Single displacement reaction

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.

It can be represented generically as:



where either


 * and are different metals (or any element that forms cation like hydrogen) and  is an anion; or


 * and are halogens and  is a cation.

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).









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:


 * (Hydrogen, carbon and ammonium &mdash; labeled in gray &mdash; are not metals.)
 * (Hydrogen, carbon and ammonium &mdash; labeled in gray &mdash; are not metals.)

Similarly, the halogens with the highest propensity to acquire electrons are the most reactive. The activity series for halogens is:



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. 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 ).

Cation replacement
Here one cation replaces another:



(Element A has replaced B in compound BC to become a new compound AC and the free element B.)

Some examples are:


 * (Blue vitriol)____(Green vitriol)
 * (Blue vitriol)____(Green vitriol)


 * (Blue vitriol)___(White vitriol)
 * (Blue vitriol)___(White vitriol)


 * (Green vitriol) (White vitriol)
 * (Green vitriol) (White vitriol)

These reactions are exothermic and the rise in temperature is usually in the order of the reactivity of the different metals.

If the reactant in elemental form is not the more reactive metal, then no reaction will occur. Some examples of this would be the reverse.


 * No Reaction

Anion replacement
Here one anion replaces another:



(Element A has replaced B in the compound CB to form a new compound CA and the free element B.)

Some examples are:

Again, the less reactive halogen cannot replace the more reactive halogen:


 * no reaction

Metal-acid reaction
Metals react with acids to form salts and hydrogen gas.





However less reactive metals can not displace the hydrogen from acids. (They may react with oxidizing acids though.)


 * No reaction

Reaction between metal and water
Metals react with water to form metal oxides and hydrogen gas. The metal oxides further dissolve in water to form alkalies.







The reaction can be extremely violent with alkali metals as the hydrogen gas catches fire.

Metals like gold and silver, which are below hydrogen in the reactivity series, do not react with water.

Metal extraction
Coke or more reactive metals are used to reduce metals by carbon from their metal oxides, such as in the carbothermic reaction of zinc oxide (zincite) to produce zinc metal:

and the use of aluminium to produce manganese from manganese dioxide:

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.





a(Haematite)



Silver tarnish


Silver tarnishes due to the presence of hydrogen sulfide, leading to formation of silver sulfide.





Extraction of halogens
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.