Single displacement reaction
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
See also
Double displacement reaction
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