Electrolysis of ionic solutions
The decomposition (breakdown) of a compound using an electric current. aqueous solutions of An ionic compound occurs when a negative ion (an atom that has gained an electron) joins with a positive ion (an atom that has lost an electron). can be more complicated than electrolysing A term used to describe a liquid substance (eg rock, glass or metal) formed by heating a solid. compounds, because the water molecules can provide hydrogen Electrically charged particle, formed when an atom or molecule gains or loses an electron/electrons. (H+) and hydroxide ions (OH鈥), in addition to the ions from the ionic compounds.
At the negative electrode
Metal ions and hydrogen ions are positively charged. Whether you get the metal or hydrogen during electrolysis depends on the position of the metal in the A list of elements in order of their reactivity, usually from most reactive to least reactive.:
- the metal will be produced if it is less reactive than hydrogen
- hydrogen will be produced if the metal is more reactive than hydrogen
For example, the electrolysis of copper(II) chloride solution produces copper at the negative A conductor used to establish electrical contact with a circuit. The electrode attached to the negative terminal of a battery is called a negative electrode, or cathode. The electrode attached to the positive terminal of a battery is the positive electrode, or anode.. However, the electrolysis of sodium chloride solution produces hydrogen. Note that the electrolysis of sodium chloride solution produces chlorine gas and hydrogen gas but also leaves a solution of sodium hydroxide as well.
At the positive electrode
If the negative ion from the ionic compound is simple (eg Cl鈥 or Br鈥), then that element is produced. If the negative ion is a compound ion (eg NO3鈥, SO42鈥, CO32鈥), then oxygen is produced from the hydroxide ion present instead.
The table summarises some of the elements you should expect to get during electrolysis.
| Negative ion in solution | Element given off at positive electrode |
| Chloride, Cl鈥 | Chlorine, Cl2 |
| Bromide, Br鈥 | Bromine, Br2 |
| Iodide, I鈥 | Iodine, I2 |
| Negative ion in solution | Chloride, Cl鈥 |
|---|---|
| Element given off at positive electrode | Chlorine, Cl2 |
| Negative ion in solution | Bromide, Br鈥 |
|---|---|
| Element given off at positive electrode | Bromine, Br2 |
| Negative ion in solution | Iodide, I鈥 |
|---|---|
| Element given off at positive electrode | Iodine, I2 |
At the negative and positive electrodes
This table shows some common ionic compounds (in solution), and the elements released when their solutions are electrolysed using Unreactive. electrodes, eg carbon electrodes:
| Ionic substance | Element at 鈥 | Element at + |
| Copper(II) chloride, CuCl2 | Copper, Cu | Chlorine, Cl2 |
| Sodium chloride, NaCl | Hydrogen, H2 | Chlorine, Cl2 |
| Hydrochloric acid, HCl | Hydrogen, H2 | Chlorine, Cl2 |
| Ionic substance | Copper(II) chloride, CuCl2 |
|---|---|
| Element at 鈥 | Copper, Cu |
| Element at + | Chlorine, Cl2 |
| Ionic substance | Sodium chloride, NaCl |
|---|---|
| Element at 鈥 | Hydrogen, H2 |
| Element at + | Chlorine, Cl2 |
| Ionic substance | Hydrochloric acid, HCl |
|---|---|
| Element at 鈥 | Hydrogen, H2 |
| Element at + | Chlorine, Cl2 |
Electrolysis of sodium chloride solution (brine)
As shown above, the electrolysis of sodium chloride solution will produce chlorine gas at the anode and hydrogen gas at the cathode.
- at the anode, 2Cl鈥 鈫 Cl2 + 2e鈥 (The gain of oxygen, or loss of electrons, by a substance during a chemical reaction.)
- at the cathode, 2H+ + 2e鈥 鈫 H2 (The loss of oxygen, gain of electrons, or gain of hydrogen by a substance during a chemical reaction.)
During the electrolysis, hydrogen and chloride ions are removed from solution whereas sodium and hydroxide ions are left behind in solution. This means that sodium hydroxide is also formed during the electrolysis of sodium chloride solution. This process is carried out on an industrial scale using sea water to produce hydrogen gas, chlorine gas and sodium hydroxide solution.