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A double displacement reaction is a type of reaction where two reactants exchange ions to form two new compounds. Double displacement reactions typically result in the formation of a product that is a precipitate.
Double displacement reactions take the form:
AB + CD → AD + CB
Key Takeaways: Double Displacement Reaction
- A double displacement reaction is a type of chemical reaction in which the reactant ions exchange places to form new products.
- Usually, a double displacement reaction results in precipitate formation.
- The chemical bonds between the reactants may be either covalent or ionic.
- A double displacement reaction is also called a double replacement reaction, salt metathesis reaction, or double decomposition.
The reaction occurs most often between ionic compounds, although technically the bonds formed between the chemical species may be either ionic or covalent in nature. Acids or bases also participate in double displacement reactions. The bonds formed in the product compounds are the same type of bonds as seen in the reactant molecules. Usually, the solvent for this type of reaction is water.
A double displacement reaction is also known as salt metathesis reaction, double replacement reaction, exchange, or sometimes a double decomposition reaction, although that term is used when one or more of the reactants does not dissolve in the solvent.
Double Displacement Reaction Examples
The reaction between silver nitrate and sodium chloride is a double displacement reaction. The silver trades its nitrite ion for the sodium's chloride ion, causing the sodium to pick up the nitrate anion.
AgNO3 + NaCl → AgCl + NaNO3
Here's another example:
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2 NaCl(aq)
How to Recognize a Double Displacement Reaction
The easiest way to identify a double displacement reaction is to check to see whether or not the cations exchanged anions with each other. Another clue, if the states of matter are cited, is to look for aqueous reactants and the formation of one solid product (since the reaction typically generates a precipitate).
Types of Double Displacement Reactions
Double displacement reactions may be classified into several categories, including counter-ion exchange, alkylation, neutralization, acid-carbonate reactions, aqueous metathesis with precipitation (precipitation reactions), and aqueous metathesis with double decomposition (double decomposition reactions). The two types most commonly encountered in chemistry classes are precipitation reactions and neutralization reactions.
A precipitation reaction occurs between two aqueous ionic compounds to form a new insoluble ionic compound. Here's an example reaction, between lead(II) nitrate and potassium iodide to form (soluble) potassium nitrate and (insoluble) lead iodide.
Pb(NO3)2(aq) + 2 KI(aq) → 2 KNO3(aq) + PbI2(s)
The lead iodide forms what is called the precipitate, while the solvent (water) and soluble reactants and products are termed the supernate or supernatant. Formation of a precipitate drives the reaction in a forward direction, as the product leaves the solution.
Neutralization reactions are double displacement reactions between acids and bases. When the solvent is water, a neutralization reaction typically produces an ionic compound--a salt. This type of reaction proceeds in the forward direction if at least one of the reactants is a strong acid or a strong base. The reaction between vinegar and baking soda in the classic baking soda volcano is an example of a neutralization reaction. This particular reaction then proceeds to release a gas (carbon dioxide), which is responsible for the fizz of the reaction. The initial neutralization reaction is:
NaHCO3 + CH3COOH(aq) → H2CO3 + NaCH3COO
You'll notice the cations exchanged anions, but the way the compounds are written, it's a bit trickier to notice the anion swap. The key to identifying the reaction as double displacement is to look at the atoms of the anions and compare them on both sides of the reaction.
- Dilworth, J. R.; Hussain, W.; Hutson, A. J.; Jones, C. J.; Mcquillan, F. S. (1997). "Tetrahalo Oxorhenate Anions." Inorganic Syntheses, vol. 31, pp. 257-262. doi:10.1002/9780470132623.ch42
- IUPAC. Compendium of Chemical Terminology (2nd ed.) (the "Gold Book"). (1997).
- March, Jerry (1985). Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (3rd ed.). New York: Wiley. ISBN 0-471-85472-7.
- Myers, Richard (2009). The Basics of Chemistry. Greenwood Publishing Group. ISBN 978-0-313-31664-7.