Precipitation Reactions (Reactions that form a precipitate)

Definition: Precipitate: A solid that forms as the result of a chemical reaction in aqueous solution.

Let’s see what this looks like: Sodium iodide + Mercury II Chloride Cadmium Nitrate + Sodium Sulfide Click here for a video of a precipitation reaction.

Precipitation Reaction: A reaction in which a solid is formed from a solution is called a precipitation reaction Solid = precipitate = insoluble What is the precipitate in the reaction below? Pb(NO3)2 (aq) + 2NaI (aq) PbI2 (s) + 2NaNO3 (aq)

Writing Equations for Precipitation Reactions

First Step: Practice Using Solubility Rules – Are These Compounds Soluble or Insoluble? Barium Nitrate Potassium Carbonate Sodium Sulfate Copper (II) Hydroxide Mercury (I) Chloride Ammonium Phosphate Chromium (III) Sulfide Lead (II) Sulfate * Use the solubility rules on your periodic table

Solubility – Why does it matter? A precipitation reaction only occurs if one of the products is insoluble.

Back to Precipitation Reactions What Happens When an Ionic Compound Dissolves in Water? NaCl (s) NaCl (aq) But what does NaCl (aq) mean? Na+ and Cl- ions in water ? NaCl particles in water ? Na and Cl atoms in water?

When an ionic compound dissolves in water, it breaks apart into ions: NaCl (aq)  Na+ + Cl-

Predicting whether a reaction happens What happens when solutions of BaCl2 and Na2SO4 are made? The ionic compounds break up into their cations and anions: BaCl2 becomes Ba2+ and Cl- Na2SO4 becomes Na+ and SO42-

When the two solutions are mixed together, there will be 4 ions floating around: Ba2+ Cl- Na+ SO42- A double replacement reaction will occur in this case because when the ions switch partners, there is a precipitate: BaCl2(aq) + Na2SO4(aq) BaSO4(s) + NaCl(aq) How do we know that it was the barium sulfate that was the precipitate?? From the solubility rules!

Another example: Will a reaction occur between solutions of NaCl and AgNO3? NaCl (aq) + AgNO3 (aq)  ? The reactants separate into ions.

Using the solubility rules, we see that NaNO3 is soluble, so those ions will continue to float around in the water. However, AgCl is insoluble:

Because we got a new substance (AgCl) , a chemical reaction took place: NaCl (aq) + AgNO3 (aq)  NaNO3 (aq) + AgCl (s)

Practice: Will this reaction occur? 2 NaNO3 (aq) + Li2SO4 (aq)  Na2SO4 + 2 LiNO3 Is Na2SO4 soluble in water? Is LiNO3 soluble in water? Will the reaction occur?

Steps for writing a precipitation reaction Step 1: Identify the reactants Step 2: Split each reactant into its positive and negative ions. Step 3: Identify the possible products of the reactants by swapping the positive and negative ions of the reactants. Step 4: Write the equation and balance it so that equal numbers of all atoms occur on both sides. Step 5: Use the Solubility Rules to find if one product is insoluble. If one product is insoluble, the reaction occurs. If both products are soluble, no reaction occurs.

Practice: Switch the partner ions and write balanced equations for each possible reaction. Use your Solubility Rules to see whether a precipitate forms. NaOH + CaCl2 CuBr2 + (NH4)2CO3 K2SO4 + Fe(NO3)3

Practice: A precipitate forms when a colourless solution of lead II nitrate Pb(NO3)2 is added to a colourless solution of potassium iodide (KI). A dense yellow colour appears that is solid lead iodide. Write a balanced chemical equation, identifying the precipitate.