how is rust different from oxides?

Question

Can someone explain what rust is exactly please? Is it just the metal reacting with oxygen?

If so, how is this different from the oxides formed when a metal is heated and combined with oxygen?

Why can you reverse one and not the other?

Answer 1

Rust generally refers to the mixture of FeO , Fe2O3 and Fe(OH)2 formed by the action of O2 and H2O upon Fe.

Answer 2

rust has the general formula Fe2O3

STEP 1 it is generally formed by the oxidation of Fe(0) to ferrous (Fe2+) or ferric (3+) ions

STEP 2 these ions can then react with water is a step wise fashion as follows

2Fe(3+) + 3 H2O -> Fe2O3 + 6H+

to my knowledge, rust forms from oxidation (step-1) followed by hydrolysis (2-step)

there are many various iron oxides, depending on the way the lattice packing of the atoms and depending on the stochiometry, for instance FeOOH is also a balance form called iron oxyhydroxide

iron ozide can be broken down in acids like HCl or HNO3, driving the reaction in step-2 to the left

Answer 3

Rust is generally considered to be FeO2, and yes it is generally the metal reacting with oxygen in the atmosphere or in water.

As far as I know, it isn't all that different from a heated metal, heating just accelerates the process.

And you can reverse the process, it just takes large amounts of heat, and (almost explosive) reactions.

Answer 4

Rust is the oxidation of metal due to long exposure to the elements. Corrosion is when something starts dissolving or falling apart due to anything - rust, salt water, acid, sand... Something that rusts is corroding but something that is corroding does not necessarily rust.

Answer 5

Rust is a general term for iron oxides formed by the reaction of iron with oxygen. Several forms of rust are distinguishable visually and by spectroscopy, and form under different circumstances.[1] The chemical composition of rust is typically hydrated iron(III) oxide (Fe2O3.nH2O), and under wet conditions may include iron(III) oxide-hydroxide (FeO(OH)). Rusting is the common term for corrosion of iron and its alloys, such as steel. Although oxidation of other metals is equivalent, these oxides are not commonly called rust.

As rust has higher volume than the originating mass of iron, its buildup may force apart adjacent parts - a phenomenon known as rust smacking.

The rusting of iron is one of the more widely used examples of corrosion. This electrochemical process requires the presence of water, oxygen and an electrolyte and leads to the formation of hydrated iron oxides.


Net reactions
The overall outcome of rust formation involves reaction of iron with varying amounts of oxygen and water.

2Fe + O2 + 2H2O = 2Fe(OH)2
4Fe + O2 + 6H2O = 4Fe(OH)3
4Fe + O2 + 2H2O = 2Fe2O3•H2O
6Fe + 4O2 = 2Fe3O4




An oxide is a chemical compound containing at least one oxygen atom and other elements. Most of the earth's crust consists of oxides. Oxides result when elements are oxidized by air. Combustion of hydrocarbons affords the two principal oxides of carbon, carbon monoxide and carbon dioxide. Even materials that are considered to be pure elements often contain a coating of oxides. For example, aluminium foil has a thin skin of Al2O3 that protects the foil from further corrosion.
The oxide ion, O2−, is the conjugate base of the hydroxide ion, OH−, and is encountered in ionic solid such as calcium oxide. O2− is unstable in aqueous solution − its affinity for H+ is so great (pKb ~ -22) that it abstracts a proton from a solvent H2O molecule:

O2− + H2O → 2 OH−
Although many anions are stable in aqueous solution, ionic oxides are not. For example, sodium chloride dissolves readily in water to give a solution containing the constituent ions, Na+ and Cl-. Oxides do not behave like this. If an ionic oxide dissolves, the O2− ions become protonated. Although Calcium oxide, CaO, is said to "dissolve" in water, the products include hydroxide:

CaO + H2O → Ca2+ + 2 OH-
In fact, no monoatomic dianion is known to dissolve in water - all are so basic that they undergo hydrolysis. Concentrations of oxide ion in water are too low to be detectable with current technology.

Authentic soluble oxides do exist of course, but they release oxyanions, not O2-. Well known soluble salts of oxyanions include sodium sulfate (Na2SO4), potassium permanganate (KMnO4), and sodium nitrate (NaNO3).