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.[1][2][3]

It can be represented generically as:

${\displaystyle {\ce {A + B-C -> A-C + B}}}$

where

• ${\displaystyle {\ce {A}}}$ and ${\displaystyle {\ce {B}}}$ are different metals (or any element that forms cation like hydrogen) and ${\displaystyle {\ce {C}}}$ is an anion;[2] or

• ${\displaystyle {\ce {A}}}$ and ${\displaystyle {\ce {B}}}$ are halogens and ${\displaystyle {\ce {C}}}$ is a cation.[2]

This will most often occur if ${\displaystyle {\ce {A}}}$ is more reactive than ${\displaystyle {\ce {B}}}$, thus giving a more stable product.

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).[1]

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

When a copper wire is dipped in a silver nitrate solution, copper displaces silver and solid silver precipitates out: Cu + AgNO₃ → Cu(NO₃)₂ + Ag↓

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]

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

(Hydrogen and Carbon are non-metals that form cations.)

Similarly, the halogens with the highest propensity to acquire electrons are the most reactive. The activity series for halogens is: [1][2][3] ${\displaystyle {\ce {F2>Cl2>Br2>I2}}}$

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.[4] 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 ${\displaystyle {\ce {0}}}$).