4.4.1.4,4.1.1.1 - Electron transfer and ionic equations

Oxidation and reduction by electrons (Higher)

A change in electron number explains the conversion between a metal atom and its positive ion.

A zinc atom becomes a zinc ion by losing two electrons. It retains the same nucleus, so it is still zinc; its charge changes because there are now two fewer negative electrons than positive protons. A copper ion becomes a copper atom by gaining two electrons, which removes its positive charge.

The memory aid “OIL RIG” means oxidation is loss, reduction is gain. Name the species that actually changes. In the reaction between zinc and copper sulfate, zinc atoms are oxidised and copper ions are reduced. Copper atoms are the product of that reduction, not the species receiving electrons at the start.

These definitions are more general than oxygen transfer. When magnesium reacts with oxygen, magnesium loses electrons to form positive ions while oxygen gains electrons to form oxide ions. Gain of oxygen and loss of electrons describe the same magnesium oxidation from two viewpoints. But zinc displacing copper also involves electron transfer, even though oxygen is not transferred between the reacting species.

Redox half equations (Higher)

A half equation represents one electron-transfer change and balances both atoms and charge.

Electrons are written as $\ce{e-}$. They appear on the product side when lost and on the reactant side when gained:

The first has one zinc atom on each side. Its left-hand charge is zero; its right-hand charge is $+2-2=0$. The second has total charge $+2-2=0$ on the left and zero on the right. Balancing atoms alone would not detect an incorrect electron coefficient.

To write a metal-ion half equation, first write the atom and ion with the correct ion charge. Then put electrons on the side needed to make the total charges equal. Finally check atom numbers. Thus aluminium forming $\ce{Al^3+}$ loses three electrons:

A coefficient before an ion counts particles; a superscript charge belongs to each particle. Do not change an ion's charge to balance an equation. If two iron atoms each lose two electrons, the equation needs four electrons overall:

Ionic equations and spectator ions (Higher)

An ionic equation shows the reacting particles and leaves out ions whose chemical state does not change.