One of the most important applications of electrolytic cells is the process of electroplating, in which a thin layer of a metal is deposited on an electrically conducting surface.
Electroplating has many commercial applications including decorative and protective coatings.
The purpose of this project is to get familiar with electroplating process and learn how it works, however you can expand your study and also determine the amount of electricity used to plate certain mass of copper. You may also calculate atomic mass of copper.
In electroplating, the metal to be plated is used as the anode and the electrolytic solution contains an ion derived from that metal. In this experiment, a copper anode (pre-1982 US penny) will be used in a solution of copper sulfate. Copper will be plated out onto a second penny at the cathode.
The old pre-1982 copper pennies were 95 percent copper and 5 percent zinc, but the new pennies are 97.6 percent zinc, coated with a thin layer of copper that forms 2.4 percent of the total weight of each coin.
What is Electroplating?
Electroplating is one kind of surface finishing. There are many other kinds. Everyone has seen and handled electroplated objects, even if they didn’t know it. Examples include kitchen and bathroom faucets, inexpensive jewelry and the trim on some automobiles. There are thousands of examples. In fact, it is certain that nearly every piece of metal you have ever seen has been through some kind of surface finishing process. There are three basic reasons for surface finishing: to improve appearance, to slow or prevent corrosion (rust) and to increase strength and resistance to wear (in the case of “engineering” finishes). An object may be processed for any or all of these reasons.
The term electroplating means the coating of an object with a thin layer of metal by use of electricity. The metals most often used are gold, silver, chromium, copper, nickel, tin and zinc, but many others are also used . The object to be plated is usually a different metal, but can be the same metal or a non-metal, such as a plastic grille for an automobile.
Electroplating usually takes place in a “tank” of solution containing the metal to be deposited on an object. This metal is in a dissolved form called ions. An ion is an atom that has lost or gained one or more electrons and is thus electrically charged. You cannot see ions, but the solution may show a certain color; a nickel solution, for example, is typically emerald green. The deposited metal, however, will be gray or silver in appearance.
When certain metallic chemicals dissolve in water, the metal atoms of these chemicals are freed to move about, but lose one or more electrons (negative charges) and, as a result, are positively charged. The object to be plated is negatively charged and attracts the positive metal ions, which then coat the object to be plated and regain the lost electrons to become metal again.
A familiar example of this process is the experiment often performed in which a key is plated with copper. The key (called the cathode) is connected to the negative terminal of a battery and is placed in a solution of vinegar, a weak acid. The positive terminal of the battery is connected to a piece of copper (called the anode–and often just a copper wire), which is placed in the solution. The acid slowly dissolves the wire, making copper ions that are then attracted to the key, regaining their lost electrons and becoming copper metal again, but now in the form of a thin coating on the key. The battery forces all this to happen and prevents the deposited copper from re-dissolving.
Now look at the illustration. Positively charged copper ions are free in the solution, but are being attracted by the negatively charged key. As the ions contact the key, they regain their lost electrons and become copper metal and stick to the key wherever they touched it. This is the basic process of electroplating, and all forms of it work the same way.
Other Common Finishing Processes
There is another plating process, discovered in 1946, called electroless plating. It earned that name because it operates without using electricity; the action is purely chemical and runs by itself, once started.
Electrical and chemical processes are not the only ways to coat an object. Another important process is called vapor deposition or vacuum coating. In this process, the metal to be deposited is converted into a vapor that is allowed to condense on the object to be coated. Many beautiful finishes can be obtained by this process, which is carried out in a vacuum chamber.
Another increasingly important surface finishing process is powder coating. It depends on the fact that opposite charges attract, just as in electroplating. The object to be coated is electrically charged and is sprayed with a non-metallic powder that sticks to it. The object is then passed through an oven where the powder particles melt and run together to make a smooth finish. Articles commonly coated in this way include lawn mower frames, sports equipment, playground equipment, lawn furniture and the insides of refrigerators, washing machines and dryers.
