1. 4.5 Precipitation Reactions

2. 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

4. 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) 

5. 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

6. 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)

7. 4.6 Describing Reactions in Solution

8. 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

9. 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)

10. 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)

11. 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)

12. 4.7 Stoichiometry of Precipitation Reactions

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

14. 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

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

16. 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)

17. 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)

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

19. 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-

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

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

22. 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

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