Water molecules break down to form hydrogen and hydroxide ions:
H2O → 2H+ + OH-
The ions compete at each electrode to gain or lose electrons.
Whether hydrogen or a metal is produced at the cathode depends on the position of the metal in the metal reactivity series:
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.
Oxygen is produced (from hydroxide ions), unless halide ions (chloride, bromide or iodide ions) are present. In that case, the negatively charged halide ions lose electrons and form the corresponding halogen (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|
Predict the product formed at the positive electrode during the electrolysis of concentrated sodium chloride solution.
Chlorine will be produced.
Predict the product formed at the positive electrode during the electrolysis of concentrated sodium sulfate solution.
Oxygen will be produced.
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|
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-