A single-displacement reaction, also known as single replacement reaction or exchange reaction, is a chemical reaction in which one element is replaced by another in a compound. It can be represented generically as: A + BC -> AC + B where either A and B are different metals (or any element that forms cation like hydrogen) and C is an anion; or A and B are halogens and C is a cation. 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, BC and AC are usually aqueous compounds (or very rarely in a molten state) and C is a spectator ion (i.e. remains unchanged). A(s) + \underbrace{B+(aq) + C^{-}(aq)}_{BC(aq)} -> \underbrace{A+(aq) + C^{-}(aq)}_{AC(aq)} + B(s) 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: Similarly, the halogens with the highest propensity to acquire electrons are the most reactive. The activity series for halogens is: F2>Cl2>Br2>I2 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. 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 + BC -> AC + B (Element A has replaced B in compound BC to become a new compound AC and the free element B.) Some examples are: Fe + CuSO4 -> FeSO4 + Cu(v) (Blue vitriol)(Green vitriol) Zn + CuSO4 -> ZnSO4 + Cu(v) (Blue vitriol)(White vitriol) Zn + FeSO4 -> ZnSO4 + Fe(v) (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. Fe + ZnSO4 -> No Reaction Anion replacement Here one anion replaces another: A + CB