A double replacement reaction is a type of chemical reaction where two reactants exchange ions to form two new products with the same type of chemical bonds. Usually, one of the products forms a precipitate. Double displacement reactions take the form:
AB + CD → AD + CB
Double replacement reactions may involve reactants that contain either ionic or covalent bonds, but the type of reaction is more common with ionic compounds. Acids and bases may participate in double replacement reactions. Usually, the solvent is water.
Other names for a double replacement reaction are a double displacement reaction, exchange reaction, or salt metathesis reaction. A double replacement reaction is called a double decomposition reaction, but the term is reserved for when one or both of the reactants doesn’t dissolve in a solvent.
Double Replacement Reaction Examples
An example of a double replacement reaction is the reaction between silver nitrate and sodium chloride in water. Both silver nitrate and sodium chloride are ionic compounds. Both reactants dissolve into their ions in aqueous solution. The silver ion picks up the sodium’s chloride ion to form silver chloride, while the sodium ion picks up the nitrate anion to form sodium nitrate. Like the reactants, both products are ionic compounds. Silver chloride has a low solubility in water, so it precipitates out of solution.
AgNO3 + NaCl → AgCl + NaNO3
Another example is the reaction between barium chloride and sodium sulfate to form barium sulfate and sodium chloride:
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2 NaCl(aq)
All of the reactants and products contain ionic bonds. The product barium sulfate precipitates out of solution as a solid.
How to Recognize a Double Replacement Reaction
You can recognize a double replacement reaction in a chemical equation by checking whether the cations exchange anions with each other. If the states of matter of the reactants and products are listed, look for a reaction between two aqueous solutions that yields one aqueous product (aq) and one that precipitates to form a solid product (s). If you don’t know the reactants but you see precipitate formation upon mixing them, suspect a double replacement reaction.
If you can’t visually observe the reaction, you can predict whether or not the reactants will dissolve and a precipitate will form (indicating a double replacement reaction) using solubility rules.
Types of Double Replacement Reactions
There are several categories of double replacement reactions, including neutralization, alkylation, acid-carbonate reactions, counter-ion exchange, aqueous metathesis with precipitation (precipitation reactions), and aqueous metathesis with double decomposition (double decomposition reactions). However, the two types most often seen in general chemistry are neutralization reactions and precipitation reactions.
A neutralization reaction is a double displacement reaction between acids and bases. When water is the silver, the reaction usually produces an ionic compound—a salt. If one or both of the reactants is a strong acid or strong base, the reaction proceeds in the forward direction.
The reaction between hydrofluoric acid and sodium hydroxide in water to form water and sodium fluoride is an example of a neutralization reaction. Hydrofluoric acid is (naturally) an acid, while sodium hydroxide is a base. The general form of the reaction is:
acid + base → water + salt
In this case, the reaction is:
HF(aq) + NaOH(aq) → H2O + NaF(aq)
Another example of a neutralization reaction is the reaction between baking soda and vinegar in the baking soda volcano. The reaction ultimately produces a gas (carbon dioxide) and a salt (sodium carbonate), but the initial neutralization reaction forms carbonic acid (H2CO3) and sodium acetate (NaCH3COO)
NaHCO3 + CH3COOH(aq) → H2CO3 + NaCH3COO
The cations exchange anions, but it’s trickier to notice the swap because of the way the compound formulas are written. You can identify the reaction as double replacement when you compare the atoms in the anions of the reactants and products.
In a precipitation reaction, two aqueous ionic compounds form an insoluble ionic product. An example is the reaction between lead(II) nitrate and potassium iodide to form potassium nitrate and (insoluble) lead iodide.
Pb(NO3)2(aq) + 2 KI(aq) → 2 KNO3(aq) + PbI2(s)
You can recognize precipitate formation because of the (s) following the chemical formula. While lead iodide is the precipitate, the solvent (water) and soluble reactants and product are called the supernate or supernatant. Precipitate formation drives the reaction in the forward direction, as product leaves solution.
- Dilworth, J. R.; Hussain, W.; Hutson, A. J.; Jones, C. J.; Mcquillan, F. S. (1997). “Tetrahalo Oxorhenate Anions.” Inorganic Syntheses. 31: 257–262. doi:10.1002/9780470132623.ch42
- IUPAC (1997). “Metathesis.” Compendium of Chemical Terminology (2nd ed.) (the “Gold Book”). doi:10.1351/goldbook.M03878
- 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.