Electrolysis Products at Electrodes

An ionic compound contains positively and negatively charged particles called ions. In a solid ionic compound, these ions are held in fixed positions, so they cannot move through the solid to carry charge. When the compound is molten or dissolved in water, its ions can move freely.

Passing an electric current through this liquid or solution can break down the compound. This process is called electrolysis, and the conducting liquid or solution is the electrolyte. Ions carry charge through the electrolyte; electrons carry charge through the connecting wires.

The electrodes and their signs

An electrolysis cell has two conducting electrodes connected to a power supply and placed in the electrolyte. The cathode is negative, and the anode is positive.

Here we are considering inert electrodes, such as graphite or platinum. They conduct electricity and provide surfaces where products form, but do not themselves take part chemically in the reactions. This condition matters: electrodes that react can change what happens during electrolysis.

Cations and anions move in opposite directions

A cation is a positively charged ion. Opposite charges attract, so cations move towards the negative cathode. For example, a copper ion, $Cu^{2+}$, is a cation: the superscript $2+$ shows its positive charge. A hydrogen ion, $H^+$, is also a cation.

An anion is a negatively charged ion. Anions move towards the positive anode. For example, a chloride ion, $Cl^-$, is an anion because it has a negative charge. A hydroxide ion, $OH^-$, is also an anion, even though it contains more than one atom.

The charge tells you which electrode an ion moves towards. It does not, by itself, tell you which product will form when several types of ion are present.

What forms at each electrode?

With inert electrodes, the central product rule is:

• At the cathode: a metal or hydrogen forms.
• At the anode: a non-metal forms.

At the cathode, ions that react gain electrons. Metal ions can form metal atoms, producing a deposit of metal on the electrode. Alternatively, hydrogen can form as a gas, giving bubbles at the cathode.

At the anode, ions that react lose electrons. Non-metal products include chlorine and oxygen. These form gases, although the general rule is non-metal, not necessarily gas.

Hydrogen is itself a non-metal, but it forms at the cathode. This is why the cathode rule must include both possibilities: a metal or hydrogen. Keep the direction of ion movement and the type of product connected: positive cations head towards the negative cathode; negative anions head towards the positive anode.