Predicting products of electrolysis of aqueous solutions

When an ionic compound is dissolved in water, the solution contains ions from the compound. Water also provides hydrogen ions, $$\ce{H+}$$, and hydroxide ions, $$\ce{OH-}$$. During electrolysis, these ions compete to be discharged at the electrodes: they gain or lose electrons and form products.

With inert electrodes, the electrodes themselves do not supply the products. To predict the products, consider the positive and negative ions separately.

Product at the cathode

Positive ions move to the cathode, where they gain electrons. The metal ions from the dissolved compound compete with hydrogen ions from water.

Compare the metal with hydrogen in the reactivity series:

• If the metal is more reactive than hydrogen, hydrogen ions are discharged and hydrogen gas is produced.
• If the metal is less reactive than hydrogen, metal ions are discharged and the metal is deposited at the cathode.

Product at the anode

Negative ions move to the anode, where they lose electrons. Check whether the dissolved compound contains a halide ion:

• If chloride, bromide or iodide ions are present, the corresponding halogen—chlorine, bromine or iodine—is produced.
• If no halide ions are present, hydroxide ions from water are discharged and oxygen gas is produced.

Applying the rules to aqueous solutions

For aqueous sodium chloride, sodium is more reactive than hydrogen, so hydrogen is produced at the cathode. Chloride is a halide, so chlorine is produced at the anode.

For aqueous copper sulfate, copper is less reactive than hydrogen, so copper is deposited at the cathode. Sulfate is not a halide, so oxygen is produced at the anode.