Section 8.1
Understanding Reactions in Aqueous Solutions
Objectives
1. To learn more about some of the results of chemical
reactions
2. To learn to predict the solid that forms in a precipitation
reaction
3. To learn to write molecular, complete ionic, and net ionic
equations
Section 8.1
Understanding Reactions in Aqueous Solutions
A. Common Results of a Reaction
• Four common events accompany chemical reactions.
These often occur when a reaction happens
– Formation of a solid
– Formation of water
– Formation of a gas
– Transfer of electrons
Can you give examples of each of these?
Section 8.1
Understanding Reactions in Aqueous Solutions
B. Precipitation Reactions
What Happens When an Ionic Compound Dissolves
in Water?
NaCl (s)
H 2O
NaCl (aq)
But what does NaCl (aq) mean?
NaCl molecules
in water ?
Na and Cl atoms
in water?
Na+ and Cl- ions
in water ?
Section 8.1
Understanding Reactions in Aqueous Solutions
B. Precipitation Reactions
What Happens When an Ionic Compound Dissolves in
Water?
• The ions separate and move around independently –
a strong electrolyte
Section 8.1
Understanding Reactions in Aqueous Solutions
B. A Reaction in Which a Solid Forms
• A reaction in which a solid is formed from a solution is
called a precipitation reaction
– Solid = precipitate = insoluble
When positive and negative ions form a precipitate
the net charge of the precipitate is zero
Pb(NO3)2 (aq) + 2NaI (aq)
PbI2 (s) + 2NaNO3 (aq)
Section 8.1
Understanding Reactions in Aqueous Solutions
Stalactites in Yosemite
Ca(HCO3)2(aq)
→ CaCO3(s) +
H2O(l) + CO2(aq)
Section 8.1
Understanding Reactions in Aqueous Solutions
Section 8.1
Understanding Reactions in Aqueous Solutions
B. Precipitation Reactions
What Happens When Two Ionic Compounds Dissolve in
Water?
• K2CrO4(aq) + Ba(NO3)2(aq)  Products
Section 8.1
Understanding Reactions in Aqueous Solutions
B. Precipitation Reactions
How to Decide What Products Form
• Determine the possible products from the ions in the
reactants.
• In our example
K2CrO4(aq) + Ba(NO3)2(aq)  Products
The possible ion combinations are
Section 8.1
Understanding Reactions in Aqueous Solutions
What are the potential products in the reactions
between the following aqueous solutions? – Write
balanced equations for each
• NaOH + CaCl2
• CuBr2 + (NH4)2CO3
• K2SO4 + Fe(NO3)3
Section 8.1
Understanding Reactions in Aqueous Solutions
B. Precipitation Reactions
How Would We Decide What Product Has Formed?
• What is most likely to be the yellow solid formed in the
following reaction?
K2CrO4(aq) + Ba(NO3)2(aq)
• The possible product combinations are KNO3 and BaCrO4
– KNO3
white solid
– BaCrO4
yellow solid
Section 8.1
Understanding Reactions in Aqueous Solutions
B. Precipitation Reactions – Solubility Rules
Section 8.1
Understanding Reactions in Aqueous Solutions
B. Precipitation Reactions
Using Solubility Rules
• Predicting Precipitates
– Soluble solid
– Insoluble solid
– (Slightly soluble solid)
Why are some
compounds
soluble and
others not?
Section 8.1
Understanding Reactions in Aqueous Solutions
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
Section 8.1
Understanding Reactions in Aqueous Solutions
B. Precipitation Reactions
Let’s try some predictions: WOC P271 Q10
Section 8.1
Understanding Reactions in Aqueous Solutions
C. Describing Precipitation Reactions in Aqueous Solutions
• Three types of equations:
– Molecular (formula) equation 
complete formulas of all reactants and products
– Complete ionic equation 
all strong electrolytes are shown as ions
– Net ionic equation 
only those components of the solution that undergo a
change
• Spectator ion (those that remain unchanged) 
not shown in the net ionic equation
Section 8.1
Understanding Reactions in Aqueous Solutions
C. Describing Precipitation Reactions in Aqueous
Solutions
• Molecular Equation
Pb(NO3)2 (aq) + 2NaI (aq)
PbI2 (s) + 2NaNO3 (aq)
• Complete Ionic Equation
Pb2+(aq) + 2NO3-(aq) + 2Na+(aq) + 2I-(aq)
PbI2(s) + 2Na+(aq) + 2NO3-(aq)
Section 8.1
Understanding Reactions in Aqueous Solutions
C. Describing Precipitation Reactions in Aqueous
Solutions
• Net Ionic Equation
Pb2+(aq) + 2I-(aq)
PbI2(s)
• What are the spectator ions in this reaction?
A solution of sodium chloride in water reacts with
aqueous silver nitrate to give a precipitate of silver
chloride – write down the three equations that describe
this reaction and identify the spectator ions
(P272 Q14 – all three equations and identify spectators)