Predicting products and half-equations

Last time

Before we start: last time, in Electroplating and fuel cells, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A crowd of ions in water

In a molten compound there are only two kinds of ion, so the products are easy. In a solution, there are four kinds, because water adds its own hydrogen ions and hydroxide ions. Then only one ion at each electrode is discharged, and you must decide which.

Ions in an aqueous electrolyte

Water is very slightly ionised, so every solution contains hydrogen ions, H⁺, and hydroxide ions, OH⁻. In aqueous sodium chloride, there are four ions: Na⁺, Cl⁻, H⁺ and OH⁻. At each electrode, only one kind of ion is discharged. The rules decide which.

The cathode rule

At the cathode, positive ions gain electrons. The competition is between the metal ion and the hydrogen ion. If the metal is more reactive than hydrogen, such as sodium or potassium, hydrogen is formed. If the metal is less reactive than hydrogen, such as copper or silver, the metal is formed.

The anode rule

At the anode, negative ions lose electrons. The competition is between the other anion and the hydroxide ion. If the solution contains halide ions, chloride, bromide or iodide, in a concentrated solution, the halogen is formed. If the solution is dilute, or the anion is sulfate or nitrate, hydroxide ions lose electrons and oxygen is formed.

The two rules together

Learn them as a pair. At the cathode: reactive metal gives hydrogen, unreactive metal gives the metal. At the anode: concentrated halide gives the halogen, anything else gives oxygen.

Half-equations

A half-equation shows what happens at one electrode. The electrons are written in the equation. At the cathode, electrons are on the left, because ions gain them. At the anode, electrons are on the right, because ions lose them. It must balance twice: the atoms, and the charge.

The half-equations to know

Hydrogen at the cathode: two H⁺ plus two electrons gives H₂. Metals at the cathode: Cu²⁺ plus two electrons gives copper, and Ag⁺ plus one electron gives silver. Chlorine at the anode: two Cl⁻ gives Cl₂ plus two electrons. Oxygen at the anode: four OH⁻ gives two H₂O plus O₂ plus four electrons.