CHEMICAL & ELECTROCHEMICAL PLATING
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. 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 an electric 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 or
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 a 3 M NaOH solution and a 0.5 M ZnSO4
solution.
Part A: Chemical Plating of a Penny
One solution of the following needs to be created for all the students in the lab section:
Pour ~ 100 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 pair of tongs for a few
seconds until it turns gold. Turn the penny around so that both sides are heated equally, overheating will cause
the penny to tarnish. Run the penny under water to completely 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.
The penny is yours to keep. Do NOT, under any circumstances, throw the penny in the trash. If you do not
wish to keep the penny, I’ll keep it.
Part B: Electrochemical Plating of a Penny
Weigh the penny and record the mass.
Attach one of the wires to the penny and the positive pole of the power supply and attach a second wire to the
paper clip and the positive pole of a 9.0 V battery or variable power supply.
Penny (cathode)
Paper clip (anode)
Pour ~100 mL of 0.5 M ZnSO4 solution into a 250 mL beaker. Immerse the electrodes (the penny and the paper
clip, make sure the electrodes don’t touch) into the solution.. Remove the penny form the solution after (exactly)
60 sec. Dry the penny carefully and record the final mass
Pour the zinc solution into the waste bottle in the hood. Do not flush it down the drain. The paper clip should be
placed in the beaker in the collection beaker in the hood. The penny is yours to keep. Do NOT, under any
circumstances, throw the penny in the trash.
DATA & DISCUSSION
(1) Describe how an alloy of Cu and Zn is formed. What are some other alloys commonly used in industry?
(2) Brass contains 30 wt.% Zn and 70 wt.% of Cu. How many mols of Cu and Zn are present in 100 g of brass?
(3) Is the plating of Zn(s) on Cu(s) a spontaneous process? What is the purpose of the battery?
(4) Which is more effective in the chemistry lab – chemical plating or electroplating and why?
(https://eee.uci.edu/programs/hongchem/5hMANPlating.pdf)