Burns Night: Combustion

Abstract

Burning, more formally combustion, denotes burning in oxygen and more commonly in air (which is 20 per cent oxygen). Combustion is a special case of a more general term, ‘oxidation’, which originally meant reaction with oxygen, not necessarily accompanied by a flame. The rusting of iron is also an oxidation, but we don’t normally think of it as a combustion because no flame is involved. Oxidation now has a much broader meaning than reaction with oxygen, as I shall unfold in Reaction 5. For now, I shall stick to combustion itself. To achieve combustion, we take a fuel, which might be the methane, CH4, 1, of natural gas or one of the heavier hydrocarbons, such as octane, C8H18, 2, that we use in internal combustion engines, mix it with air, and ignite it. The outcome of the complete combustion of any hydro-carbon is carbon dioxide and water but incomplete combustion can result in carbon monoxide and various fragments of the original hydrocarbon molecule. All combustions are ‘exothermic’, meaning that they release a lot of energy as heat into the surroundings. We use that energy for warmth or for driving machinery. Another example of an exothermic combustion is provided by the metal magnesium, which gives an intense white light as well as heat when it burns in air. A part of the vigour of this reaction is due to the fact that magnesium reacts not only with oxygen but also with nitrogen, the major component of air. You should be getting a glimpse of the broader significance of the term ‘oxidation’ in the sense that the reaction need not involve oxygen; in magnesium’s case, nitrogen can replace oxygen in the reaction. Magnesium foil was used in old-fashioned photographic flashes and in fireworks. The latter now mostly use finely powdered aluminium, which is much cheaper than magnesium and reacts in much the same way. In what follows you could easily replace aluminium with magnesium if you want to think fireworks. For the whole of the following discussion you need to be familiar with oxygen, O2, 3, a peculiar molecule in several respects.