1. Aqueous Reactions and Solution Stoichiometry

2. 4.1 General Properties of Aqueous Solutions
Aqueous solutions: solutions in which water is the solvent
Solvent: dissolving medium
Solute: dissolved by solvent
Electrolytic solutions:
Aqueous ionic solns
Conduct electrical current
Electrolyte: solute that produces ions in solution

3. Electrolyte vs. Nonelectrolyte
Electrolytes
form ions in solution
Conduct electrical current when dissolved
NaCl, KNO3
Nonelectrolytes
Do not form ions in solution
Do not conduct electrical current
sucrose C11H22O11

4. Conduction of Electrical Current by an Electrolytic Solution

5. Ionic Compounds in Water
Dissociate into separate ions
Because water is a polar solvent, ions are dispersed and surrounded by water molecules (fig 4.3a)

6. Electrical Current is a movement of charged particles
Because ions can now move in solution, they can carry an electrical current
(electrical current is a net movement of charges)

7. Molecular Cpds in Water
Molecular = Covalent bonding
Entire molecules are dispersed (fig 4.3b)
Because molecules are uncharged, they cannot conduct an electrical current

8. Some molecular cpds form ions
Exceptions: some molecular compounds react with water to form ions, such as acids
HCl + H2O  H3O+ + Cl-

9. Strong vs. Weak Electrolytes
Many ionic substances dissolve completely (≈100% dissociated)
More ions in solution = greater conductivity
These are strong electrolytes
HCl + H2O → H3O+ + Cl-
Substances that partially dissolve form fewer ions = lesser conductivity
These are weak electrolytes
CH3COOH + H2O ↔ H3O+ + CH3COO-

10. Identifying Strong and Weak Electrolytes

11. Strong Electrolytes Are…
Strong acids
Strong bases
Soluble ionic salts

12. Strong Electrolytes Are…
Strong acids, strong bases, soluble ionic salts

13. Relating relative numbers of ions to chemical formulas
MgCl2  Mg Cl-
one mole  one mole + 2 moles = 3 moles
How many cations & anions would be formed from the following?
6 NiSO4
6 Ca(NO3)2
6 Na3PO4
6Al2(SO4)3

14. 4.2 Precipitation Reactions
Precipitate: insoluble compound formed from a reaction of solutions.
When KI dissolves  K+ (aq) + I- (aq)
When Pb(NO3)2 dissolves Pb2+(aq) + 2NO3-(aq)
When the solutions are added together:
2K+(aq) + 2I-(aq) + Pb2+(aq) + 2NO3-(aq)  2K+(aq) + 2NO3-(aq) + PbI2 (s)

15. Metathesis (Exchange) Reactions
Metathesis comes from a Greek word that means “to transpose.”
It appears the ions in the reactant compounds exchange, or transpose, ions.
AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)

16. AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)
Molecular Equation
The molecular equation lists the reactants and products in their molecular form.
AgNO3 (aq) + KCl (aq)  AgCl (s) + KNO3 (aq)

17. Ionic Equation
In the ionic equation all strong electrolytes (strong acids, strong bases, and soluble ionic salts) are dissociated into their ions.
This more accurately reflects the species that are found in the reaction mixture.
Ag+ (aq) + NO3- (aq) + K+ (aq) + Cl- (aq) 
AgCl (s) + K+ (aq) + NO3- (aq)

18. Net Ionic Equation
To form the net ionic equation, cross out anything that does not change from the left side of the equation to the right.
Ag+(aq) + NO3-(aq) + K+(aq) + Cl-(aq) 
AgCl (s) + K+(aq) + NO3-(aq)

19. Solubility of Ionic Compounds
Solubility is measured as the mass of solute that will dissolve in a given volume of solvent
For solids & liquids it is often measured as g solute/100 g 25°C
Examples
CaCl g per100g water
MnSO
KNO
Ethanol infinite
PbI

20. Solubility Table

21. Effect of Temperature on Salt Solubility

22. The Dissolving Process: Solvation
Dipoles of water molecules align with ions
Ions removed from solid
Ions surrounded by water molecules forming “hydration shell”

23. 4.3 Acid-Base Reactions Acids
Ionize in aqueous soln to form hydrogen ion (H+)
HCl (aq) → H+ (aq) + Cl- (aq)
Acids produce H+ (Arrhenius’ definition)
Acids donate H+ in chem rxns (Bronsted-Lowry)
Monoprotic acids produce one H+ per molecule
Diprotic acids produce two H+ per molecule
H2SO4(aq) → H + + HSO4-
HSO4- (aq) ↔ H + + SO42-
Polyprotic acids?
Reduce pH
pH inversely proportional to [H+]
pH = -log [H+]

