Electrolysis: decomposition of a compound in solution or molten state by passing electricity through it.
Conductor: a solid substance that allows electricity to pass through it. All metals are included in this class.
Non-conductor or insulator: a solid substance that does not allow electricity to flow through it. All non-metals fall in this class.
Electrolyte: a substance which, when dissolved or molten, conducts electricity and is decomposed by it.
Non-electrolyte: a compound which cannot conduct electricity, be it in molten or solution state.
Electrode: a graphite or metal pole (rod) or plate through which the electric current enters or leaves the electrolyte.
Cathode: a negative electrode which leads electrons into the electrolyte.
Anode: a positive electrode which leads electrons out of the electrolyte.
Ion: a positively or negatively charged atom or radical (group of atoms).
Cation: a positive ion which moves to the cathode during electrolysis.
Anion: a negative ion which moves to the anode during electrolysis.
Electrolytes and non-electrolytes
Liquids such as ethanol, paraffin, petrol and methylbenzene do not conduct electricity. The bonding in these compounds is covalent. These substances consist of molecules. There are no free electronsor charged particles to flow through them. Solutions of covalent compounds, for example sugar solution, do not conduct electricity.
These compounds are non-electrolytes. Non-electolytes exist only in the form of molecules and are incapable of ionization.
Ionic compounds contain charged particles (ions), but in solid state, the ions are firmly held in place and they are not free to move. An ionic solid does not conduct electricity. However, the ions present can become free to move if the solid is melted or dissolved in water. Then they can conduct electricity.
For example, solid sodium chloride cannot conduct electricity but when melted or dissolved in water, the ions, Na+ and Cl– are set free. Then these ions are free to move in solution and hence conduct electricity. These compounds are called electrolytes.
Weak and Strong Electrolytes
Categorize weak and strong electrolytes
Weak electrolytes are compounds that are only partially or slightly ionized in aqueous solutions. Some substances, for example, ethanoic acid solution ionize partially.
CH3COOH(aq) ⇔CH3COO–(aq) + H+(aq)
Most of the electrolytes exist in solution in the form of unionized molecules. For example, in ordinary dilute (2M) ethanoic acid, out of every 1000 molecules present, only 4 are ionized and 996 are unionized.
A solution of ammonia water is also a weak electrolyte, containing a relatively small proportion of ammonium and hydroxyl ions.
NH4OH(aq) ⇔NH4+(aq) + OH–(aq)
Most of the organic acids are weak electrolytes, e.g. tartaric, citric and carbonic acids.
However, there is no sharp dividing line between weak and strong electrolytes.Water is also a weak electrolyte. It ionizes only slightly.
H2O(l)⇔H+(aq) + OH–(aq)
Study shows that for every molecule of water ionized, there are 6 million molecules of water not ionized.Strong electrolytes are compounds that are completely ionized in aqueous solutions. When sodium chloride is dissolved in adequate water it ionizes completely into Na+ and Cl– ions. There are no NaCl solid particles left unionized. All strong electrolytes (salts, the mineral acids and caustic alkalis) ionize completely in solutions.
The Mechanisms of Electrolysis
Electrolytic Cells of Different Electrolytes in the Molten and Aqueous States
Set up electrolytic cells of different electrolytes in the molten and aqueous states
The conductivity of ionic compounds is explained by the fact that ions move in a particular direction in an electric field. This can be shown in experiments with coloured salts. For example, copper (II) chromate (VI) (CuCrO4) dissolves in water to give a green solution. This solution is placed in a U-tube.
A colourless solution of dilute hydrochloric acid (HCl) is then layered on top of the salt solution in each arm. Graphite rods are fitted as shown in figure 13.3. These rods (electrodes) carry the current into and out of the solution.
After passing the current for a short time, the solution around the cathode becomes blue. Around the anode, the solution becomes yellow. These colours are produced by the movement (migration) of the ions in the salt.
The positive copper ions (Cu2+) are blue in solution. They are attracted to the cathode (negative electrode). The negative chromate ions (CrO42-) are yellow in solution. They are attracted to the anode (the positive electrode). The use of coloured ions in solution has shown the direction that positive and negative ions move in an electric field. Always positive ions (cations) move to the cathode and negative ions (anions) move to the anode.