The concept of equilibrium constants is fundamental in Chemistry Topics as it quantitatively describes the extent of chemical reactions at equilibrium.
How does Electroplating Process Work?
The process of depositing a thin layer of a desired metal over a ‘metal object’ with the help of electric current is called electroplating. The purpose of electroplating is to protect the metal objects from corrosion (or rusting) or to make the metal objects look more attractive.
In other words, electroplating is done (i) for protection against corrosion (or rusting), and (ii) for decorative purposes. For example, bathroom taps made of iron (or steel) are electroplated with chromium metal to prevent their corrosion (or rusting). And at the same time, chromium plating gives a shining appearance to the bathroom taps due to which they look more attractive (see Figure).
The metal objects (or metal articles) are usually electroplated with chromium, tin, nickel, silver, gold or copper metals. When a metal object is electroplated with chromium, it is called chromium plating (or chrome plating). When a metal object is electroplated with tin, it is called tin plating. When a metal object is electroplated with nickel, it is called nickel plating.
When a metal object is electroplated with silver, it is called silver plating. When a metal object is electroplated with gold, it is called gold plating. And when a metal Figure. This tap has been object is electroplated with copper, it is called copper plating. The metals which electroplated with chromium are used for electroplating are those which are resistant to corrosion and also give a shiny finish to the object. The following points should be remembered while electroplating :
- The ‘metal object on which electroplating is to be done’ is made the negative electrode (cathode) : It is connected to the negative terminal of the battery.
- The ‘metal whose layer is to be deposited’ is made the positive electrode (anode): It is connected to the positive terminal of the battery.
- A water soluble salt of the ‘metal to be deposited’ is taken as the electrolyte (The electrolyte contains the metal to be deposited in the form of a soluble salt).
We will now describe the process of electroplating by taking the example of copper plating.’ We will electroplate a key made of iron metal with a thin layer of copper metal (Any other object made of iron can also be used in place of key). For electroplating an iron object with copper metal (or copper plating) :
(a) The iron object is made negative electrode (cathode). This means that the iron object is connected to the negative terminal of the battery.
(b) A copper plate is made positive electrode (anode). This means that a copper plate is connected to the positive terminal of the battery.
(c) Copper sulphate solution (CuSO4 solution) is taken as electrolyte (It contains copper metal in dissolved form as copper ions, Cu2+).
Electroplating (Copper Plating)
We will electroplate an iron object in the form of an iron door key with copper metal. In other words, we will describe the copper plating of an iron key. The experimental set-up for copper plating an iron key is shown in Figure. Take 250 mL of distilled water in a clean beaker. Dissolve two teaspoonfuls of copper sulphate in it. This will give us a blue coloured copper sulphate solution.
Add a few drops of dilute sulphuric acid to copper sulphate solution to make it more conducting. Take a copper plate of about 10 cm x 4 cm size and a door key made of iron. Clean the surfaces of copper plate and iron key by rubbing with sand paper. Then wash them with water and dry them.
- Immerse the cleaned copper plate in copper sulphate solution in the beaker. Connect the copper plate to the positive terminal of a battery through a switch (see Figure). This copper plate becomes the positive electrode (or anode).
- Immerse the cleaned iron key also in copper sulphate solution at a small distance from the copper plate. Connect the negative terminal of the battery to the iron key (see Figure). This iron key becomes the negative electrode (or cathode).
- Switch on the electric current by closing the switch. Allow the current to pass for about 15 minutes.
- Now remove the copper plate and iron key from the copper sulphate solution and look at them carefully.
We will find that the copper plate has dissolved a little and the iron key has got a reddish layer of copper metal all over its surface. Thus, the iron key has been electroplated with copper. That is, the iron key has been copper plated. The copper sulphate solution remains unchanged.
The process of electroplating copper on an iron key can be explained in the most simple way as follows :
The copper sulphate solution (taken as electrolyte) has copper metal in the dissolved form. In fact, the copper sulphate solution consists of free positively charged copper ions (Cu2+) and negatively charged sulphate ions (SO42-). When electric current is passed through copper sulphate solution, then the following changes take place :
(a) The dissolved copper metal present in copper sulphate solution as positively charged copper ions (Cu2+) gets attracted to the negatively charged electrode ‘iron key’. The positively charged copper ions (Cu2+) lose their positive charge on coming in contact with negatively charged iron key and form copper atoms (Cu).
These copper atoms deposit on the iron key to form a thin layer (or coating) of copper metal all over the surface of iron key. In this way, copper metal in the electrolyte comes out of the solution and forms a thin layer on the iron key (which is the cathode here).
