Dissolve an ionic compound in water and the solution contains extra ions from the water: hydrogen ions and hydroxide ions. At each electrode two kinds of ion now compete, and two simple rules tell you which one wins.
An aqueous solution also contains H+ and OH− ions from water. At the cathode, the metal forms only if it is less reactive than hydrogen (such as copper or silver); otherwise hydrogen gas forms. At the anode, a halogen forms from a concentrated halide solution; otherwise oxygen gas forms. So concentrated aqueous sodium chloride gives hydrogen at the cathode and chlorine at the anode, and dilute sulfuric acid gives hydrogen and oxygen.
Switch the simulator to sodium chloride solution. Look at the four kinds of ion present and see which two are discharged.
Sodium is never produced at the cathode from an aqueous solution: it is more reactive than hydrogen, so hydrogen gas is released instead. Sodium metal only forms when molten sodium chloride is electrolysed.
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Concentrated aqueous sodium chloride is electrolysed with inert electrodes. What forms at the cathode?
What forms at the anode when concentrated aqueous sodium chloride is electrolysed?
Dilute sulfuric acid is electrolysed. Which gases form at the cathode and anode?
Which test identifies the gas formed at the anode during electrolysis of concentrated brine?
Sulfate ions are never discharged at the anode, so oxygen forms there instead. What forms at the cathode depends on the metal ion.
| Aqueous electrolyte (inert electrodes) | Cathode (−) | Anode (+) |
|---|---|---|
| concentrated sodium chloride | hydrogen | chlorine |
| dilute sodium chloride | hydrogen | oxygen |
| dilute sulfuric acid | hydrogen (twice the volume) | oxygen |
| concentrated hydrochloric acid | hydrogen | chlorine |
| copper(II) sulfate (Extended) | copper: pink-brown coating | oxygen; the blue colour fades |
In dilute sulfuric acid the volume of hydrogen collected is twice the volume of oxygen, because water is effectively split into H2 and O2 in a 2 : 1 ratio.
Extended. The same rule applies: reduction (gain of electrons) at the cathode, oxidation (loss of electrons) at the anode.
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(Extended) Aqueous copper(II) sulfate is electrolysed using carbon electrodes. Which observations are made?
(Extended) Which half-equation shows the formation of oxygen at the anode?
Dilute sulfuric acid is electrolysed and 20 cm3 of oxygen is collected. What volume of hydrogen is collected at the same time?
(Extended) Dilute aqueous potassium bromide is electrolysed with inert electrodes. What are the main products?
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