Salt metathesis (from the Greek μετάθεσις "transposition") is a molecular process involving the exchange of bonds between the two reacting chemical species, which results in the creation of products with similar or identical bonding affiliations.[1] This is represented by the general reaction scheme:
AB+CD → AD+CB
These chemical species can either be ionic or covalent. When referring to precipitation reactions between solutions of ions in inorganic chemistry, these were formerly referred to as double displacement or double replacement reactions, and these terms are still encouraged. In older literature, the term double decomposition is frequently encountered. It seems that the term double decomposition is more specifically used when at least one of the substances does not dissolve in the liquor, as the ligand or ion exchange takes place in the solid state of the reactant, i.e. AX(aq) + BY(s) → AY(aq) + BX(s).
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A neutralization reaction is a specific type of double displacement reaction. Neutralization occurs when an acid reacts with an equal amount of a base. A neutralization reaction creates a solution of a salt and water. For example, hydrochloric acid reacts with sodium hydroxide to produce salt and water:
Metathesis reactions can occur between two inorganic salts when one product is insoluble in water, driving the reaction forward. For example, the precipitation of silver chloride from a mixture of silver nitrate and sodium chloride causes sodium nitrate to be left in solution:
The formation of an insoluble gas that bubbles out of the solution, or a molecular compound such as water, also drives the reaction to completion. Therefore, a solubility chart (or general knowledge of solubility rules) can be used to predict whether two aqueous solutions will react. HSAB theory can also be used to predict the products of a metathesis reaction.
The reactants need not to be dissolved for metathesis reactions to take place. An example of this is the formation of barium thiocyanate when boiling a mixture of copper(I)thiocyanate and barium hydroxide in water:
Ba(OH)2 (s) + 2 CuCNS (s) → Ba(CNS)2 (aq) + 2CuOH (s)
A subcategory of aqueous metathesis reactions is the reaction of an acid with a carbonate or bicarbonate. Such a reaction always yields carbonic acid as a product, which spontaneously decomposes into carbon dioxide and water. The release of carbon dioxide gas from the reaction mixture drives the reaction to completion. For example, a common, science-fair "volcano" reaction involves the reaction of acetic acid with sodium bicarbonate: