1. Unit 5 – Aqueous Reactions & Solution Stoichiometry
Properties of Aqueous Solutions
Predicting Products of Chemical Reactions
Concentrations of Solutions
Solution Stoichiometry and Chemical Analysis

2. Aqueous Solutions
Many chemical reactions take place in an aqueous solution:
a solution in which water is the dissolving medium
(aq)
Solution: a homogeneous mixture of two or more substances

3. Aqueous Solutions Examples of Aqueous Reactions or Processes
Biological or physiological processes
photosynthesis
metabolism of food
transportation of oxygen or nutrients in blood
Rust formation
Acid rain

4. Aqueous Solutions Components of Solutions Solvent: the dissolver
usually present in the greatest quantity
Solute:
the substance that dissolves

5. Aqueous Solutions Sodium Chloride/Water Solute: sodium chloride
Solvent: water
Sugar/water
Solute: sugar
Denatured alcohol
Solutes: methyl alcohol, water
Solvent: ethyl alcohol

6. Aqueous Solutions
NaCl
sucrose
H2O
NaCl in H2O
sucrose in H2O

7. Aqueous Solutions Similarities: Differences:
Compare and contrast salt (NaCl) water solution and sucrose (C12H11O22) dissolved in water
Similarities:
Differences:

8. Aqueous Solutions Salt water solutions are good electrical conductors.
Sucrose (C12H11O22)/water solutions are not good electrical conductors.
Salt (NaCl) forms ions when dissolved in water
ions carry electrical charge
Sugar is a molecular compound.
Molecules stay intact when the compound dissolves so no ions are formed.

9. Aqueous Solutions Electrolyte:
a substance that forms ions when dissolved in water
resulting solutions conduct electricity
all ionic compounds
MgCl2
NaHCO3
Ca(OH)2
some molecular compounds
HCl
Acetic acid

10. Aqueous Solutions Nonelectrolyte:
a substance that does not form ions when dissolved in water
solutions do not conduct electricity
most molecular compounds
sugar
methyl alcohol
acetone

11. Aqueous Solutions
When an ionic compound like NaCl dissolves in water, the ions are separated from their orderly crystalline structure in a process called dissociation.
The water molecules form a
“shell” around the ions
which helps stabilize the
ions and keeps them from
recombining.

12. Aqueous Solutions
Ions and their “shell” of water molecules are free to move around in solution.
The ions become uniformly dispersed throughout the solution.
a homogeneous solution forms

13. Aqueous Solutions How does water stabilize ions in solution?
Water is an electrically neutral, molecular compound but the electrons are not shared equally between the H and O atoms.
Oxygen attracts the electrons in the molecule more strongly than the hydrogen atoms.
The O atom in water has a partial negative charge (high electron density).
The H atoms in water have a partial positive charge (low electron density). d+ d- O H

14. Aqueous Solutions Water solvates (surrounds and stabilizes) the ions.
the H’s (d+) interacts with the negatively charged anions
the O (d-) interacts with the positively charged cations - +
Solvation of anion
Solvation of cation

15. Aqueous Solutions
Example: What ions (and how many of each) are formed when KBr dissolves in water?
Example: What ions (and how many of each are formed when Na2SO4 dissolves in water?

16. Aqueous Solutions
Example: What ions (and how many of each form when Ba(NO3)2 dissolves in water?
Example: What ions (and how many of each form when Al2(SO4)3 dissolves in water?

17. Aqueous Solutions
Most molecular compounds dissolve in water without forming ions.
Water molecules solvate the molecular compound, but the structural integrity of the molecule is maintained.
The dissolution (dissolving) of methanol.

18. Aqueous Solutions
Some molecular compounds interact strongly with water and form ions.
Acids
HCl (g) + H2O  H+ (aq) + Cl- (aq)
Ammonia
NH3 (g) + H2O  NH4+ (aq) + OH- (aq)
Molecular compounds that ionize are electrolytes.

19. Aqueous Solutions There are two kinds of electrolytes:
Strong electrolyte:
a substance that ionizes completely when dissolved in water
ionic compounds
strong acids
strong bases

20. Aqueous Solutions Weak Electrolyte:
a substance that partially ionizes when dissolved in water
part of the substance exists as ions and part exists as molecules
HC2H3O2 (aq) H+ (aq) + C2H3O2- (aq)

21. Aqueous Solution and (at the same time)
The double arrow indicates that the reaction goes in both directions.
HC2H3O2 is ionizing to form H+ and C2H3O2-
H+ and C2H3O2- are recombining to form HC2H3O2
A single arrow indicates the reaction goes only in one direction.
and (at the same time)

22. Predicting Chemical Reactions
Precipitation Reactions
Acid/Base Reactions
Oxidation/Reduction (Redox) Reactions

23. Precipitation Reactions
One visible sign of a chemical reaction is the formation of a solid:
AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq)
One driving force for a chemical reaction (i.e. thing that pushes a reaction to occur) is the formation of a solid product (precipitate).

24. Precipitation Reactions
A reaction that forms an insoluble product is called a precipitation reaction.
AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq)
Precipitate: the insoluble product formed by a reaction in solution
In the reaction above, AgCl is the precipitate.

25. Precipitation Reactions
AgCl forms a precipitate because it has very low solubility in water.
Solubility:
the amount of a substance that can be dissolved in a given quantity of solvent
AgCl is insoluble in water while NaNO3 is soluble.
Soluble: “dissolves”
Insoluble: “does not dissolve”

26. Precipitation Reactions
In order to predict whether a precipitation reaction will occur, you must know if an ionic compound is soluble in water or not.
Experimental data have led to a series of guidelines used to predict solubility.
On your exam, you must be able to use the guidelines in Table 4.1 to predict the solubility of a compound.
I will give you a copy of these guidelines on the exam.

