Double Replacement Reaction Definition

A double replacement reaction is a chemical reaction where two reactant ionic compounds exchange ions to form two new product compounds with the same ions.

Double replacement reactions take the form:

A⁺B^- + C⁺D^- → A⁺D^- + C⁺B^-

In this type of reaction, the positive-charged cations and the negative-charged anions of the reactants both trade places (double displacement), to form two new products.

Also Known As: Other names for a double displacement reaction are a metathesis reaction or a double replacement reaction.

Examples of Double Replacement Reactions

The reaction:

AgNO₃ + NaCl → AgCl + NaNO₃

is a double replacement reaction. The silver traded its nitrite ion for the sodium's chloride ion.

Another example is the reaction between sodium sulfide and hydrochloric acid to form sodium chloride and hydrogen sulfide:

Na₂S + HCl → NaCl + H₂S

Types of Double Displacement Reactions

There are three classes of metathesis reactions: neutralization, precipitation, and gas formation reactions.

Neutralization Reaction - A neutralization reaction is an acid-base reaction which yields a solution with a neutral pH.

Precipitation Reaction - Two compounds react to for a solid product called a precipitate. The precipitate is either slightly soluble or else insoluble in water. 

Gas Formation - A gas formation reaction is one which yields a gas as a product. The example given earlier, in which hydrogen sulfide was produced, was a gas formation reaction.