Double displacement reactions are a fundamental type of chemical reaction where two compounds exchange their ions to form two entirely new compounds. The general formula is AB plus CD yields AD plus CB, where A and C are typically cations, while B and D are anions. These reactions involve ionic compounds and are characterized by the exchange of positive and negative ions between the reactants, always producing exactly two products from two reactants.
The mechanism of double displacement reactions involves three distinct steps. First, the ionic compounds dissociate completely in aqueous solution, breaking apart into their constituent cations and anions. For example, sodium chloride separates into sodium cations and chloride anions, while silver nitrate dissociates into silver cations and nitrate anions. Second, these ions move freely throughout the solution and interact based on their opposite charges. Finally, new ionic combinations form as the ions recombine in different arrangements, creating entirely new compounds with different properties than the original reactants.
Precipitation reactions are a major type of double displacement reaction where one of the products formed is an insoluble solid called a precipitate. A classic example is the reaction between silver nitrate and sodium chloride, which produces silver chloride precipitate and sodium nitrate. The formation of the precipitate occurs because silver chloride is insoluble in water, according to solubility rules. Most chlorides are soluble, but silver chloride is a notable exception. The driving force for this reaction is the formation of the insoluble precipitate, which removes ions from solution and pushes the reaction forward.
Acid-base neutralization is another important type of double displacement reaction where an acid reacts with a base to produce a salt and water. In the classic example of hydrochloric acid reacting with sodium hydroxide, the hydrogen ions from the acid combine with hydroxide ions from the base to form water molecules. Meanwhile, the remaining sodium and chloride ions form sodium chloride salt. The driving force for neutralization reactions is the formation of water from the highly reactive hydrogen and hydroxide ions. This process results in a neutral solution with a pH of seven, as indicated by the color change in pH indicators.
Gas formation reactions represent another important category of double displacement reactions where one of the products is a gas that escapes from the solution. A classic example is the reaction between sodium carbonate and hydrochloric acid, which produces sodium chloride, water, and carbon dioxide gas. The carbon dioxide gas bubbles out of solution, which can be observed as effervescence or fizzing. Common gas-forming combinations include carbonates reacting with acids to produce carbon dioxide, sulfites with acids producing sulfur dioxide, and sulfides with acids generating hydrogen sulfide. The driving force for these reactions is the escape of gas from the solution, which removes the product and pushes the equilibrium forward, ensuring the reaction proceeds to completion.