Iron(III) oxide

Iron(III) oxide, often referred to as red iron oxide, is the chemical compound with chemical formula Fe₂O₃. It is the main component of rust and acts as the oxidizing agent in the classic thermite reaction with aluminium.

Properties

Chemical

Iron oxide is a typical metal oxide. It will react with acids to form the iron salt, so iron(III) oxide is a useful starting point to make chemicals such iron acetate.

A mixture of red iron oxide and aluminium powders is the classic thermite mixture. This mixture, while difficult to initiate, burns in excess of 1500 °C, producing a slag of red hot molten iron and and aluminium oxide.

Red iron oxide is sometimes added to KNO₃ and sugar mixtures to help accelerate the reaction, which is needed when making 'rocket candy' rockets.

Physical

Iron(III) oxide is a deep red compound usually encountered in powdered form. It is insoluble in water but readily reacts with acids. It is noticeably ferromagnetic, accumulating thickly on the surface of magnets. While ferrofluid, a magnetic liquid suspension, is typically made with iron(II,III) oxide, the magnetic properties of iron(III) oxide may also allow for this to be made.

Availability

Sometimes used as a colouring agent for things such as concrete. Pottery supply stores will have this chemical for the same reason.

Due to the well known thermite reaction, it is commonly available in large amounts online.

Preparation

Iron can be slowly oxidized by oxygen in the presence of salt water to red iron oxide.

A more effective way to produce large amounts of iron oxide is through electrolysis. A solution of sodium chloride is electrolyzed with two iron electrodes, producing insoluble iron(II) oxide.

This is filtered out and heated until dry and then strong heating in air converts the black iron(II) oxide to the red iron(III) oxide.

A quick way to whip up a lot of this oxide is reacting bulk iron (such as nails) with concentrated (60-70%) nitric acid. The reaction is very vigorous and exothermic, and the nails are quickly converted to iron (III) nitrate (and already a lot of iron (III) oxide, the liquid becomes very brown and opaque).

Fe + 6HNO₃(conc) = Fe(NO₃)₃ + NO₂ + 3H₂O (main reaction)

2Fe + 6HNO₃ = Fe₂O₃ + 6NO₂ + 3H₂O (useful side reaction)

Add alkali, such as sodium hydroxide, to precipitate more iron (III) hydroxide, which can be calcined to yield red iron oxide.

Projects

• Thermite with aluminium powder.
• Make elemental iron
• Ferrofluid, a "liquid magnet"
• Make magnetite
• Make ferrates

Handling

Safety

As an insoluble compound of iron, iron(III) oxide is not substantially toxic unless intentionally swallowed in larger amounts. Obviously, be wary of problems associated with tetanus.

Storage

No special storage is required, though it will stain many materials and clothing and may induce rusting in steels.

Disposal

No special disposal is required, unless is contaminated with heavy metals.