Electrolysis of water | Electroplating | Electrorefining process |How does it works|
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Chemical effect of electric current
Electrolysis of water
On passing electric current in acidified water (impure water), water is broken into hydrogen and oxygen gases. This happen due to the chemical effect of electric current on water. The splitting of water into its compound gases by using electricity is called electrolysis of water.
An apparatus called voltmeter or electrolytic cell is used to perform electrolysis. A voltmeter contains electrodes and the electrolytic solution ( here acidified water).
The electrode are connected to a battery via or switch and depending on their connection to the battery are classified -anode(+terminal) and cathode(-ve terminal).
Explanation :- Water is a compound of hydrogen and oxygen and under normal conditions, has positively charged hydrogen ions (H+) called cation and negatively charged hydroxyl (OH-) ions called anions. Both the cation and anions being in number, water remains electrically neutral.
On passing electric current in impure water (to make it a conductor), the H+ ions move towards negative terminal (cathode) and get collected over it. The electrons at the cathode make H+ ions neutral hydrogen atoms.
2H+ + 2e- →2H
Hydrogen atoms combine to form a molecule and this comes out as hydrogen gas. The One ions move towards positive terminal (anode) and lose electrons to become neutral OH which combine to form water with the libration of oxygen.
4OH- →4OH + 4e-
4OH → 2H2O + O2
The oxygen is collected at the anode .
Thus, ultimately it is seen that hydrogen gas bubbles have collected on the cathode and oxygen gas bubble on the anode. It is found that by volume hydrogen gas collected over cathode is twice volume of oxygen gas collected over anode.
• The chemical effect of electric current depends on the electrolytic and on the nature of electrodes used.
For example,
• As you have seen above, bubbles of a gas (or gases) may be formed on the electrodes.
• Deposits of metals may form on electrodes. Thus, when electric current is passed through copper sulphate solution, e.g., during electroplating.
• Change of colour takes place when electric current passes through a cut potato.
Electroplating
Electroplating is an electrolytic process of deposition of a thin film of now reactive or superior metal, like nickel, silver, chromium, gold on the surface of metallic article of a more reactive or inferior or a base metal, e.g., iron, copper, brass with the help of electricity.
Objectives of electroplating
1:- protects the surface of the base metal from rusting and Corrosion E.g. Nickel and chromium plating is done on steel to prevent it from rusting.
2:- Makes the article attractive, E.g. a brass or copper or any other article made of cheaper metal os electroplated with silver or gold.
Conditions for electroplating
• The surface of the metal to be electroplated should be thoroughly cleaned with an alkali and then made free of grease or oil.
• A direct current should be passed as A.C. current causes periodic change of polarity of electrodes thus giving no effective coating.
• The metallic article to be electroplated is always placed at the cathode because during electrolytic reaction, the metal is always deposited at the cathode by gain of electrons.
• The metal to be electroplated on a metallic article is always made the anode and needs to be periodically replaced because the metal at the anode continuously dissolve as ions in solution.
• A small current should be applied for a long period of time to get the proper thickness and consistent coating.
• Suitable temperature is necessary around the electroplating equipment. Conduction of electricity through electrolytes increases with the rise in temperature.
Process of electroplating
Copper plating :- Electroplating an iron object with copper metal
• crystals of copper sulphate (blue vitriol) are dissolved in pure metal to prepare a concentrated solution. Now, a few drops of concentrated sulphuric acid are added to increase the conductivity of electricity through this electrolyte.
• The iron object is made the negative electrode (cathode) by connecting it to the negative terminal of the battery.
• The copper plate is made the positive electrode (anode) by connecting it to the positive of the battery.
• Switch on the current and let it remain in the 'ON' positions for about 15minutes.
• After switching of the current, take out the iron object from the solution.
You will find that copper plate has dissolved a little and the iron, object is coated with shiny and uniform reddish layer of copper. The copper sulphate remains unchanged
Explanation :- When electric current passes through the copper sulphate solution, the copper ions(Cu++) migrate from the copper sulphate solution toward the cathode (-ve terminal). These copper get deposited on the iron object (the cathode) and forma coating of fine layer on it.
The loss of copper ions from the copper sulphate solution is replenished by the copper atoms released from the copper plate (the anode). Thus the concentration of the copper sulphate solution (the electrolyte) remains unchanged. The copper plate gradually gets degenerated due to loss of copper atoms.
Electrolytic process in equation
CuSO4 → Cu²+ + SO4²-
Electroplating of brass with silver
In this case, the solution of 𝘴𝘰𝘥𝘪𝘶𝘮 𝘢𝘳𝘨𝘦𝘯𝘵𝘰 𝘤𝘺𝘢𝘯𝘪𝘥𝘦 or 𝘱𝘰𝘵𝘢𝘴𝘴𝘪𝘶𝘮 𝘢𝘳𝘨𝘦𝘯𝘵𝘰 𝘤𝘺𝘢𝘯𝘪𝘥𝘦 is taken as the electrolyte. It is prepared by adding sodium cyanide to a solution of silver nitrate until the precipitate formed jus dissolve
The silver plate (the metal whose coating is to be done) is at the +terminal (anode) and the object to be electroplated (brass spoon) is at the cathode (-ve terminal).
Explanation. On passing the electric current the electrolyte ( sodium argento cyanide solution) is dissolved into sodium (Na+) and silver cyanide ions [Ag (CN)2]- which further dissociate into silver ions [Ag+] and cyanide ions (CN-).
Process of electroplating
The Ag+ ions migrate to the cathode and get deposited as a thin metallic film on the brass spoon.
Ag+ + e- → Ag
( electron at the cathode)
The silver atoms from the anode lose electrons enter the solution as silver ions.
Ag – e- → Ag+
[Silver atoms from [ goes into the the silver plate(anode)] electrolyte]
CN- and OH- ions migrate to the anode but none of them discharged.
Note:- Silver nitrate (AgNO3) solution is not used because in this case the deposition of silver will be very fast and hence not very smooth and uniform
Electroplating with Nickel
Rusting of iron can be prevented if iron article is coated with a metal like nickel, zinc or tin. The process is similar to described in I and II above. The electrolyte is the aqueous solution of nickel sulphate [ NiSO4].
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| Electroplating of an article with Metal |
Electrorefing
The chemical effect of electricity is also used in the refining of impure metal like copper, zinc and aluminium.
Refining of copper
Method:- A thick rod of the impure metal (copper) is made the positive electrode (anode) and a thin strip of pure metal (copper) is made the cathode. A water soluble salt of the metal to be purified (here, copper sulphate and dilute sulphuric acid) is taken as the electrolyte.
An electric current is passed through the solution for about half an hour. When this is done the impure copper rod will gradually become thinner and thinner whereas the pure plate will become thicker and thicker.
This happens because impure copper metal at the anode gradually dissolves in the solution whereas the pure metal from copper sulphate solutions gets deposited on the copper plate at cathode.
Electrometallurgy
Electrometallurgy is the process of extraction of metals by electrolysis. Reactive metals like sodium, potassium, calcium and magnesium are extracted from their aqueous solutions by electrolysis.
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