Electrolysis of solutions
Electrolysis of dissolved ionic compounds
Water molecules break down to form hydrogen and hydroxide ions:
H2O → 2H+ + OH-
Therefore, 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 (H+) from the water, and positive ions from the compound
- hydroxide ions (OH-) 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 is produced at the cathode if it is less The tendency of a substance to undergo a chemical reaction. than hydrogen
- hydrogen is produced at the cathode if the metal is more reactive than hydrogen
Question
Predict the A substance formed in a chemical reaction. formed at the negative electrode during the The decomposition (breakdown) of a compound using an electric current. 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 | 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 | Chloride, Cl- |
|---|---|
| Element given off at anode | Chlorine, Cl2 |
| Negative ion | Bromide, Br- |
|---|---|
| Element given off at anode | Bromine, Br2 |
| Negative ion | Iodide, I- |
|---|---|
| Element given off at anode | Iodine, I2 |
| Negative ion | Sulfate, SO42- |
|---|---|
| Element given off at anode | Oxygen, O2 |
| Negative ion | Nitrate, NO3- |
|---|---|
| Element given off at anode | Oxygen, O2 |
Question
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.
Testing for gases
Electrolysis of solutions can produce hydrogen, oxygen and chlorine. It is easy to test for these gases in the laboratory.
| Gas | How to test for the gas | Positive result |
| Chlorine | Put damp blue litmus paper into the test tube of gas | Turns red then white |
| Hydrogen | Hold a burning splint near to the neck of the test tube | Squeaky pop sound |
| Oxygen | Put a glowing splint into the test tube of gas | Relights the splint |
| Gas | Chlorine |
|---|---|
| How to test for the gas | Put damp blue litmus paper into the test tube of gas |
| Positive result | Turns red then white |
| Gas | Hydrogen |
|---|---|
| How to test for the gas | Hold a burning splint near to the neck of the test tube |
| Positive result | Squeaky pop sound |
| Gas | Oxygen |
|---|---|
| How to test for the gas | Put a glowing splint into the test tube of gas |
| Positive result | Relights the splint |
Half equations during electrolysis - Higher
During the electrolysis of aqueous solutions, positive ions from the unreactive metal or the hydrogen ion (from the water) are attracted to the negative electrode (the cathode) and they gain electrons to form neutral atoms. For example:
Cu2+ + 2e- → Cu
2H+ + 2e- → H2
Meanwhile, negative ions are attracted to the positive electrode (the anode) and they lose electrons to form neutral atoms or molecules. The ions discharged at the anode will either be halide ions (chloride, bromide or iodide) or they will be hydroxide ions:
2Cl- → Cl2 + 2e-
2Br- → Br2 + 2e-
2I- → I2 + 2e-
4OH- → 2H2O + O2 + 4e-
