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A 'single-displacement reaction', also called 'single-replacement reaction', is when one element appears to move 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' or 'A + BX → AB + X'
This will occur if A is more reactive than B. You can refer to the reactivity series to be sure of this.
A and c must either be different metals (hydrogen's behavior as a cation renders it as a metal here), in which case X represents an anion. However, A and B may also be 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).
:2AgNO_3(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) → MgCl_2(aq) + H_2(g)