Electrolysis - solutions
Electrolysis of acidified water
Water is a poor conductor of electricity, but it does contain some hydrogen Electrically charged particle, formed when an atom or molecule gains or loses electrons., H+, and hydroxide ions, OH-. These ions are formed when a small proportion of water molecules naturally The breaking up of a molecule into ions when dissolved in water.. If water is acidified with a little dilute sulfuric acid:
- H+ ions are attracted to the The negative electrode during electrolysis., gain Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. and form hydrogen gas
- OH- ions are attracted to the The positive electrode during electrolysis., lose electrons and form oxygen gas
The overall balanced equation for the process is: 2H2O(l) → 2H2(g) + O2(g)
The volume of hydrogen given off is twice the volume of oxygen given off.
Learn more on electrolysis of aqueous solutions in this podcast.
Listen to the full series on ±«Óătv Sounds.
Electrolysis of dissolved ionic compounds
An A substance which, when molten or in solution, will conduct an electric current. formed by dissolving an 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). contains:
- hydrogen ions from the water and positive ions from the compound
- hydroxide ions from the water and negative ions from the compound
The ions compete at each 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. to gain or lose electrons.
At the cathode
Whether hydrogen or a metal is produced at the cathode depends on the position of the metal in the metal A list of elements in order of their reactivity, usually from most reactive to least reactive.:
- the metal will be produced if the metal is less The tendency of a substance to undergo a chemical reaction. than hydrogen
- hydrogen will be produced if the metal is more reactive than hydrogen
Example
Predict the A substance formed in a chemical reaction. formed at the negative electrode during the electrolysis of sodium chloride solution.
Hydrogen will be produced because sodium is more reactive than hydrogen.
Question
Predict the product formed at the negative electrode during the electrolysis of copper chloride solution.
Copper will be produced because copper is less reactive than hydrogen.
At the anode
Oxygen is produced (from hydroxide ions), unless A halide ion is an ion formed when a halogen atom (an atom from group 7) gains one electron. Halide ions have a single negative charge. Ionic compounds containing halide ions may be called halides. ions (chloride, bromide or iodide ions) are present. In that case, the negatively charged halide ions lose electrons and form the corresponding An element placed in group 7 of the periodic table, which starts with fluorine and ends with astatine. The name 'halogen' means 'salt-producing' because halogens produce a range of salts when they react with metals. (chlorine, bromine or iodine).
The table summarises the product formed at the anode during the electrolysis of different electrolytes in solution.
| Negative ion from electrolyte | Element given off at anode |
| chloride, Cl- | chlorine, Cl2 |
| bromide, Br- | bromine, Br2 |
| iodide, I- | iodine, I2 |
| sulfate, SO42- | oxygen, O2 |
| nitrate, NO3- | oxygen, O2 |
| Negative ion from electrolyte | chloride, Cl- |
|---|---|
| Element given off at anode | chlorine, Cl2 |
| Negative ion from electrolyte | bromide, Br- |
|---|---|
| Element given off at anode | bromine, Br2 |
| Negative ion from electrolyte | iodide, I- |
|---|---|
| Element given off at anode | iodine, I2 |
| Negative ion from electrolyte | sulfate, SO42- |
|---|---|
| Element given off at anode | oxygen, O2 |
| Negative ion from electrolyte | nitrate, NO3- |
|---|---|
| Element given off at anode | oxygen, O2 |
Example
Predict the product formed at the positive electrode during the electrolysis of concentrated sodium chloride solution.
Chlorine will be produced.
Question
Predict the product formed at the positive electrode during the electrolysis of concentrated sodium sulfate solution.
Oxygen will be produced.
Very dilute solutions of halide compounds
If a halide solution is very dilute, oxygen is given off instead of the halogen. This is because its halide ions are outnumbered by hydroxide ions from the water.