CHEMICAL & ELECTROCHEMICAL PLATING
PRE-LAB
Read through the lab experiment. You will need to, as a class, determine the volume and concentration of any solutions that will need to be made prior to you starting the lab. The actual plating should not require a great deal of time.
SAFETY
Safety goggles and aprons must be worn in lab at all times. NaOH is corrosive and causes burns to any area of contact. Liquid ZnSO4 is harmful and can cause severe irritation when in contact with eyes. When handling NaOH and ZnSO4 wear gloves and if in contact with eyes and skin flush with water for at least 15 min. When solid NaOH and Zn come into contact, ignition can occur. All zinc should be rinsed away from paper towels. Be careful when handling the burner, extra caution should be taken when an open flame is used in a chemistry lab.
INTRODUCTION
Chemical Plating
Turning metal into silver or gold was one of the goals of the ancient alchemists. Although thisgoal was never achieved, the alchemists were able to manipulate the appearance and the color of the metal. In this part of the experiment, you will perform your own alchemy - making a copper penny into silver, and then into gold in appearance. To begin the process, zinc powder is dissolved in a hot sodium hydroxide solution to form sodium zincate, Na2Zn(OH)4, according to the reaction below:
Zn(s) + 2 NaOH(aq) + 2 H2O(l) Na2Zn(OH)4(aq) + H2(g)
When the copper penny is added to the solution an electrochemical couple is formed by the copper-zinc contact, this causes the zincate ions to migrate onto the surface of the penny where they are reduced to Zn(s). It is believed that the reducing agent in this process is zinc itself and the driving force is the different reduction potentials for the zincate ions on the different surfaces.
The following reduction occurs on the surface of the penny:
[Zn(OH)4]-2(aq) + 2e- Zn(s) + 4 OH-(aq)
When the “silver penny” is heated, the zinc coating mixes with the outer layer of the copper atoms on the penny, producing the alloy brass - which looks like gold. This process demonstrates diffusion in the solid state: zinc atoms diffuse (or mix) with copper atoms creating a uniformed distribution of each element. Diffusion in the solid state is commonly used in industry to engineer hardness, wear resistance, and resistance to oxidation and corrosion in metallic materials. For example, the creation of alloys of carbon with different metals such as Fe, Cr and Ti leads to the formation of strong light weight metal carbides that resistance wear and or corrosion.
Electrochemical Plating (Electroplating)
Electrochemical plating is different method used to apply a metallic coating on the surfaces (fenders, grills, and toasters) to protect against corrosion by using an electric current. The first step in electroplating is to negatively charge the object (to be coated) by connecting it to anelectric current. The object is then dipped into a solution containing a metal salt. The metal cation, in the solution, is eventually reduced to a neutral metal and deposited onto the object’s surface to become a protective coating. In this experiment, a penny will be the object and ZnSO4 is the metal salt. The positive pole of a current probe will be connected to the penny and the negative pole to a battery. The positive pole of the battery will be connected to a paper clip. The paper clip is used as the anode since it is made out of metal and therefore has the ability to transfer electrons into the solution. In practice the anode could be any type of small object made out of metal (Zn, Cu, Fe etc).
The following reaction is observed at the cathode (penny):
ZnSO4(aq) + 2e- Zn(s) + SO4-2 (aq)
As seen in the reaction above the zinc ions in the solution are reduced at the cathode to Zn(s), which is deposited onto the penny.
As described above, electrochemical and chemical plating are two different methods used for coating of a metal surface. These methods are commonly used in industry and they both have their advantages and disadvantages:
1) Electrochemical plating is less expensive; thickness of the coating can be controlled,
no chemical spills and less waste disposal.
2) Chemical plating, a more even coating on the surface can be achieved and no power supply nor
electrodes are needed.
PROCEDURES
Work in pairs. Wear safety goggles, lab aprons and gloves. Bring 4 shiny pennies to lab, if the pennies are dull, get some steel wool and polish the surface. The instructor will provide solid sodium hydroxide in order for you to prepare a 3 M NaOH solution.
Chemical Plating of a Penny
One solution of the following needs to be created for all the students in the lab section:
Pour ~ 50 mL of the 3 M NaOH solution in a 250 mL beaker. Weigh out ~ 2 g of Zn(s) and add to the NaOH solution. Heat until the solution is almost boiling.
Once the solution has reached its boiling point, each student should place a penny in the solution (to a maximum of 4 pennies at a time) and turn down the heat of the hot plate to 3 or 4. Leave the penny in the solution for ~ 5 min. Once a zinc layer has been coated on the surface of the penny, carefully remove the penny with plastic tweezers (metal tweezers will react with excess sodium zincate in solution), wash with distilled H2O and dry. Place the penny on a watch glass or surface that is heat resistant.
Then a burner can be set up on the work bench. Hold the penny over the flame with a pairof tongs for a few seconds until it turns gold. Turn the penny around so that both sides areheated equally, overheating will cause the penny to tarnish. Run the penny under water tocompletely cool it down. You may need to wipe off some of the soot on the surface to see the penny’s new luster.
The zinc solution must be completely transferred to the collection bottle in the hood. Any zinc residue should be rinsed out of the beaker and placed in the waste. Zinc should not come in contact with paper towels.
DATA & DISCUSSION
Analysis: Submit your plated pennies upon completion. (2 marks each) Submit a formal lab report and include all previous labs from the semester for comparison (6 marks)
1.Write a balanced chemical equation for the formation of Zn(OH)2 from Zn(s) and OH- (aq). (4 marks)
2.Is the reaction in question 1 an oxidation ½ reaction or a reduction ½ reaction.
How do you know? (3 marks)
3.In 1996 the composition of Canadian pennies changed dramatically. Indicate, upon conducting some brief research, why this is the case. (2 marks)
4.Repeat the experiment and change only one of the variables. Explain which variable was changed and the result of this change. (3 marks)
5.Provide the name for Zn(OH)3 H2O. (2 marks)
6.Draw the 3- dimensional VSEPR shape for Zn(OH)4. Name this shape. Is this molecule polar or non- polar? (4 marks)
7.Why is the reaction heated? Provide reasoning based on collision theory and reaction rates. (4 marks)
8.Explain whether or not the formation of the zinc coating is an equilibrium.
Justify your answer.(3 marks)
9.Is the reaction more similar to a galvanic cell or an electrolytic cell. Explain. (2 marks)
TOTAL/35