Some metals including aluminum can not be coated by electroplating method. In order to protect aluminum from rust, we usually anodize it. Anodizing process involves producing a very thin, invisible layer of the oxide on the aluminum, which protects it from the kinds of corrosion that affect aluminum, such as salt air from the ocean. The oxide is a chemical combination of aluminum and oxygen. Anodized aluminum can be dyed to produce any desired color.
Electroplating uses a small electric current to drive atoms of one metal from one object onto the surface of another.
(An atom is the smallest amount of an element — the basic building blocks of all matter. An atom is the smallest amount of stuff you can have, and still be able to call it an element.)
The process was discovered by physicist Michael Faraday, who did most of his work dealing with the connections between electricity and magnetism.
To “plate” one material onto another, you need to have a setup like the top diagram. The anode is the metal you want to plate onto the cathode. The electrolyte is a liquid that conducts electricity and that will dissolve the anode. It will also contain dissolved ions of the anode metal in it. (An ion is any atom, or group of atoms, with an electrical charge, either positive or negative.) The key to this process is the battery. It supplies the energy needed for the atoms in the anode to dissolve into the electrolyte, and eventually “stick” to the cathode.
If you let enough electrical current pass through the solution, you will get a layer of anode metal on the cathode, with the anode dissolving into the electrolyte. The thickness of the layer can be precisely controlled by varying the amount of current that passes through the electrolyte.
Quick Reference Electroplating Guide
|GOLD||140ºF||6V||30 sec||24K gold|
|RHODIUM||80 – 100ºF||4V||20 – 30 sec||14K or platinum|
|COPPER||100ºF||6V||30 + sec||copper|
|NICKEL||70ºF||2V||3 – 4 min||nickel|
|SILVER||70ºF||2V||30 + sec||silver|
Electrodeposition of metals is performed by immersing a conductive surface in a solution containing ions of the metal to be deposited. The surface is electrically connected to an external power supply, and current is passed through the surface into the solution. This causes reaction of the metal ions with electrons to from metal.
As copper is plated out at the cathode (negative electrode), copper goes into solution at the anode (positive electrode) as copper(II) ions, maintaining a constant concentration of copper(II) ions in the electrolytic solution.
cathode: Cu2+(aq) + 2 e- —> Cu(s)
anode: Cu(s) —> Cu2+(aq) + 2 e-
Commercial plating is done very slowly in order to obtain a smooth, even coating of the plated metal. Although this experiment does not produce plating of commercial quality, it gives you the opportunity to study the chemistry of an important commercial process. This general method is also used in purifying copper. A small cathode of pure copper is used with a larger anode of impure copper. As the electrolytic cell operates, pure copper is transferred to the cathode.
You should be introduced to Faraday’s law before doing this experiment. From this law, you will note that 2 x 96,485 coulombs of charge are required to produce one mole of copper from copper(II) ion. If an ammeter reading is taken, the number of coulombs that actually passed through the electrolytic cell can be calculated by using the formula; q = It, where q is the charge in coulombs, I is the current in amperes, and t is the time in seconds. From the coulombs of charge that pass through the cell, you can calculate the theoretical number of moles of copper that should have plated out and compare this to the actual number of moles that were plated out. If one assumes that the theoretical yield is equal to the actual yield, the atomic mass of copper can be calculated.
If an ammeter reading is not taken, you can compare the changes in mass of the two electrodes and from the number of moles of copper plated, calculate the number of coulombs of charge passed through the cell and the average current through the cell.
Faraday’s constant = 96485 Coulombs/mole
The purpose of this experiment is to demonstrate the process of electroplating and a commercial method used to purify copper. We will also use the Faraday’s constant to determine the atomic mass of copper or calculate to see how much electricity we have used to create certain amount of copper plating.
When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.
Variables that can affect the electroplating process are:
- Concentration of the electrolyte
- Temperature of electrolyte
- Voltage used in electroplating
Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.
