When one element replaces another in a compound a N has occurred?

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:

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

where either

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

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

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

In the first case, when A{\displaystyle {\ce {A}}} and B{\displaystyle {\ce {B}}} are metals, BC{\displaystyle {\ce {BC}}}

and AC{\displaystyle {\ce {AC}}} are usually aqueous compounds (or very rarely in a molten state) and C{\displaystyle {\ce {C}}} is a spectator ion (i.e. remains unchanged).[1]

A(s)+B+(aq)+C−(aq)⏟BC(aq)⟶A+(aq)+C−(aq)⏟AC(aq)+B(s){\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, turning the solution blue and solid silver precipitates out ("silver tree"): Cu + AgNO₃ → Cu(NO₃)₂ + Ag↓

Formation of tin crystals as zinc displaces tin, seen under microscope.

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]

K>Na>Ca>Mg>Al>C>Zn>Fe>NH4+>H+>Cu>Ag>Au{\displaystyle {\ce {K}}>{\ce {Na}}>{\ce {Ca}}>{\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: [1][2][3]

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

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

to −1{\displaystyle {\ce {-1}}}) and B{\displaystyle {\ce {B}}} is oxidized (from −1{\displaystyle {\ce {-1}}} to 0{\displaystyle {\ce {0}}}).

Here one cation replaces another:

A+BC⟶AC+B{\displaystyle {\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:

Fe+CuSO4⟶Fe(SO4)+Cu↓{\displaystyle {\ce {Fe + CuSO4 -> Fe(SO4) + Cu(v)}}}(Blue vitriol)____(Green vitriol)Zn+CuSO4⟶ZnSO4+Cu↓{\displaystyle {\ce {Zn + CuSO4 -> ZnSO4 + Cu(v)}}}(Blue vitriol)___(White vitriol)Zn+FeSO4⟶ZnSO4+Fe↓{\displaystyle {\ce {Zn + FeSO4 -> ZnSO4 + Fe(v)}}}(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.[5]

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.

Fe+ZnSO4⟶{\displaystyle {\ce {Fe + ZnSO4 ->}}} No Reaction

Anion replacement[edit]

Here one anion replaces another:

A+CB⟶CA+B{\displaystyle {\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: Cl2+2NaBr⟶2NaCl+Br2↓{\displaystyle {\ce {Cl2 + 2NaBr -> 2NaCl + Br2(v)}}}

Br2+2KI⟶2KBr+I2↓{\displaystyle {\ce {Br2 + 2KI -> 2KBr + I2(v)}}}

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

I2+2KBr⟶{\displaystyle {\ce {I2 + 2KBr ->}}} no reaction

Common reactions[edit]

Metals react with acids to form salts and hydrogen gas.

Liberation of hydrogen gas when zinc reacts with hydrochloric acid.

Zn(s)+2HCl(aq)⟶ZnCl2(aq)+H2↑{\displaystyle {\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.)

Cu+HCl⟶{\displaystyle {\ce {Cu + HCl ->}}} No reaction

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

Fe(s)+H2O(g)⟶Fe2O3(s)+H2↑{\displaystyle {\ce {Fe(s) + H2O (g) -> Fe2O3(s) + H2 ^}}}Ca(s)+H2O(l)⟶CaOH(aq)+H2↑{\displaystyle {\ce {Ca(s) + H2O (l) -> CaOH(aq) + H2 ^}}}

Sodium explodes in water breaking the glass vessel

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.

Coke or more reactive metals are used to reduce metals by carbon from their metal oxides,[6] such as in the carbothermic reaction of zinc oxide (zincite) to produce zinc metal:

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

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

3MnO2+4Al⟶3Mn+2Al2O3{\displaystyle {\ce {3MnO2 + 4Al -> 3Mn + 2Al2O3}}}

Even for reactions that run in the direction opposite of their intrinsic reactivities, displacement can be driven to occur, as in the Acheson process for displacing silicon from silicon dioxide using carbon:

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

Thermite reaction[edit]

Using highly reactive metals as reducing agents leads to exothermic reactions that melt the metal produced. This is used for welding railway tracks.[6]

Thermite reaction proceeding for a railway welding: Shortly after this, the liquid iron flows into the mould around the rail gap

Fe2O3(s)+2Al(s)⟶2Fe(l)+Al2O3(s){\displaystyle {\ce {Fe2O3(s) + 2 Al(s) -> 2 Fe(l) + Al2O3(s)}}}

a(Haematite)

3CuO+2Al⟶3Cu+Al2O3{\displaystyle {\ce {3CuO + 2Al -> 3Cu + Al2O3}}}

Silver tarnish[edit]

Silver tarnishes due to the presence of hydrogen sulfide, leading to formation of silver sulfide.[7][2]

What is the reaction called in which an element is displaced by another element?

How can you tell if a double displacement reaction has occurred?

What is it called when two compounds switch places?

What type of chemical reaction is double replacement?