Section 4.2
Precipitation Reactions
Aqueous
Reactions
Precipitation Reactions
Mixing ions that form
insoluble compounds
The insoluble solid formed is
a precipitate
Aqueous
Reactions
Solubility
• The amount of substance
that can dissolve in a
given quantity of solvent
at a given temperature
Aqueous
Reactions
Solubility Guidelines
Aqueous
Reactions
Steps for Predicting Precipitation
1. Note ions present in reaction
2. Consider possible combinations of cations
and anions
3. Use Table 4.1 to determine if any of those
combinations are insoluble
Aqueous
Reactions
Example
• Will a precipitate form when solutions of
Mg(NO3)2 and NaOH are mixed?
Aqueous
Reactions
Step 1: Ions present
•
•
•
•
Mg 2+
NO3 –
Na +
OH -
Aqueous
Reactions
Step 2: Possible combinations
• Mg 2+ with OH –
• Na + with NO3 –
Aqueous
Reactions
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)
Aqueous
Reactions
Metathesis (Exchange) Reactions
• Metathesis- Greek, “to transpose”
• Pattern of precipitation formation
AX + BY  AY + BX
AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)
Aqueous
Reactions
Metathesis (Exchange) Reactions
AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)
Aqueous
Reactions
Balancing Metathesis Reactions
1. Determine ions present
2. Write chemical formulas of products by
combining cation from one reactant
with anion of the other
3. Use charges of ions to determine
subscripts
4. Balance the equation
Aqueous
Reactions
Sample Problem
• Predict the identity of the precipitate that
forms when solutions of BaCl2 and K2SO4
are mixed.
Aqueous
Reactions
Step 1: Determine ions
• BaCl2 and K2SO4 are mixed:
• Ba 2+
• Cl •K+
• SO4 2Aqueous
Reactions
Step 2: Combine
• Reactants: BaCl2 and K2SO4
BaCl2 + K2SO4  BaSO4 + K2Cl2
Aqueous
Reactions
Step 3: Subscripts
Ba 2+
Cl -
K+
SO4 2-
BaCl2 + K2SO4  BaSO4 + KCl
Aqueous
Reactions
Step 4: Balance
BaCl2 + K2SO4  BaSO4 + 2KCl
Did a precipitate form?
Aqueous
Reactions
Solution
BaCl2 (aq) + K2SO4 (aq)  BaSO4 (s) + 2KCl (aq)
Aqueous
Reactions
Molecular Equation
• Reactants and products in their molecular form
• NOT ionic character
AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)
Aqueous
Reactions
Complete Ionic Equation
• All soluble strong electrolytes shown as ions
Ag+ (aq) + NO3- (aq) + K+ (aq) + Cl- (aq) 
AgCl (s) + K+ (aq) + NO3- (aq)
• Spectator ions- appear as reactants and
products in identical form
Aqueous
Reactions
Net Ionic Equation
• Omit spectator ions
Ag+ (aq) + Cl- (aq)  AgCl (s)
Aqueous
Reactions
Writing Net Ionic Equations
1. Write a balanced molecular equation.
2. Dissociate all strong electrolytes.
3. Identify and cancel spectator ions
Aqueous
Reactions
Sample Problem
• Write the net ionic equation for
mixing calcium chloride and
sodium carbonate.
Aqueous
Reactions
Step 1: Molecular equation
• Calcium chloride and sodium carbonate
CaCl2 + Na2CO3  CaCO3 + 2Na Cl
Aqueous
Reactions
Step 2: Dissociate strong
electrolytes
Ca 2+ + 2Cl- + 2Na+ + CO3 2-  CaCO3 + 2Na+ + 2Cl-
All are strong electrolytes, but CaCO3
is insoluble in water
Aqueous
Reactions
Step 3: Omit spectators
Ca 2+ (aq) + CO3 2- (aq) CaCO3 (s)
Aqueous
Reactions
Homework
• 4.19-4.24 on page 158
Aqueous
Reactions