How do you determine which ions are discharged at electrodes?

The ions discharged at electrodes are determined by their position in the electrochemical series and the electrode's polarity.

In electrolysis, the ions that are discharged at the electrodes are determined by their relative reactivity, which is indicated by their position in the electrochemical series. The electrochemical series is a list of elements in the order of their standard electrode potentials. Elements at the top of the series are more likely to be discharged at the electrodes than those at the bottom.

At the cathode (the negative electrode), positive ions (cations) are attracted and are discharged. The cation that is less reactive or lower in the electrochemical series is preferentially discharged. For example, if the electrolyte is a solution of copper(II) sulphate, the copper(II) ions (Cu2+) are less reactive than the hydrogen ions (H+) from water, so copper is discharged at the cathode.

At the anode (the positive electrode), negative ions (anions) are attracted and are discharged. The anion that is lower in the electrochemical series is preferentially discharged. However, if the electrolyte is a solution of a halide ion (like chloride, bromide or iodide), then the halide ion is discharged in preference to the hydroxide ions (OH-) from water.

The concentration of the ions in the electrolyte also plays a role in determining which ions are discharged. If the concentration of a particular ion is very high, it may be discharged in preference to an ion that is lower in the electrochemical series.

IGCSE Chemistry Tutor Summary: In electrolysis, ions are discharged at electrodes based on their position in the electrochemical series and their reactivity. At the cathode, less reactive positive ions are discharged, while at the anode, less reactive negative ions are preferred. Factors like ion concentration also influence which ions are discharged.