double+replacement

A **double replacement** reaction is one in which two ionic compounds react in a way such that they swap ionic partners.

The general form for such a reaction is:

AB + CD --> AD + CB

Specific classes of double replacement reactions include:
 * precipitate reactions
 * acid-base reactions

As an example, let's start with a precipitate reaction. These are reactions that form a solid product from two aqueous reactants.

The toxic, water-soluble compound barium chloride (AB) can be precipitated using aqueous sodium sulfate (CD); this reaction produces solid barium sulfate (AD) and soluble sodium chloride (CB).

BaCl 2 (aq) + Na 2 SO 4 (aq) --> BaSO 4 (s) + 2 NaCl (aq)

To interpret this reaction according to AB + CD --> AD + CB, label each ion accordingly:

The toxic, water-soluble compound barium (A) chloride (B) can be precipitated using aqueous sodium (C) sulfate (D); this reaction produces solid barium (A) sulfate (D) and soluble sodium (C) chloride (B).


 * < Ba ||< Cl 2 (aq) + ||< Na 2 ||< SO 4 (aq) ||< --> ||< Ba ||< SO 4 (s) + ||< 2 Na ||< Cl (aq) ||
 * < A ||< B ||< C ||< D ||<  ||< A ||< D ||< C ||< B ||

You can use this technique to predict the products of a reaction, as well.