A single-displacement reaction, also known as single replacement reaction or exchange reaction, is an archaic concept in chemistry. It describes the stoichiometry of some
chemical reactions in which one element or ligand is replaced by atom or group.[1][2][3]
It can be represented generically as:
where either
and are different
metals (or any element that forms
cation like hydrogen) and is an
anion;[2] or
This will most often occur if is more
reactive than , thus giving a more stable product. The reaction in that case is
exergonic and spontaneous.
In the first case, when and are metals, and are usually aqueous compounds (or very rarely in a molten state) and is a
spectator ion (i.e. remains unchanged).[1]
In the
reactivity series, the metals with the highest propensity to donate their electrons to react are listed first, followed by less reactive ones. Therefore, a metal higher on the list can displace anything below it. Here is a condensed version of the same:[1]
(Hydrogen, carbon and ammonium — labeled in gray — are not metals.)
Similarly, the halogens with the highest propensity to acquire electrons are the most reactive. The activity series for halogens is: [1][2][3]
Due to the free state nature of and , single displacement reactions are also
redox reactions, involving the transfer of electrons from one reactant to another.[4] When and are metals, is always oxidized and is always reduced. Since halogens prefer to gain electrons, is reduced (from to ) and is oxidized (from to ).
Coke or more reactive metals are used to reduce metals by carbon from their metal oxides,[6] such as in the
carbothermic reaction of
zinc oxide (zincite) to produce
zinc metal:
Such reactions are also used in extraction of boron, silicon, titanium and tungsten.
Thermite reaction
Using highly reactive metals as reducing agents leads to exothermic reactions that melt the metal produced. This is used for welding railway tracks.[6]
Chlorine is manufactured industrially by the
Deacon's process. The reaction takes place at about 400 to 450 °C in the presence of a variety of catalysts such as .
Bromine and iodine are extracted from brine by displacing with chlorine.