Single displacement reaction

Template:Short description 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.^[1]^[2]

It can be represented generically as:

A + B C ⟶ A C + B

where either

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

$A$ and $B$ are halogens and $C$ is a cation.^[2]

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, $B C$ and $A C$ are usually aqueous compounds (or very rarely in a molten state) and $C$ is a spectator ion (i.e. remains unchanged).^[1]

A (s) + \underset{B C (a q)}{\underset{⏟}{B^{+} (a q) + C^{-} (a q)}} ⟶ \underset{A C (a q)}{\underset{⏟}{A^{+} (a q) + C^{-} (a q)}} + B (s)

File:Single Displacement AgNO3 and Cu.ogv

File:Zinc displaces Tin.webm

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 > N a > C a > M g > A l > C > Z n > F e > N H_{4}^{+} > H^{+} > C u > A g > A u

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Similarly, the halogens with the highest propensity to acquire electrons are the most reactive. The activity series for halogens is: ^[1]^[2]

F_{2} > C l_{2} > B r_{2} > I_{2}

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.^[3] When $A$ and $B$ are metals, $A$ is always oxidized and $B$ is always reduced. Since halogens prefer to gain electrons, $A$ is reduced (from $0$ to $- 1$ ) and $B$ is oxidized (from $- 1$ to $0$ ).

Cation replacement

Here one cation replaces another:

A + B C ⟶ A C + B

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

Some examples are:

F e + C u S O_{4} ⟶ F e S O_{4} + C u

(Blue vitriol)Template:Font color(Green vitriol)

Z n + C u S O_{4} ⟶ Z n S O_{4} + C u

(Blue vitriol)Template:Font color(White vitriol)

Z n + F e S O_{4} ⟶ Z n S O_{4} + F e

(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.^[4]

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.

F e + Z n S O_{4} ⟶

No Reaction

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Anion replacement

Here one anion replaces another:

A + C B ⟶ C A + B

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

Some examples are:

$C l_{2} + 2 N a B r ⟶ 2 N a C l + B r_{2}$

$B r_{2} + 2 K I ⟶ 2 K B r + I_{2} (v)$

$C l_{2} + H_{2} S ⟶ 2 H C l + S (v)$

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

I_{2} + 2 K B r ⟶

no reaction

Common reactions

Metal-acid reaction

Metals react with acids to form salts and hydrogen gas.

Z n (s) + 2 H C l (a q) ⟶ Z n C l_{2} (a q) + H_{2} ↑

^[2]

However, less reactive metals cannot displace the hydrogen from acids. (They may react with oxidizing acids though.)

C u + H C l ⟶

No reaction

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.

F e (s) + H_{2} O (g) ⟶ F e O (s) + H_{2} ↑

C a (s) + 2 H_{2} O (l) ⟶ C a (O H)_{2} (a q) + H_{2} ↑

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.

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Metal extraction

Coke or more reactive metals are used to reduce metals by carbon from their metal oxides,^[5] such as in the carbothermic reaction of zinc oxide (zincite) to produce zinc metal:

Z n O + C ⟶ Z n + C O

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

3 M n O_{2} + 4 A l ⟶ 3 M n + 2 A l_{2} O_{3}

Such reactions are also used in extraction of boron, silicon, titanium and tungsten.

3 S i O_{2} + 4 A l ⟶ 3 S i + 2 A l_{2} O_{3}

B_{2} O_{3} + 3 M g ⟶ 2 B + 3 M g O

T i C l_{4} + 2 M g ⟶ T i + 2 M g C l_{2}

W F_{6} + 3 H_{2} ⟶ W + 6 H F

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.^[5]

F e_{2} O_{3} (s) + 2 A l (s) ⟶ 2 F e (l) + A l_{2} O_{3} (s)

Template:Font color(Haematite)

3 C u O + 2 A l ⟶ 3 C u + A l_{2} O_{3}

Silver tarnish

Silver tarnishes due to the presence of hydrogen sulfide, leading to formation of silver sulfide.^[6]^[2]

4 A g + 2 H_{2} S + O_{2} ⟶ 2 A g_{2} S + 2 H_{2} O

3 A g_{2} S + 2 A l ⟶ 6 A g + A l_{2} S_{3}

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 $C u C l_{2}$ .

4 H C l + O_{2} ⟶ 2 C l_{2} + 2 H_{2} O

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

2 H B r + C l_{2} ⟶ 2 H C l + B r_{2} ↑

2 H I + C l_{2} ⟶ 2 H C l + I_{2} ↑

References

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External links

Reactivity series by RSC

Halogen displacement reaction by RSC

Chlorine water reacting with Iodide and Bromide, YouTube

↑ ^1.0 ^1.1 ^1.2 ^1.3 Template:Cite web
↑ ^2.0 ^2.1 ^2.2 ^2.3 ^2.4 ^2.5 ^2.6 Template:Cite web
↑ Silberberg. Chemistry, the Molecular Nature of Matter and Change, 4th ed. p. 150 McGraw Hill 2006.
↑ Template:Cite web
↑ ^5.0 ^5.1 Template:Cite web
↑ Template:Cite journal

[:1-1] 1.0 ^1.1 ^1.2 ^1.3 Template:Cite web

[:2-2] 2.0 ^2.1 ^2.2 ^2.3 ^2.4 ^2.5 ^2.6 Template:Cite web

[3] Silberberg. Chemistry, the Molecular Nature of Matter and Change, 4th ed. p. 150 McGraw Hill 2006.

[4] Template:Cite web

[:4-5] 5.0 ^5.1 Template:Cite web

[:0-6] Template:Cite journal

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Single displacement reaction

Contents

Cation replacement

Anion replacement