Electrolysis of solutions
Ions in water
Pure water can To allow electricity, heat or other energy forms to pass through. electricity because a small proportion of its A collection of two or more atoms held together by chemical bonds. The breaking up of a molecule into ions when dissolved in water. into Electrically charged particle, formed when an atom or molecule gains or loses electrons.. The two ions formed in water are, hydrogen ions, H+, and hydroxide ions, OH-.
During the electrolysis of water:
- 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 A chemical equation written using the symbols and formulae of the reactants and products, so that the number of units of each element present is the same on both sides of the arrow. for the process is:
2H2O(l) 鈫 2H2(g) + O2(g)
The volume of hydrogen given off is twice the volume of oxygen given off.
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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 two pairs of negative and positive ions:
- positive hydrogen ions from the water, and positive ions from the A substance formed by the chemical union of two or more elements.
- negative 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 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
At the anode
Either oxygen or a non-metal from the electrolyte can be produced at the anode:
- for the most common compounds oxygen is produced (from the hydroxide ions)
- if 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, then the negatively charged halide ions lose electrons to form the corresponding non-metal 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 |
| 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 |
Example
Predict the A substance formed in a chemical reaction. formed at the positive electrode during the The decomposition (breakdown) of a compound using an electric current. 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.