Single displacement reaction


A single-displacement reaction, also known as single replacement reaction or exchange reaction, is a type of chemical reaction in which one element or ligand is replaced by an atom or group.
It can be represented generically as:
where either
This will most often occur if A is more reactive than B, thus giving a more stable product. The reaction in that case is exergonic and spontaneous.
In the first case, when A and B are metals, BC and AC are usually aqueous compounds and C is a spectator ion.

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:
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 A and B, single displacement reactions are also redox reactions, involving the transfer of electrons from one reactant to another. When A and B are metals, A is always oxidized and B is always reduced. Since halogens prefer to gain electrons, A is reduced and B is oxidized.

Cation replacement

Here one cation replaces another:
Some examples are:
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.

Anion replacement

Here one anion replaces another:
Some examples are:
Cl2 + 2NaBr -> 2NaCl + Br2
Br2 + 2KI -> 2KBr + I2
Cl2 + H2S -> 2HCl + S
Again, the less reactive halogen cannot replace the more reactive halogen:

Common reactions

Metal-acid reaction

Metals react with acids to form salts and hydrogen gas.
However, less reactive metals cannot displace the hydrogen from acids.

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 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.

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 CuCl2.
Bromine and iodine are extracted from brine by displacing with chlorine.