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Double Replacement Reactions

Definition

Double replacement reactions, also known as double displacement reactions or metathesis, occur when parts of two ionic compounds are exchanged and form two new compounds. This type of reaction is represented by the general equation: AB + CD → AD + CB

Key points

  1. Exchange of ions: These reactions involve the exchange of ions between two compounds. Typically, one of the products is a precipitate, an insoluble gas, or water.

  2. Formation of precipitate: A common outcome of double replacement reactions is the formation of an insoluble solid, known as a precipitate, which falls out of solution.

  3. Acid-base neutralization: Acid-base neutralizations, where an acid reacts with a base to produce water and a salt, are a common type of double replacement reaction. An example of this is the reaction of hydrochloric acid (HCl) with sodium hydroxide (NaOH) to form water (H2O) and sodium chloride (NaCl): HCl + NaOH → H2O + NaCl

  4. Gas formation: Some double replacement reactions result in the formation of a gas, such as when a carbonate reacts with an acid to produce carbon dioxide, for example: CaCO3 + 2HCl → CaCl2 + H2O + CO2

  5. Solubility rules: The occurrence of a double replacement reaction often depends on the solubility rules for the products formed. Soluble products will remain in solution, while insoluble products precipitate out.

Example

An example of a double replacement reaction that forms a precipitate is the reaction between silver nitrate and sodium chloride in which silver chloride is the precipitate (AgNO3 + NaCl → AgCl + NaNO3).