24. Bases Produce OH- ions in aqueous solutions.
Arrhenius’ definition
Accept H+ in aqueous solutions.
Bronsted-Lowry definition
NaOH (aq)  Na+ (aq) + OH- (aq)
NH3 (aq) + H2O > NH4+ (aq) + OH- (aq)
Increase pH by neutralizing H+

25. Strong & Weak Acids & Bases
Strong = highly ionized, strong electrolyte
Weak = low ionization, weak electrolyte

26. Neutralization Reactions & Salts
Reaction of an acid with a base
Acid + base  water + salt
HCl (aq) + NaOH (aq)  H2O (l) + NaCl (aq)
Salt is a category of substances:
An ionic cpd whose cation came from a base and whose anion came from an acid
Total ionic equation:
H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq)  H2O (l) + Na+(aq) + Cl-(aq)
Net ionic equation:
H+(aq) + OH-(aq)  H2O (l)

27. Ionic Equations HCl (aq) + NaOH (aq)  H2O (l) + NaCl (aq)
Write all aqueous substances as dissociated ions
Total ionic equation:
H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq)  H2O (l) + Na+(aq) + Cl-(aq)
Cancel spectator ions (ions common to both sides)
Net ionic equation for neutralization reaction
H+(aq) + OH-(aq)  H2O (l)

28. Ionic Equations
Write the molecular, total ionic and net ionic equations for the reaction of magnesium hydroxide and hydrochloric acid.
Mg(OH)2 + 2HCl  2H2O + MgCl2
Total ionic?
Net ionic?

29. Acid-Base Rxns with Gas Formation
Two ions, in particular, react with acids to produce a gas
Sulfide, S2- and bicarbonate HCO3-
These reactions could be used as a test for acids
2HCl(aq) + Na2S(aq)  H2S(g) + 2NaCl(aq)
Write the total and net ionic equations.

30. Acid-Base Rxns with Gas Formation
HCl(aq) + NaHCO3(aq)  NaCl(aq) + H2O(l) + CO2(g)
Write the total ionic equation
Write the net ionic equation

31. 4.4 Oxidation-Reduction Reactions: Oxidation is a loss of electrons
Redox reactions involve the transfer of e-s
A substance is oxidized when it loses e-s Ca(s) + 2HCl(aq)  CaCl2 (aq) + H2(g)
In this reaction, Ca is oxidized…
Ca(s) + 2H+(aq)  Ca2+(aq)+ H2(g)
Ca0  Ca2+ + 2e-

32. Reduction is a gain of electrons
A substance is reduced when it gains e-s Ca(s) + 2HCl(aq)  CaCl2 (aq) + H2(g)
In this reaction, H+ is reduced…
Ca(s) + 2H+(aq)  Ca2+(aq)+ H2(g)
2H1+ (aq) + 2e-  H20(g)

33. Redox reactions are coupled
Oxidation & reduction always occur together
One cannot occur without the other
Ca(s) + 2H+(aq)  Ca2+(aq)+ H2(g)
Ca0  Ca2+ + 2e-
2H+(aq) + 2e-  H2(g)

34. Oxidation States
Assigning oxidation numbers is a way of keeping track of electrons in a cpd
Some rules…
Oxidation number of elements = 0 e.g. Mg0
Oxidation number of monatomic ions = charge of the ion e.g. Al3+
Nonmetals usually have negative ox #’s
examples: O2-, F 1- Exception: H1+
Sum of ox #’s in a neutral cpd = 0 e.g. MgSO4
Sum of ox #’s for a polyatomic ion = charge of the ion e.g. SO42-

35. Determining Oxidation States
Using the above rules, determine the oxidation state of the following
Mn in MnO2
Mn in MnO41-
Cl in HCl
Cl in HClO
Cl in HClO4
Cl in ClO41-
Cl in ClO31-
Fe in FeBr2
Fe in FeSO4

36. Oxidation Numbers & Redox Rxns
The oxidation number is reduced in reduction
2H1+ (aq) + 2e-  H20(g)
The oxidation number is increased in oxidation
Ca0  Ca2+ + 2e-

37. Oxidation Numbers & Redox Rxns
Identify the atoms that are oxidized and reduced in the following reactions. Identify the oxidizing and reducing agents.
Ni(s) + Cl2(g)  NiCl2(s)
3Fe(NO3)2(aq) + 2Al(s)  3Fe(s) + 2Al(NO3)3 (aq)
Cl2 (aq) + 2NaI (aq)  I2 (aq) + 2NaCl(aq)
PbS(s) + 4H2O2 (aq)  PbSO4 (s) + 4H2O(l)

38. Metathesis Reactions Displacement reactions
Some ion is displaced by another
AX + BY  AY + BX
AgNO3 + KCl  AgCl + KNO3
For the reaction to lead to a net change, ions have to be removed from solution
Driving forces for metathesis reactions:
Formation of precipitate, gas, weak or nonelectrolyte

