Salt metathesis reaction
A salt metathesis reaction (from the Greek μετάθεσις, "transposition"), sometimes called a double replacement reaction or double displacement reaction, is a chemical process involving the exchange of bonds between two reacting chemical species, which results in the creation of products with similar or identical bonding affiliations. This reaction is represented by the general scheme:
- A-B + C-D → A-D + C-B
The bond between the reacting species can either be ionic or covalent. Classically, these reactions result in the precipitation of one product.
In older literature, the term double decomposition is frequently encountered. The term double decomposition is more specifically used when at least one of the substances does not dissolve in the solvent, as the ligand or ion exchange takes place in the solid state of the reactant, for example:
- AX(aq) + BY(s) → AY(aq) + BX(s).
Types of reactions
Salt metathesis is a common technique for exchanging counter ions. Starting from aqueous solutions of inorganic anions, one can use salt metathesis reaction to isolate salts that are soluble in organic solvents. Illustrative is the conversion of sodium perrhenate to the tetrabutylammonium salt:
- NaReO4 + N(C4H9)4Cl → N(C4H9)4[ReO4] + NaCl
The tetrabutylammonium salt precipitates from the aqueous solution. In contrast to sodium perrhenate, this salt is soluble in chloroform.
- [Fe(C5H5)2]BF4 + NaB(C6F5)4 → [Fe(C5H5)2]B(C6F5)4 + NaBF4
When the reaction is conducted in non-polar solvents such as dichloromethane, the salt NaBF4 precipitates, which helps drive these reactions.
- (C5H5)2TiCl2 + 2 ClMgCH3 → (C5H5)2Ti(CH3)2 + 2 MgCl2
The salt product typically precipitates from the reaction solvent.
A neutralization reaction is a specific type of double replacement reaction. A neutralization reaction occurs when an acid reacts with an equal amount of a base. This usually creates a solution of a salt and water. For example, hydrochloric acid reacts with sodium hydroxide to produce sodium chloride and water:
- HCl (aq) + NaOH (aq) → NaCl (aq) + H
Aqueous metathesis (precipitation)
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:
3 (aq) + NaCl (aq) → AgCl (s) + NaNO
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.
Aqueous metathesis (double decomposition)
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:
2 (s) + 2CuCNS (s) → Ba(CNS)
2 (aq) + 2CuOH (s)
Acid and carbonates
A subcategory of aqueous metathesis reactions in which there is a reaction between an acid and 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:
3COOH (aq) + NaHCO
3 (s) → CH
3COONa (aq) + CO
2 (g) + H
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