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:

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

where

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

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

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

In the first case, when ${\ce {A}}$ and ${\ce {B}}$ are metals, ${\ce {BC}}$ and ${\ce {AC}}$ are usually aqueous compounds (or very rarely in a molten state) and ${\ce {C}}$ is a spectator ion (i.e. remains unchanged).^[1]

{\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↓

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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]

{\text{K}}>{\text{Na}}>{\text{Ca}}>{\text{Mg}}>{\text{Al}}>{\color {grey}{\text{C}}}>{\text{Zn}}>{\text{Fe}}>{\color {grey}{{\text{NH4}}^{+}}}>{\color {grey}{{\text{H}}^{+}}}>{\text{Cu}}>{\text{Ag}}>{\text{Au}}

(Hydrogen, Carbon and Ammonium are not metals but 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] ${\ce {F2>Cl2>Br2>I2}}$

Due to the free state nature of ${\ce {A}}$ and ${\ce {B}}$ , single displacement reactions are also redox reactions, involving the transfer of electrons from one reactant to another.^[4] When ${\ce {A}}$ and ${\ce {B}}$ are metals, ${\ce {A}}$ is always oxidized and ${\ce {B}}$ is always reduced. Since halogens prefer to gain electrons, ${\ce {A}}$ is reduced (from ${\ce {0}}$ to ${\ce {-1}}$ ) and ${\ce {B}}$ is oxidized (from ${\ce {-1}}$ to ${\ce {0}}$ ).

Cation replacementEdit

Here one cation replaces another:

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

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

Some examples are:

{\ce {Fe + CuSO4 -> Fe(SO4)2 + Cu(v)}}

(Blue vitriol)aaa(Green vitriol)

{\ce {Zn + CuSO4 -> ZnSO4 + Cu(v)}}

(Blue vitriol)aa(White vitriol)

{\ce {Zn + FeSO4 -> ZnSO4 + Fe(v)}}

(Green vitriol) (White vitriol)

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.

{\ce {Fe + ZnSO4 ->}}

No Reaction

Anion replacementEdit

Here one anion replaces another:

{\ce {A + CB -> CA + B}}

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

Some examples are: ${\ce {Cl2 + 2NaBr -> 2NaCl + Br2(v)}}$ ${\ce {Br2 + 2KI -> 2KBr + I2(v)}}$

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

{\ce {I2 + 2KBr ->}}

no reaction

Common reactionsEdit

Metal-acid reactionEdit

Metals react with acids to form salts and hydrogen gas.

Liberation of hydrogen gas when zinc reacts with hydrochloric acid.

{\ce {Zn(s) + 2HCl(aq) -> ZnCl2(aq) + H2 ^}}

^[2]^[3]

However less reactive metals can not displace the hydrogen from acids.^[3] (They may react with oxidizing acids though.)

{\ce {Cu + HCl ->}}

No reaction

Reaction between metal and waterEdit

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

{\ce {Fe(s) + H2O (g) -> Fe2O3(s) + H2 ^}}

{\ce {Ca(s) + H2O (l) -> CaOH(aq) + H2 ^}}

The reaction can be extremely violent with alkali metals as the hydrogen gas catches fire.^[2]

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

Metal extractionEdit

Coke or more reactive metals are used to reduce metals from their metal oxides.^[5]

{\ce {ZnO + C -> Zn + CO}}

a(Zincite)

{\ce {MnO2 + 4Al -> 3Mn + Al2O3}}

{\ce {SiO2 + 2C -> Si + 2CO}}

a:(Silica)

Thermite reactionEdit

Using highly reactive metals as reducing agents leads to exothermic reactions that melt the metal produced. This is used for welding railway tracks.^[5]