27. Precipitation Reactions Solubility Guidelines
Guidelines Based on Cation
All ionic compounds of the 1A metals are soluble.
NaCl, NaHCO3, K2CO3, LiOH
All ammonium salts (ionic compounds) are soluble.
NH4OH, NH4Cl, (NH4)3PO4

28. Precipitation Reactions Solubility Guidelines
SOLUBLE Compounds
Anion Important Exceptions
NO3 - None
C2H3O2 - None
ClO4- Few common exceptions
SO42- Cpds. With Ca2+, Sr2+, Ba2+, Pb2+, Ag+

29. Precipitation Reactions Solubility Guidelines
SOLUBLE Compounds
Anion Important Exceptions
Cl - Cpds. with Ag+, Pb2+, Cu+, Hg22+
Br - Cpds. with Ag+, Pb2+, Cu+, Hg22+
I - Cpds. with Ag+, Pb2+, Cu+, Hg22+
F- Cpds. With Pb2+, Group 2A

30. Precipitation Reactions Solubility Guidelines
INSOLUBLE Compounds
Anion Important Exceptions
S 2- Cpds. with NH4+, Group 1A, Ca2+, Sr2+, or Ba2+
CO3 2- Cpds. with NH4+ or alkali metal
cations
PO43- Cpds. with NH4+, alkali metal cations
OH - Cpds. with Group 1A, NH4+, Ca2+,
Sr2+, or Ba2+

31. Precipitation Reactions
Example: Use the solubility guidelines in Table 4.1 to predict the solubility of the following compounds.
Ba(NO3)2
CaCl2
Ca3(PO4)2
NH4Cl
PbSO4
Ba(OH)2

32. Precipitation Reactions Predicting Products
Consider the reaction between AgNO3 and NaCl in aqueous solution:
AgNO3 (aq) + NaCl (aq)  AgCl (s) + NaNO3 (aq)
Notice that the anions and cations appear to have exchanged partners.

33. Precipitation Reactions
Metathesis Reactions
reactions in which the positive ions and negative ions present in the reactants appear to exchange partners
also called exchange reactions
AX + BY  AY + BX
AgNO3 + NaCl  AgCl + NaNO3

34. Precipitation Reactions
To write a chemical equation for a precipitation reaction:
Write the formula including charge for the ions formed by each reactant in aqueous solution.
Switch “partners” and write the correct formulas (electrically neutral) for the possible products.
Determine the solubility of each product.
Balance the equation. Include all physical states for reactants and products.

35. Precipitation Reactions
Example: Write a balanced equation for the reaction between aqueous solutions of NaF and Pb(NO3)2.

36. Precipitation Reactions
Example: Write the chemical equation for the reaction between aqueous solutions of Na2CO3 and Fe(C2H3O2)3.

37. Precipitation Reactions
Example: Write the chemical equation for the reaction between aqueous solutions of aluminum nitrate and sodium hydroxide.

38. Precipitation Reactions
Al(NO3)3 (aq) + 3 NaOH (aq)  Al(OH)3 (s) + 3 NaNO3 (aq)
At the end of this reaction, the beaker contains two components:
The solid aluminum hydroxide that precipitates out and settles to the bottom of the beaker.
The aqueous solution that contains Na+ (aq) and NO3- (aq)

39. Precipitation Reactions
Al(NO3)3 (aq) + 3 NaOH (aq)  Al(OH)3 (s) + 3 NaNO3 (aq)
This is called a molecular equation.
It shows the chemical formulas of reactants and products but does not indicate the ionic nature of the reactants or products.
In reality, the chemicals with (aq) beside them exist as ions in solution

40. Precipitation Reactions
In order to indicate the ionic nature of the chemicals, we can write a complete ionic equation:
Explicitly indicates all of the ions in the reaction mixture
All soluble strong electrolytes are written as individual ions.

41. Precipitation Reactions
To write a complete ionic equation:
Break all soluble strong electrolytes (ionic compounds or strong acids with (aq) beside them) into their ions.
indicate the correct charge of each ion
indicate the number of each ion
write (aq) after each ion
Bring down all weak electrolytes unchanged.
Bring down all compounds with (s), (g), or (l) unchanged.

42. Precipitation Reactions
Example: Write the complete ionic equation for the following reaction:
Al(NO3)3 (aq) + 3 NaOH (aq) Al(OH)3 (s) + 3 NaNO3 (aq)

43. Precipitation Reactions
Al3+ (aq) + 3 NO3- (aq) + 3 Na+ (aq) + 3 OH- (aq) 
Al(OH)3 (s) + 3 Na+ (aq) + 3 NO3- (aq)
Notice that the Na+ and NO3- ions appear unchanged on both sides of the reaction.
Spectator ions
ions that are present but play no direct role in a chemical reaction

44. Precipitation Reactions
If we eliminate all spectator ions from the equation, we can write a net ionic equation
an equation that includes only the ions directly involved in the reaction and the insoluble products that are formed

45. Precipitation Reactions
To write a net ionic equation:
Write the complete ionic equation.
Cross out the ions that appear in the same type, number, and physical state on both sides of the equation.
Write the “leftovers” as the net ionic equation.
Don’t forget to include the physical states of the reactants and products.

46. Precipitation Reactions
Al3+ (aq) + 3 NO3 - (aq) + 3 Na+ (aq) + 3 OH- (aq) 
Al(OH)3 (s) + 3 Na+ (aq) + 3 NO3- (aq)
Net ionic equation: