Single displacement reaction

A single-displacement reaction, also known as single replacement reaction or exchange reaction, is a chemical reaction in which one element is replaced by another in a compound.

It can be represented generically as:

${\displaystyle {\ce {A + BC -> AC + B}}}$

where either

• ${\displaystyle {\ce {A}}}$ and ${\displaystyle {\ce {B}}}$ are different metals (or any element that forms cation like hydrogen) and ${\displaystyle {\ce {C}}}$ is an anion; or

• ${\displaystyle {\ce {A}}}$ and ${\displaystyle {\ce {B}}}$ are halogens and ${\displaystyle {\ce {C}}}$ is a cation.

This will most often occur if ${\displaystyle {\ce {A}}}$ is more reactive than ${\displaystyle {\ce {B}}}$, thus giving a more stable product. The reaction in that case is exergonic and spontaneous.

In the first case, when ${\displaystyle {\ce {A}}}$ and ${\displaystyle {\ce {B}}}$ are metals, ${\displaystyle {\ce {BC}}}$ and ${\displaystyle {\ce {AC}}}$ are usually aqueous compounds (or very rarely in a molten state) and ${\displaystyle {\ce {C}}}$ is a spectator ion (i.e. remains unchanged).

${\displaystyle {\ce {A(s) + \underbrace{B+(aq) + C^{-}(aq)}_{BC(aq)}-> \underbrace{A+(aq) + C^{-}(aq)}_{AC(aq)}+ B(s)}}}$

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:

${\displaystyle {\ce {K}}>{\ce {Ca}}>{\ce {Na}}>{\ce {Mg}}>{\ce {Al}}>{\color {gray}{\ce {C}}}>{\ce {Zn}}>{\ce {Fe}}>{\color {gray}{\ce {NH4^+}}}>{\color {gray}{\ce {H+}}}>{\ce {Cu}}>{\ce {Ag}}>{\ce {Au}}}$

(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:

${\displaystyle {\ce {F2>Cl2>Br2>I2}}}$

Due to the free state nature of ${\displaystyle {\ce {A}}}$ and ${\displaystyle {\ce {B}}}$, single displacement reactions are also redox reactions, involving the transfer of electrons from one reactant to another. When ${\displaystyle {\ce {A}}}$ and ${\displaystyle {\ce {B}}}$ are metals, ${\displaystyle {\ce {A}}}$ is always oxidized and ${\displaystyle {\ce {B}}}$ is always reduced. Since halogens prefer to gain electrons, ${\displaystyle {\ce {A}}}$ is reduced (from ${\displaystyle {\ce {0}}}$ to ${\displaystyle {\ce {-1}}}$) and ${\displaystyle {\ce {B}}}$ is oxidized (from ${\displaystyle {\ce {-1}}}$ to ${\displaystyle {\ce {0}}}$).