Combustion and corrosion

Reactions with oxygen

Oxygen is reactive and makes up 21% of the air, so reactions with oxygen are all around us. A reaction in which a substance gains oxygen is called an oxidation reaction. (In senior Chemistry you will extend this idea to the loss of electrons.)

Metals react with oxygen to form metal oxides, which are bases:

2Mg(s) + O₂(g) → 2MgO(s)   (burns with a dazzling white light)
4Na(s) + O₂(g) → 2Na₂O(s)   (a freshly cut surface tarnishes in seconds)

Non-metals react with oxygen to form non-metal oxides, many of which dissolve in water to make acids:

C(s) + O₂(g) → CO₂(g),   then CO₂ + H₂O → H₂CO₃ (carbonic acid)
S(s) + O₂(g) → SO₂(g),   which forms sulfurous and sulfuric acid in rain

Sulfur dioxide from burning coal and smelting sulfide ores, and nitrogen oxides formed in hot engines and power stations, are the main causes of acid rain. Clean rain already has a pH of about 5.6 because of dissolved carbon dioxide; acid rain can fall below pH 4.5.

Burning fuels

Combustion is a fast reaction with oxygen that releases energy as heat and light. Most fuels are hydrocarbons (compounds of hydrogen and carbon only), such as methane in natural gas, propane in LPG and octane in petrol. With plenty of oxygen, every carbon atom ends up in carbon dioxide and every hydrogen atom in water:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)   (complete combustion)
C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(g)   (propane in a barbecue)

When oxygen is short, some carbon only gets as far as poisonous carbon monoxide, or is left as soot. This is incomplete combustion, for example 2CH₄ + 3O₂ → 2CO + 4H₂O. Combustion is an oxidation reaction too: the fuel's carbon and hydrogen end up as non-metal oxides, just like the burning carbon and sulfur above.

Chapter 4 (4B) compares fuels by the energy they release, explains the yellow and blue Bunsen flames, and looks closely at carbon monoxide, soot and the other pollutants from burning. This page now turns to the slow reactions of metals with oxygen.

Corrosion and rust

Corrosion is the slow reaction of a metal with substances in its surroundings, such as oxygen, water, acids and salts. The corrosion of iron and steel is called rusting. It needs both water and oxygen, and it forms a flaky, reddish-brown solid, hydrated iron(III) oxide:

4Fe(s) + 3O₂(g) + 2xH₂O(l) → 2Fe₂O₃·xH₂O(s)

(The "x" shows that the amount of water in rust varies.) Rusting is an electrical process. Under a drop of water, iron atoms in the middle, where oxygen is scarce, lose electrons and dissolve as Fe²⁺ ions, leaving a pit. The electrons flow through the metal to the edge of the drop, where oxygen and water take them up. Rust forms where the two meet. Salt makes the water a much better conductor, which is why cars and bridges near the sea, or on roads salted against ice, rust faster.

Not all corrosion is harmful. Aluminium is more reactive than iron, but it is instantly coated with a thin, tough layer of aluminium oxide that sticks to the metal and stops further reaction. Copper roofs slowly turn green as a layer of copper carbonate and sulfate (a patina) forms and protects the metal beneath. Rust, in contrast, is crumbly and flakes off, exposing fresh iron underneath.

Protecting steel

• Barriers keep out water and oxygen: paint, oil or grease, plastic coatings and a layer of a less reactive metal such as tin (as in food cans). They only work while the barrier is unbroken.
• Sacrificial protection uses a more reactive metal that corrodes instead of the iron. Steel coated with zinc is galvanised: even when scratched, the zinc gives up electrons in place of the iron, and its corrosion products help seal the scratch. Blocks of zinc or magnesium, called sacrificial anodes, are bolted to ship hulls, jetty piles and the insides of hot-water tanks, and replaced when they are used up.
• Alloying changes the metal itself. Stainless steel contains at least about 10.5% chromium, which forms a thin, self-repairing layer of chromium oxide on the surface.