APChemistryCollege BoardRevision NotesUnit 4: Chemical Reactions4.7 Types of Chemical ReactionsPrecipitation Reactions

Precipitation Reactions (College Board AP Chemistry)

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Precipitation Reactions

  • A precipitation reaction occurs when two homogeneous solutions are mixed and an insoluble solid is produced
    • Precipitation reactions are double replacement reactions
    • Double replacement reactions involve the exchange of cations or anions, making two new chemical compounds
      • There need to be an exchange from cations or anions, not both at the same time because you would end up with the same reactants
    • E.g. Below is showed a general equation, where A and C are cations and B and D are anions

 AB space plus space CD space rightwards arrow space AD space plus space BC

  • The insoluble solid that forms in the mixture is also known as a precipitate
  • The formation of a precipitate can be predicted using some rules of solubility summarized in the table below:

Solubility of Ionic Compounds

Soluble ions

Insoluble

Compounds of sodium, potassium, and ammonium

-

All nitrates (NO3-)

-

All chlorides (Cl-), except

… silver and lead(II)

All sulfates (SO4-), except

… barium, calcium, and lead(II)

Sodium, potassium and ammonium carbonates

All other carbonates

Sodium, potassium, and calcium hydroxides

All other hydroxides

Worked example

Write a balanced equation with state symbols for the reaction between a solution of silver nitrate (AgNO3) and a solution of sodium chloride (NaCl). If there is a precipitate identify it.

Answer:

  • Step 1: Write down a balanced equation for the reaction
    • Since precipitation reactions are double replacement reactions, the cations can be switched

 AgNO subscript 3 left parenthesis aq right parenthesis end subscript plus NaCl subscript left parenthesis aq right parenthesis end subscript rightwards arrow NaNO subscript 3 plus AgCl

  • Step 2: Using the rules of solubility, identify the soluble and insoluble products
    • NaNO3 is a sodium salt and it is also a nitrate. Therefore, it is soluble and it will be in aqueous state in the mixture
    • AgCl is a chloride, however, silver chloride is an exception to the rule. Therefore, it is insoluble and it will be a solid in the mixture. Therefore, it is the precipitate
  • Step 3: Write down the balanced equation with all the state symbols

begin mathsize 16px style Ag N O subscript 3 open parentheses aq close parentheses end subscript and Na Cl subscript open parentheses aq close parentheses end subscript yields Na N O subscript 3 open parentheses aq close parentheses end subscript and Ag Cl subscript open parentheses s close parentheses end subscript end style

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