In order to fully understand the meaning of the word electrolysis, one must start with the word itself.
The suffix ''lysis'' is of Greek origin and denotes breaking up or splitting. The prefix ''electro'' refers to the flow of electrical current. Therefore, electrolysis is a process in which a compound is split into its component elements by passing electricity through it.
Electrolysis does not take place on its own. For example, the compound sodium chloride does not decompose by itself. Electrolysis is a non-spontaneous chemical change; in other words, electrolysis can only occur if electrical energy is supplied. Electrical energy is the force that drives the electrolysis process.
Where is electrolysis used? Here are some real-world applications of electrolysis.
- Elements like aluminum, used in daily life. are obtained from their natural resources by the process of electrolysis.
- Crude metals are refined by electrolysis.
- Cosmetic jewelry can be gold plated to give a more expensive look.
- Metals are electroplated to protect them from corrosion.
- Electrolysis is used in hair removal.
If electrical energy can cause a chemical change, as in electrolysis, can chemical changes produce electrical energy?
- In a beaker, take some copper sulfate solution and place a copper rod in it.
- Take zinc sulfate solution with a zinc rod in another beaker.
- Connect the two beakers with a salt bridge.
- Connect the two metal rods with a voltmeter in between.
- The voltmeter shows deflection, indicating a potential difference exists between the two rods. An electric current flows through the setup.
This is an example of an electrochemical cell or Galvanic cell.
The cells used in remotes and flashlights produce electrical energy from chemical energy.
 |
A cell where electrolysis takes place is called an electrolytic cell.
- Insert two connecting wires into a cylinder of water.
- Add a few drops of dilute sulfuric acid to the water.
- Connect the two metal wires to a power source like a battery. It has to be a DC source.
- The connecting wires in the cylinder of water are called electrodes.
- The metal wire connected to the negative terminal of the power source is the cathode
- The one connected to the positive terminal of the power source is the anode.
- Place an inverted test tube of water over the cathode.
- With time, bubbles of gas will emerge in the cylinder and the water level will be pushed down in the inverted test tube as the gas collects in it.
- Water has been decomposed into hydrogen and oxygen by electrolysis.
 |
In a sense, electrochemical cells are the converse of electrolytic cells.
Electrolytic cells and electrochemical cells can be compared as follows:
| No/ Cell | Electrolytic Cell | Electrochemical cell |
|---|---|---|
| 1 | Electrical energy is used to cause chemical reaction | Electrical energy is produced from chemical reactions |
| 2 | Non-spontaneous | Spontaneous |
| 3 | External energy source needed | External energy source not needed |
| 4 | Electrical energy converted to chemical energy | Chemical energy converted to electrical energy |
| 5 | Metal rods dip in a suitable salt solution | Each metal rod dips in its own salt solution |
What are Electrolytes?
Electrolysis is a process to decompose neutral compounds by passing electricity through them. Not all compounds can undergo the process of electrolysis. The compounds selected for the electrolysis process must be ionic; the compounds must be made up of ions.
Not only this, but the ions should be free to move. Only then electrolysis takes place. Ions in a solid are tightly held together by electrical forces of attraction. For the process of electrolysis to occur, the ions must be loosened from their positions. This is done by dissolving the compound in water or by heating the compound to a high temperature that it becomes a liquid. Such solutions or molten forms of ionic compounds conduct electricity and are called electrolytes.
If a solution of the compound produces a large number of ions, it is termed a strong electrolyte.
Examples are salt solutions, acids like sulfuric and hydrochloric acid, and alkalis like sodium and potassium hydroxide.
Weak electrolytes are not able to produce many ions in solution. They ionize to a limited extent. Some examples of weak electrolytes are acetic acid (ethanoic acid), carbonic acid, nitrous acid, and others. Such solutions do not conduct electricity very well.
For the process of electrolysis to take place easily, the electrolyte used should be strong. A large number of ions in the solution makes the conduction of electricity easy.
| No/ Electrolyte | Strong Electrolytes | Weak Electrolytes |
|---|---|---|
| 1 | Ionic compounds that produce a large number of ions in solution | Ionic compounds that produce few ions in solution |
| 2 | Dissociation extent is high. Solution contains mostly ions | Dissociation extent is iow. Solution contains few ions and many molecules. |
| 3 | Examples are salt olutions of reactive metals, mineral acids | Examples are acids like acetic acid, nitrous acid |
Electrorefining
Electrorefining is done by electrolysis.
 |
- In the electrorefining of copper, the anode is impure copper.
- The cathode is a thin strip of pure copper.
- The electrolyte is a copper sulfate solution containing a small quantity of acid.
- Wires connect the apparatus to a power source.
- The electrolyte ionizes as follows:
$$CuSO{_{4(aq))}}\rightarrow Cu^{2+}_{(aq))} + SO_{4(aq))}^{2-} $$
- On passing electricity, the ions move toward the oppositely charged electrodes.
- The copper ions or cations, move toward the cathode, get reduced as each ion accepts two electrons, and becomes an atom.
- So, during electrolysis, reduction takes place at the cathode.
- The copper atoms are deposited on the cathode giving it a fresh reddish color.
$$Cu^{2+}_{(aq)} + 2^{e-}\rightarrow Cu_{(s)} $$
- The negative ions or anions move toward the anode,
- Each anion strips a copper atom from the rod to form the copper sulfate solution.
- Oxidation takes place at the anode.
- The whole process continues until all the copper anode is stripped.
- The impurities collect at the bottom as anode mud.
$$Cu_{(s))}-2e^{-}\rightarrow Cu^{2+}_{(aq))} $$
Other metals can be purified similarly by electrolysis, with suitable electrodes and electrolytes.
To unlock this lesson you must be a Study.com Member.
Create your account
