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.
It can be represented generically as:
:<chem>A + BC -> AC + B</chem>
where either
- <chem>A</chem> and <chem>B</chem> are different metals (or any element that forms cation like hydrogen) and <chem>C</chem> is an anion; When <chem>A</chem> and <chem>B</chem> are metals, <chem>A</chem> is always oxidized and <chem>B</chem> is always reduced. Since halogens prefer to gain electrons, <chem>A</chem> is reduced (from <chem>0</chem> to <chem>-1</chem>) and <chem>B</chem> is oxidized (from <chem>-1</chem> to <chem>0</chem>).
Cation replacement
Here one cation replaces another:
:<chem> A + BC -> AC + B </chem>
(Element A has replaced B in compound BC to become a new compound AC and the free element B.)
Some examples are:
:<chem>Fe + CuSO4 -> FeSO4 + Cu</chem>
:::(Blue vitriol)(Green vitriol)
:<chem>Zn + CuSO4 -> ZnSO4 + Cu</chem>
:::(Blue vitriol)(White vitriol)
:<chem>Zn + FeSO4 -> ZnSO4 + Fe</chem>
:::(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.
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.
:<chem>Fe + ZnSO4 -> </chem> No Reaction
:
:
:
Anion replacement
Here one anion replaces another:
:<chem> A + CB -> CA + B </chem>
(Element A has replaced B in the compound CB to form a new compound CA and the free element B.)
Some examples are:
<chem> Cl2 + 2NaBr -> 2NaCl + Br2 </chem>
<chem> Br2 + 2KI -> 2KBr + I2(v) </chem>
<chem> Cl2 + H2S -> 2HCl + S(v) </chem>
Again, the less reactive halogen cannot replace the more reactive halogen:
:<chem>I2 + 2KBr -> </chem> no reaction
<!-- ("unsubstantiated examples", 2017-07-19)
Examples of single displacement reactions
:<math chem>\begin{array}{l}
\ce{3Na + AlCl3 -> 3NaCl + Al(v)}\\
\ce{3Na + FeCl3 -> 3NaCl + Fe(v)}\\
\ce{2Na + PbCl2 -> 2NaCl + Pb(v)}\\
\ce{3Na + ScCl3 -> 3NaCl + 3Sc(v)}\\
\ce{4Na + TiCl4 -> 4NaCl + Ti(v)}\\
\ce{4Na + VCl4 -> 4NaCl + V(v)}\\
\ce{4Na + ZrCl4 -> 4NaCl + Zr(v)}\\
\ce{3Ca + 2AlCl3 -> 3CaCl2 + 2Al(v)}\\
\ce{Ca + PbCl2 -> CaCl2 + Pb(v)}\\
\ce{2Ca + 3ScCl3 -> 2CaCl2 + 3Sc(v)}\\
\ce{2Ca + TiCl4 -> 2CaCl2 + Ti(v)}\\
\ce{2Ca + VCl4 -> 2CaCl2 + V(v)}\\
\ce{2Ca + ZrCl4 -> 2CaCl2 + Zr(v)}\\
\ce{3Mg + 2FeCl3 -> 3MgCl2 + 2Fe(v)}\\
\ce{3Mg + 2CrCl3 -> 3MgCl2 + 2Cr(v)}\\
\ce{Mg + PbCl2 -> MgCl2 + Pb(v)}\\
\ce{3Mg + 2AlCl3 -> 3MgCl2 + 2Al(v)}\\
\ce{2Mg + TiCl4 -> 2MgCl2 + Ti(v)}\\
\ce{2Mg + 4VCl4 -> 2MgCl2 + V(v)}\\
\ce{2Mg + ZrCl4 -> 2MgCl2 + Zr(v)}\\
\ce{Zn + CuSO4 -> ZnSO4 + Cu(v)}\\
\ce{Zn + 2HCl -> ZnCl2 + H2(^)}\\
\end{array}</math>
-->
Common reactions
Metal-acid reaction
Metals react with acids to form salts and hydrogen gas.
Liberation of hydrogen gas when zinc reacts with hydrochloric acid.|thumb
:<chem>Zn(s) + 2HCl(aq) -> ZnCl2(aq) + H2 ^</chem>
thumb|[[Thermite reaction proceeding for a railway welding: Shortly after this, the liquid iron flows into the mould around the rail gap]]
:<chem>Fe2O3(s) + 2 Al(s) -> 2 Fe(l) + Al2O3(s)</chem>
(Haematite)
:<chem>3CuO + 2Al -> 3Cu + Al2O3</chem>
Silver tarnish
thumb|A tarnished silver coin
Silver tarnishes due to the presence of hydrogen sulfide, leading to formation of silver sulfide.