4.5 Precipitation Reactions

Precipitation Reactions When 2 solutions are mixed and an insoluble substance forms Precipitate- this insoluble substance To figure out what the solid is, we need to know what individual species are present in the solution after the mixture occurs

Example We add potassium chromate and barium nitrate together and a yellow solid forms. What is it? Write the reactants as they exist in solution. K2CrO4 (aq) + Ba(NO3)2(aq)  2K+(aq) + CrO42- (aq) + Ba2+(aq) + 2NO3-(aq) 

Example Find the possibilities for the solid. Since K2CrO4 and Ba(NO3)2 are the beginning solutions, they cannot be the yellow solid. The only other options are: KNO3 OR BaCrO4

Example Look at the solubility rules to figure out if either of these are soluble. Since KNO3 is soluble, the solid must be: BaCrO4 So equation must be: K2CrO4 (aq) + Ba(NO3)2(aq)  KNO3(aq) + BaCrO4(s)

4.6 Describing Reactions in Solution

3 Types of Equations Molecular - overall reaction but not the actual species in solution Complete Ionic – represents all strong electrolytes as ions Net Ionic – Only species that undergo change are included, no spectators

Example Aqueous potassium chloride is added to aqueous silver nitrate. Molecular Equation: Check solubility rules to figure out what symbols to use after products. KCl(aq) + AgNO3(aq)  KNO3(aq) + AgCl(s)

Example Complete Ionic Equation: Break all strong electrolytes apart into ions Leave solids, gases, liquids, and weak electrolytes alone. K+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq)  K+(aq) + NO3-(aq) + AgCl(s)

Example Net Ionic Equation: Cancel out all spectator ions: species that are identical on both sides of the equation Must have same charges, states, formulas, etc. K+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq)  K+(aq) + NO3-(aq) + AgCl(s) Cl-(aq) + Ag+(aq)  AgCl(s)

4.7 Stoichiometry of Precipitation Reactions

Stoichiometry Determine limiting reactant 1st Always write down species present Use volume and molarity to find moles

Steps Identify species present Write balanced net ionic equation Calculate moles of reactants Determine limiting reactant Calculate moles of product Convert to grams or other unit

Example Sodium sulfate and lead (II) nitrate are mixed and a precipitate forms. Find the mass and identity of precipitate.

Example Identify species present Molecular Equation: Na2SO4(aq) + Pb(NO3)2(aq)  2NaNO3(aq) + PbSO4(s) Complete Ionic Equation: 2Na+(aq) + SO42-(aq) + Pb2+(aq) + 2NO3-(aq)  2Na+(aq) + 2NO3-(aq) + PbSO4(s)

Example 2. Write balanced net ionic equation Spectator ions: sodium and nitrate 2Na+(aq) + SO42-(aq) + Pb2+(aq) + 2NO3-(aq)  2Na+(aq) + 2NO3-(aq) + PbSO4(s) Complete Ionic Equation: SO42-(aq) + Pb2+(aq)  PbSO4(s)

Example Calculate moles of reactants Sodium Sulfate: Lead (II) Nitrate:

Example Determine the limiting reactant Limiting reactant is SO42- since extra Pb2+ is available. OR Limiting reactant is SO42- since there is not enough SO42-

Example 5. Calculate moles of product 6. Convert to grams Always start with LR 6. Convert to grams

Example 2 Calculate the mass of solid NaCl that must be added to 1.50 L of a 0.100 M AgNO3 solution to precipitate all the Ag+ ions in the form of AgCl.

Example 2 Write balanced net ionic equation NaCl(aq) + AgNO3(aq)  NaNO3(aq) + AgCl(s) Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq)  Na+(aq) + NO3-(aq) + AgCl(s) Cl-(aq) + Ag+(aq)  AgCl(s) Find moles of Ag+ that need to be precipitated

Example 2 Find the moles of Ag+ needed Find grams of NaCl needed