Single displacement reaction

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A single-displacement reaction, also called single-replacement reaction, is when an element or ion moves out of one compound and into another. (One element is replaced by another in a compound.) This is usually written as

A + BX → AX + B

This will occur if A is more reactive than B. You can refer to the reactivity series to be sure of this.

A and B must be either:

different metals (hydrogen's behavior as a cation renders it as a metal here), in which case X represents an anion; or
halogens, in which case X represents a cation.

In either case, when AX and BX are aqueous compounds (which is usually the case), X is a spectator ion.

Due to the free state nature of A and B, all single displacement reactions are also oxidation-reduction reactions. 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 a 0 to −1) and B is oxidized (from −1 to 0) when A and B represent those elements.

A and B may not have the same charge when ions are formed therefore some balancing of the equation may be necessary. For example the reaction between silver nitrate, AgNO₃, and zinc, Zn, forms silver, Ag, and zinc nitrate, Zn(NO₃)₂.

2AgNO₃(aq) + Zn(s) → 2Ag(s) + Zn(NO₃)₂(aq)

All simple metal with acid reactions are single displacement reactions. For example the reaction between magnesium, Mg, and hydrochloric acid, HCl, forms magnesium chloride, MgCl₂, and hydrogen, H₂.

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

Cation replacement

One cation replaces another. A cation is a positively charged ion or a metal. When it is written in generic symbols, it is

AX + Y → YX + A

Element Y has replaced A (in the compound AX) to form a new compound YX and the free element A. This is an oxidation-reduction reaction wherein element A is reduced from a cation into the elemental form and element Y is oxidized from the elemental form into a cation.

Some examples are:

Cu + 2AgNO₃ → 2Ag + Cu(NO₃)₂
Fe + Cu(NO₃)₂ → Fe(NO₃)₂ + Cu
Ca + 2H₂O → Ca(OH)₂ + H₂
Zn + 2HCl → ZnCl₂ + H₂

Anion replacement

One anion replaces another. An anion is a negatively charged ion or a nonmetal. Written using generic symbols, it is:

A + XY → XA + Y

Element A has replaced Y (in the compound XY) to form a new compound XA and the free element Y. This is an oxidation-reduction reaction wherein element A is reduced from the elemental form into an anion and element Y is oxidized from an anion into the elemental form.

Some of the only examples that involve halogens are here, so here are the two examples:

Cl₂ + 2NaBr → 2NaCl + Br₂
Br₂ + 2KI → 2KBr + I₂

Single displacement reaction

Cation replacement

Anion replacement

See also