Double Replacement Reactions

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  DOUBLE REPLACEMENT REACTIONS Double replacement reactions — also called double displacement , exchange , or metathesis reactions  - occur when parts of two ionic compounds are exchanged, making two new compounds. The overall pattern of a double replacement reaction looks like this:  A+B−+C+D−→A+D−+C+B We can think of the reaction as swapping the cations or the anions, but not swapping both since we would end up with the same substances you started. The solvent for a double replacement reaction is usually water, and the reactants and products are usually ionic compounds — but they can also be acids or bases . Here is an example of a double replacement reaction: BaCl2(aq)+Na2SO4 (aq)→ BaSO4(s)+2NaCl(aq) In this example, the cations are Ba2 +  and Na +  and the anions are Cl -  and SO 2/4 . If we swap the anions, or cations, we get as our products BaSO 4  and NaCl. 2 TYPES OF DOUBLE REPLACEMENT 1. Precipitation Reactions 2. Neutralization Reactions   A precipitation reaction  is when two aqueous ionic compounds form a new ionic compound that is not soluble in water. One example is the reaction between lead (II) nitrate and potassium iodide. Both compounds are white solids that can be dissolved in water to make clear, colorless solutions. When you combine the two clear solutions, you get the following reaction: Pb(NO 3 ) 2 (aq)+2KI(aq)→2KNO 3 (aq)+PbI 2 (s) The insoluble product compound is called the precipitate . The solvent and soluble components of the reaction are called the supernatant  or supernate . We can use solubility rules to predict whether a precipitation reaction will take place. The formation of a solid precipitate is the driving force that makes the reaction proceed in the forward direction.    Neutralization reactions  are  a type of double replacement reaction that occurs between an acid and a base. The following is an example of a neutralization reaction:   HF(aq)+NaOH(aq)→H 2O+NaF(aq) acid+base→H2O+salt     An aqueous neutralization reaction  generally produces water and a new ionic compound, also called a salt. The hardest part of identifying a neutralization reaction is recognizing that you have an acid and a base for the reactants. Once you know you have a neutralization reaction, you can generally predict the reaction will occur in the forward direction as long as you have a strong acid and/or a strong base as a reactant.    A fun neutralization reaction  that you may have tried is the combination of baking soda — sodium bicarbonate, NaHCO3 - and vinegar  — mostly water with acetic acid, CH3COOH(aq) — which produces carbonic acid — H 2 CO 3 — and sodium acetate — NaCH3COO. If you have tried this reaction at home, you probably remember a lot of fizzing because the neutralization reaction is accompanied by a gas-producing reaction , where the carbonic acid decomposes into carbon dioxide gas — bubbles! — and water. Note that double replacement reactions can be written as molecular, complete ionic, or net ionic equations. In this article we are only writing out the molecular equation, but you probably want to be familiar with writing the other forms of the equation as well .  
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