competition reaction

Chemical process that uses oxygen to compare the reactivity of metals, hydrogen, and carbon. A competition reaction is a redox reaction, in which one reactant is reduced and the other reactant oxidized. More reactive metals will compete for the oxygen combined to less reactive metals. By comparing how different metals compete for oxygen a reactivity series can be made. For example, a common form of the reactivity series, beginning with the most reactive metal, is: potassium, sodium, calcium, magnesium, aluminium, zinc, iron, tin, lead, copper, silver, gold.

Hydrogen and carbon are good reducing agents that can remove oxygen from certain metal oxides. If hydrogen or carbon are more reactive than the metal in the oxide, the oxygen will be removed leaving the reduced metal. A simple example of a redox reaction is the reduction of copper(II) oxide, which can be carried out by heating a mixture of charcoal and copper(II) oxide over a Bunsen burner flame. The copper(II) oxide is reduced to copper by losing oxygen, and the carbon is oxidized to carbon dioxide by gaining oxygen.

2CuO + C → 2Cu + CO2

Hydrogen undergoes a similar reaction but is oxidized to water. Lead(II) oxide can be reduced by hydrogen to form lead:

PbO + H2 → Pb + H2O

Very reactive magnesium powder mixed with copper(II) oxide will react explosively when the mixture is heated strongly. The magnesium competes with the less reactive copper, taking the oxygen from it:

Mg + CuO → MgO + Cu

Industrial application The thermite process, used in early welding, is an example of a redox reaction in industrial use. Aluminium powder is mixed with iron(III) oxide and made to react by the heat from a burning magnesium fuse. The reaction is so exothermic that the iron that forms is molten. The molten iron was used to weld rail tracks together, before efficient welding methods became available. Reactions of this sort have been used to extract the metals chromium and manganese from their ores by heating with aluminium powder.