1. Precipitation Reactions
Section 4.2
Precipitation Reactions

2. Precipitation Reactions
Mixing ions that form insoluble compounds
The insoluble solid formed is a precipitate

3. Solubility
The amount of substance that can dissolve in a given quantity of solvent at a given temperature

4. Solubility Guidelines

5. Steps for Predicting Precipitation
Note ions present in reaction
Consider possible combinations of cations and anions
Use Table 4.1 to determine if any of those combinations are insoluble

6. Example
Will a precipitate form when solutions of Mg(NO3)2 and NaOH are mixed?

7. Step 1: Ions present
Mg 2+
NO3 –
Na +
OH -

8. Step 2: Possible combinations
Mg 2+ with OH –
Na + with NO3 –

9. Step 3: Table 4.1
Hydroxides generally insoluble, and Mg is not an exception
NaNO3 is soluble
Mg(NO3)2 (aq) + 2NaOH (aq)  Mg(OH)2 (s) + 2NaNO3 (aq)

10. Metathesis (Exchange) Reactions
Metathesis- Greek, “to transpose”
Pattern of precipitation formation
AX + BY  AY + BX
AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)

11. Metathesis (Exchange) Reactions
AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)

12. Balancing Metathesis Reactions
Determine ions present
Write chemical formulas of products by combining cation from one reactant with anion of the other
Use charges of ions to determine subscripts
Balance the equation

13. Sample Problem
Predict the identity of the precipitate that forms when solutions of BaCl2 and K2SO4 are mixed.

14. Step 1: Determine ions BaCl2 and K2SO4 are mixed: Ba 2+ Cl - K +

15. Step 2: Combine Reactants: BaCl2 and K2SO4
BaCl2 + K2SO4  BaSO4 + K2Cl2

16. Step 3: Subscripts
Ba Cl K SO4 2-
BaCl2 + K2SO4  BaSO4 + KCl

17. Step 4: Balance
BaCl2 + K2SO4  BaSO4 + 2KCl
Did a precipitate form?

18. BaCl2 (aq) + K2SO4 (aq)  BaSO4 (s) + 2KCl (aq)
Solution
BaCl2 (aq) + K2SO4 (aq)  BaSO4 (s) + 2KCl (aq)

19. AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)
Molecular Equation
Reactants and products in their molecular form
NOT ionic character
AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)

20. Complete Ionic Equation
All soluble strong electrolytes shown as ions
Spectator ions- appear as reactants and products in identical form
Ag+ (aq) + NO3- (aq) + K+ (aq) + Cl- (aq) 
AgCl (s) + K+ (aq) + NO3- (aq)

21. Ag+ (aq) + Cl- (aq)  AgCl (s)
Net Ionic Equation
Omit spectator ions
Ag+ (aq) + Cl- (aq)  AgCl (s)

22. Writing Net Ionic Equations
Write a balanced molecular equation.
Dissociate all strong electrolytes.
Identify and cancel spectator ions

23. Sample Problem
Write the net ionic equation for mixing calcium chloride and sodium carbonate.

24. Step 1: Molecular equation
Calcium chloride and sodium carbonate
CaCl2 + Na2CO3  CaCO3 + 2Na Cl

25. Step 2: Dissociate strong electrolytes
Ca Cl- + 2Na+ + CO3 2-  CaCO3 + 2Na+ + 2Cl-
All are strong electrolytes, but CaCO3 is insoluble in water

26. Ca 2+ (aq) + CO3 2- (aq) CaCO3 (s)
Step 3: Omit spectators
Ca 2+ (aq) + CO3 2- (aq) CaCO3 (s)

27. Homework
on page 158