(b) The copper metal of positively charged copper plate electrode dissolves by forming positively charged copper ions, Cu2+ (This happens because copper atoms lose their negative charge to positively charged electrode). The copper ions thus formed go into the copper sulphate solution.
In this way, the loss of copper ions from copper sulphate solution (utilised in copper plating) is made up and the process continues. Since the copper ions are taken out from the solution at the negative electrode (iron key) but put into solution at the positive electrode (copper plate), therefore, the concentration of copper sulphate solution (or electrolyte) remains constant.
From the above discussion we conclude that during copper plating of an iron key, copper metal is transferred from the copper plate to the iron key through the copper sulphate solution (with the help of electric current). In other words, during electroplating with copper, copper metal gets transferred from positive electrode to the negative electrode (the negative electrode being the metal object to be electroplated).
Carbon (in the form of graphite) is a non-metal which is a good conductor of electricity. So, if we take a carbon rod (in place of iron key) in the above activity and connect it to the negative terminal of the battery, then a layer of copper metal will be formed on the carbon rod. Thus, a carbon rod can also be electroplated with copper (or copper plated). This is because carbon rod is a good conductor of electricity. Some special plastics have now been made which are good conductors of electricity. Such plastics can also be electroplated with metals.
Uses of Electroplating
Electroplating is a very useful process. It is widely used in industry for coating metal objects (or metal articles) with a thin layer of a desired metal. The metal ‘which is deposited’ in the form of a thin layer has some desired properties which the metal of the object does not possess.
For example, the metal which is deposited during electroplating is less reactive (than the metal of the object) and hence resists corrosion. It has also a shiny appearance (whereas the metal of the object has a dull appearance). This will become more clear from the following examples.
(i) Chromium metal has a shiny appearance and it does not corrode (it does not rust). Chromium metal is, however, quite expensive and hence it is not economical to make whole object out of chromium.
So, the object (or article) is made of a cheaper metal (like iron or steel) and only a thin coating of chromium metal is deposited all over its surface by electroplating (chromium plating). After chromium plating, it looks as if the whole iron (or steel) object is made of the chroihium metal. For example, chromium plating is done on many objects made of iron metal (or steel) such as bicycle handlebars, bicycle bells, wheel rims, bathroom fittings (taps, etc.), LPG stoves, motor cycle parts, and many, many other objects (see Figure).
Metallic car bumpers and car grills are also chromium plated. We know that an iron (or steel) object is not very attractive to look at. Moreover, iron (and steel) objects corrode or rust gradually. So, a thin coating (or layer) of chromium metal on iron and steel objects (deposited by electroplating) makes the iron and steel objects look shiny and attractive, as well as protects them from corrosion (or rusting).
If, however, the thin layer of chromium plating on an object is accidently scratched, then the shiny coating of chromium comes off and the iron or steel surface beneath it gets exposed. Rusting of iron and steel objects can then take place.
From the above discussion we conclude that electroplating is used to cover iron and steel objects with a thin layer of chromium metal. This chromium layer gives an attractive, shiny surface and also protects iron and steel objects from rusting.
(ii) Tin metal has a shiny appearance, it does not corrode and it is non-poisonous. It is less reactive than iron. Tin ‘cans’ used for storing food are made by electroplating tin metal on to iron. Due to tin plating over the surface of iron, the food does not come in contact with iron and is protected from getting spoilt.
In fact, the less reactive and shiny metals (like chromium, tin and nickel) are electroplated on more reactive and dull looking metals (like iron and steel) to protect them from corrosion and give them an attractive finish.
(iii) Electroplating is used to give objects made of a cheap metal a coating of a more expensive metal to make them look more attractive. For example, less expensive metals are electroplated with more expensive metals like silver and gold to make jewellery (or ornaments). These ornaments have the appearance of silver or gold but they are much less expensive (than ornaments made of pure silver or gold).
So, many times, the ornaments worn by women may appear to be made of gold but they are not really of gold. They are gold plated ornaments. However, with repeated use, the gold coating of these electroplated ornaments wears off revealing the cheaper metal silver (or some other metal) beneath it. These ornaments have then to be gold plated again. Similarly, aluminium objects look more attractive when electroplated with nickel.
The conducting solutions (or electrolytes) used in electroplating contain various type of salts which may be poisonous. The conducting solutions are also usually acidic in nature. So, in electroplating factories, the disposal of used conducting solutions (or electrolytes) is a major problem.
The conducting solutions (or electrolytes) used in electroplating process are polluting wastes and hence should be disposed of in a proper way so as to protect the environment. There are specific guidelines issued by the Government for the safe disposal of electrolyte wastes produced by the electroplating factories.