I think I can use a regular 6 volt battery known as lantern battery as a power source and perform copper plating on small objects such as coins, keys and pins. I can then weight the object and calculate added mass as a result of copper plating. I can then use the added mass to calculate how much electricity is used for the plating process.
- Pour 200 mL of the electrolyte solution into the beaker. Electrolyte solution is a strong solution of copper sulfate plus a few drops of sulfuric acid in order to keep the solution acidic.
- Attach connecting wires with alligator clips to the terminals of the power supply or a 6-volt battery.
- Clean the pennies with a mixture of 3 g NaCl and 15 mL vinegar; rinse and dry.
- Tightly wrap one end of a 10-cm length of copper around each penny, leaving 5-6 cm of wire free.
- Mass each penny-copper wire assembly and record the masses.
- Push the free end of each wire through the cardboard square and place the square over the beaker so that the penny “electrodes” are immersed in the electrolyte solution as illustrated below. Note: the two electrode assemblies must not touch.
- Attach the connecting wires to the top of the copper wire assemblies.
- Allow the electroplating cell to operate for 30-60 minutes. Record the exact time the cell was operating (optional).
- Record the ammeter reading from the meter or power supply (optional).
- Remove each “electrode;” dry, being careful not to lose any of the copper plating; mass each and record.
- Perform the following calculations.
- If an ammeter reading is taken, calculate:
- the number of coulombs of charge passed through the electrolytic cell,
- the theoretical number of moles of copper that should have plated out,
- the actual number of moles of copper that plated out, and
- the % yield of copper.
- If an ammeter is not used, calculate:
- the number of moles of copper removed from the anode,
- the number of moles of copper added to the cathode,
- the % of copper conserved,
- the average current that must have passed through the cell.
- If an ammeter reading is taken, calculate:
WARNING: Sulfuric acid can cause severe burns; handle with care. Goggles must be worn throughout this experiment. Although the power source is relatively weak, the electrodes and connecting wires should not be handled when the cell is operating. If a 9-V battery is used as the power source, it will become quite hot during use; caution should be exercised.
Materials and Equipment:
electrolyte solution (200 g CuSO4· 5H2O + 25.0 mL concentrated H2SO4 solution in enough distilled or deionized water to make l.00 L of solution)*
NaCl (Sodium Chloride/ table salt)
power supply (6.0-9.0 volts, 0.60-1.0 amps)*
connecting wires with alligator clips
16-18 gauge copper wire
cardboard square (approx. 15 cm on a side)
- Copper(II) sulfate pentahydrate is available from garden supply stores as root eater.
- Sulfuric acid is available from auto supply stores as battery acid. Substitute 95 mL of battery acid for 25 mL of concentrated sulfuric acid.
- A battery charger or 9-V battery may be substituted for the power supply. If a 9-V battery is used, it will be nearly “dead” after completing the experiment.
- A 16-oz plastic glass may be substituted for the beaker.
Results of Experiment (Observation):
Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.
Imagine we plated one gram copper. We know that 64 grams of copper requires 2 x 96485 Coulombs. [The number 2 is because copper ion (Cu++) gets 2e- to become copper metal.] So one gram copper is produced using 2 x 96485 / 64 coulombs. (=3015 Coulombs). If the process took 300 seconds, using the formula of q=I x t we can calculate that the average current during the process has been 10 Amps.
If we have used a 6-Volt battery, using the formula of W=V x I we can calculate that 60 watts electricity has been used for this process.
Summary of Results:
Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.
It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.
Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Related Questions & Answers:
What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.
- If you are using the mass of plated copper to determine the consumed electricity during this process, note that part of electricity is also being wasted in the form of heat in battery, in power supply or in the plating bath.
Masterton, W.L., Slowinski, E.J., and Stanitski, C.L., Chemical Principles, Saunders College Publishing, 1985, p. 701.
– This work describes the chemistry of electroplating and specific commercial electroplating solutions.
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