Introduction:
Electrolysis is a process of breaking down (or decomposes) a chemical compound into its elements by using an electric current. The electric current is passed through an electrolytic cell, which consists of electrodes and electrolyte. Oxidation or reduction reactions occur in these electrodes. Electrolyte is a substance which contains free ions that make the substance electrically conductive. Electrolyte can be aqueous solution of molten ionic compound or the aqueous solution of ionic compound, alkali, or an acid. Hence electrolysis can be of two types
i) electrolysis of molten ionic compound
ii) Electrolysis of aqueous solutions.
Consider the example of sodium chloride (NaCl). Molten sodium chloride is obtained when we heat it till it melts. Whereas if, sodium chloride is dissolved in water, gives the aqueous solutions of sodium chloride. Solution of water of a substance is called the aqueous solution. Since the aqueous solution contains more than one type of ions, electrolysis of aqueous solutions is entirely different from that of molten electrolyte.
i) Electrolysis of aqueous sodium chloride :
In the electrolysis of aqueous solutions, not only sodium chloride will dissociate into Na+ and Cl-, but some of the water molecules will also decompose to give hydrogen (H+) and hydroxide ions (OH-). The reaction of electrolysis of aqueous solutions (of NaCl) can be written as follows,
NaCl ---> Na+ + Cl-
H2O ---> H+ + OH-
In an aqueous solution, there can be more than one positive and one negative ion. There is a selective discharge, which means when there is a movement of ions to cathode or anode, only one negative ion and one positive ion will be selected to be discharged.
ii) Electrolysis of Aqueous Sulphuric Acid
It is one of the examples of electrolysis of aqueous solutions.
The aqueous sulphuric has three types of ions. They are hydrogen ions (H+), sulphate ions (SO42-), and also hydroxide ions (OH-) from the water. The equation can be written as follows,
H2SO4 + H2O --> 2H+ + SO42- + H+ + OH-
Diagram showing electrolysis of water:
The apparatus used for the Electrolysis of Aqueous Sulphuric Acid
There is more than one type of ion moving to the electrode, there is preferential discharge (also called as selective discharge) takes place.
The chemical reactions occur at anode and cathode is given in the table below.
Note:
When the electrolysis of aqueous solutions of dilute acids or alkalis is done, the volume of hydrogen given off at the cathode is approximately twice that of the oxygen gas at the anode.
Electrolysis is a process of breaking down (or decomposes) a chemical compound into its elements by using an electric current. The electric current is passed through an electrolytic cell, which consists of electrodes and electrolyte. Oxidation or reduction reactions occur in these electrodes. Electrolyte is a substance which contains free ions that make the substance electrically conductive. Electrolyte can be aqueous solution of molten ionic compound or the aqueous solution of ionic compound, alkali, or an acid. Hence electrolysis can be of two types
i) electrolysis of molten ionic compound
ii) Electrolysis of aqueous solutions.
Consider the example of sodium chloride (NaCl). Molten sodium chloride is obtained when we heat it till it melts. Whereas if, sodium chloride is dissolved in water, gives the aqueous solutions of sodium chloride. Solution of water of a substance is called the aqueous solution. Since the aqueous solution contains more than one type of ions, electrolysis of aqueous solutions is entirely different from that of molten electrolyte.
Electrolysis of aqueous solutions:
i) Electrolysis of aqueous sodium chloride :
In the electrolysis of aqueous solutions, not only sodium chloride will dissociate into Na+ and Cl-, but some of the water molecules will also decompose to give hydrogen (H+) and hydroxide ions (OH-). The reaction of electrolysis of aqueous solutions (of NaCl) can be written as follows,
NaCl ---> Na+ + Cl-
H2O ---> H+ + OH-
In an aqueous solution, there can be more than one positive and one negative ion. There is a selective discharge, which means when there is a movement of ions to cathode or anode, only one negative ion and one positive ion will be selected to be discharged.
Electrolysis of aqueous solutions:
ii) Electrolysis of Aqueous Sulphuric Acid
It is one of the examples of electrolysis of aqueous solutions.
The aqueous sulphuric has three types of ions. They are hydrogen ions (H+), sulphate ions (SO42-), and also hydroxide ions (OH-) from the water. The equation can be written as follows,
H2SO4 + H2O --> 2H+ + SO42- + H+ + OH-
Diagram showing electrolysis of water:
There is more than one type of ion moving to the electrode, there is preferential discharge (also called as selective discharge) takes place.
The chemical reactions occur at anode and cathode is given in the table below.
At cathode | At anode |
Here, each hydrogen ion (H+) gains electron and becomes hydrogen atom.Two of the hydrogen atom will combine and produce hydrogen gas molecule.Equation:2H+ + 2e ---> H2 | Here there is a choice of sulphate ions (SO42-) or hydroxide ions (OH-)Oxygen is gives off at anode, because of hydroxide ion is easier to discharge.Equation:OH- + 4e ---> O2 + H2O |
Note:
When the electrolysis of aqueous solutions of dilute acids or alkalis is done, the volume of hydrogen given off at the cathode is approximately twice that of the oxygen gas at the anode.
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