer of the two electrons and the ionic equations of the reaction. The conventional setup and the mechanism were incorporated from published presentations on the internet, found through GOOGLE Search.
 
There seems to be internationally  a new trend to supplement the steps of any experimental procedure with photographs of the devices applied and the actual results. Further charts for every step in the  above investigation will be supplied later. The photos were captured with an ordinary digital camera, without intricate lighting appliances. I hope that students will submit their laboratory reports with such photos. The predicted weights of the reactants and products: [Zn  (65mg)  &  ZnSO4   (161mg)   &  the Cu deposited (64 mg )],  were determined experimentally with a sensitive small (pocket size) balance.  (Charts A and C)

Reactivity Series of Metals including displacement reactions and an introduction to redox reactions
 

The reaction is driven to completion in a short time (about 10 minutes). To achieve this, the reaction is carried out at about 70C^o, by heating on a small (12W) electric plate .The original blue solution does not boil under these conditions. The completion of the reaction is clearly indicated by the full decolorisation of the original solution.  (Chart B)

All the products of the complete reaction are separated quantitatively, i.e. both the ZnSO₄, replacing the original CuSO₄, and the resulting solid copper, characterized by its reddish brownish colour.    (Charts C&D)

Reactivity Series of Metals including displacement reactions and an introduction to redox reactions

The separation of the products: the supernatant colorless ZnSO₄ solution & the spongy reddish brown precipitate of Cu metal is achieved by a decantation technique. The supernatant solutions, as well as the subsequent washings are removed quantitatively by careful manipulation and by suction with a thin, long stem plastic dropper. Thus avoiding the laborious conventional filtration technique.     To recover the solid ZnSO₄ the supernatant solution was evaporated to complete dryness in a vial heated by a gas flame. The vial was loosely capped by an Aluminium small sheet to prevent loss of the product at the end, by being ejected out of the vial. It was then transferred to a weighed aluminium sheet, to measure its mass with the sensitive small balance. C. Similarly the copper deposit was carefully dry heated after washing and discarding the washings, and the dried mass was determined.   (CHART C)

Reactivity Series of Metals including displacement reactions and an introduction to redox reactions

The 1 milli-mole Cu²⁺ solution in 1ml water was prepared by weighing exactly 250 mg CuSO₄.5H₂O s, in a vial and dissolving in 1 ml water. The solution has to be heated   for complete dissolution.   (Chart E)  Note: Heating could also be achieved by the electric hot plate, or hot water could be added from the start. The 1ml of water was manipulated by a graduated plastic syringe with its needle removed.

The following table summarises the experimental results and compares these results with those predicted from the reaction equations.

* 250 mg copper sulphate pentahydrate (CuSO₄.5H₂O) s was dissolved in 1 ml water.

These deviations from the anticipated values are due to limitations in the experimental technique adopted:

·   The sensitive balance utilized has a sensitivity of 10 milligrams. It is suggested that improvements would be achieved by using a sensitive balance of 1mg accuracy.

·    The use of one milli-mole of copper sulphate solution is restricting the accuracy of the results of the reacting masses.  Therefore it is recommended to use five milli-moles of CuSO₄(s) to overcome these limitations.

 It is recommended also to change the vials utilized in this investigation, and replace them with vials with wide upper openings, and also wider bottoms. Also to use a low wattage Electric Hot Plate (12 watts) for evaporating to dryness the solutions until constant weighs. This modification is expected to reduce the errors due to abrupt violent boiling towards the end of evaporation, which causes ejection of the residual solids out of the vials. The heating of the copper deposit under these conditions will not cause any oxidation to copper oxide, as would happen if it is dried using normal Bunsen burners. The evaporation of the solutions heated by this hot plate in wider top vials, or small beakers is expected to proceed at the approximate rate of 2.5 ml per hour. The evaporated solution will never boil under these conditions until complete dryness.

Reactivity Series of Metals including displacement reactions and an introduction to redox reactions

The full replacement of the Cu²⁺ ion with Zn²⁺ ion is verified by the reaction of a drop of the original and final supernatant solutions with a drop of ammonia solution, in separate cavities of a multi-cavity plastic strip. (Chart F) Chart F includes also an extension experiment where the displacement of Cu2⁺ (aq) ion is achieved by use of an iron nail.

Summary of the experiment

Reactivity Series of Metals including displacement reactions and an introduction to redox reactions

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