39. Oxidation of Metals by Acids
Single displacement rxn
A + BX  AX + B
Zn + 2HBr  ZnBr2 + H2
Write the ionic eqn

40. Oxidation of Metals by Acids
Write the half-reactions (oxidation, reduction)
Zn0  Zn2+ + 2e-
2H1+ + 2e-  H20
Write the redox reaction by adding the half reactions
Zn0 + 2H1+ + 2e-  Zn2+ + 2e- + H20
Zn0 + 2H1+ +  Zn2+ + H20

41. Oxidation of metals by acids or salts
Oxidation of metals by acids & salts
Mg(s) + 2HCl(aq)  MgCl2(aq) + H2(g)
Fe(s) + Ni(NO3)2(aq)  Fe(NO3)2(aq) + Ni(s)
How can you predict whether either of these reactions will occur?
Use Activity Series (Table 4.5)

42. Activity Series
A list of metals arranged according to the ease with which they are oxidized.
If a metal is higher on the list, it is easier to oxidize
If a metal is higher on the list, it will displace any ion lower on the list
Elements at the top of the list are referred to as active metals

43. Activity Series
A list of metals arranged according to the ease with which they are oxidized.

44. 4.5 Concentrations of Solutions
Molarity
Moles solute / liters of solution
What mass of NaCl is required to make 1.00L of 1.00M solution of NaCl?
58.45 g NaCl
Calculate the molarity of a soln when 23.4g Na2SO4 is dissolved to a volume of solution of 125 mL
1.32M Na2SO4

45. Molar Concentration of Electrolytes
Molar concentration of ions relates to the mole ratio of ions in the formula
In 2.0M Na2SO4 ....
[Na+] = 4.0M
[SO42-] = 2.0M
What is the [K+] in 0.025M potassium carbonate?
K2CO3, therefore [K+] = 0.050M

46. Dilutions Mi x Vi = Mf x Vf Where Mi is initial concentration
Vi is initial volume
Mf is final concentration
Vf is final volume
C1V1 = C2V2

47. 4.6 Solution Stoichiometry & Chemical Analysis
Solution stoichiometry is used to determine unknown concentrations of solutions, quantities of reactants, products, etc.
These are classical stoichiometry problems, except that molar quantities must be determined using molar concentrations and volumes

48. Using Molarities in Stoichiometric Calculations

49. Solution Stoichiometry
Determine the number of moles of HNO3 in 25 mL of 6.0 M HNO3
M = moles solute / L of solution
Moles = M x V
Moles = 6.0 moles / L x .025 L
Moles = 0.15 moles HNO3

50. Sample Problem
Determine the moles of water formed when 25.0mL of M HNO3 is completely neutralized by NaOH.
Calculate moles of HNO3
(0.100 moles/L) x (0.025L) = mol
Write balanced equation
HNO3 + NaOH  NaNO3 + H2O
Determine yield of water using stoich methods
2.5 E-3 mol HNO3 x (1 H2O /1 HNO3) = 2.5 E-3 mol H2O

51. Titrations
Process of determining the unknown concentration of a solute by reacting it with one of known concentration (a standard solution)
Equivalence point
Stoichiometrically equivalent quantities are present
End point
Point at which an indicator changes color, approximating the equivalence point.
Indicator
A substance that tells when the equivalence point is achieved
E.g. acid-base indicators, formation of a precipitate, etc.

52. Acid-Base Titration
Titration is an analytical technique in which one can calculate the concentration of a solute in a solution.

53. Titration Example
Determine the [Cl-] in a sample of water by titrating with M AgNO3
Data: mL of standard was required to titrate 45.0 mL water
Write net ionic equation
Ag+(aq) + Cl-(aq)  AgCl (s)
Determine moles of Ag+(aq) used
0.0225L x mol/L = mol Ag+
Determine moles of Cl-(aq) consumed
mol Ag+ x (1 mole Cl- / 1mol Ag+) = mol Cl-
Calculate [Cl-]
mol Cl-/0.045L = 0.05 mol/L

54. Titration Problem
25.00 mL of M HBr is titrated with M NaOH. How many mL of the NaOH solution are required to reach the equivalence point?
Write the net ionic equation.
Determine moles of H+ ion present.
Determine moles of OH- ion needed.
Calculate volume of NaOH solution containing necessary amount of OH- ion.

55. Acid-Base Titration
Indicator: phenolphtalein, colorless when acidic, pink when basic
Equivalence point: [H3O+] = [OH-]
Solution remains faintly pink
20.0 mL HCl is titrated
A few drops of phenolpthalein added as indicator
0.100 M NaOH standard solution of is used to titrate the sample.
45.0 mL of NaOH solution was required to turn the acid solution pink
What was the concentration of the acid?

56. Titration Example HCl + NaOH  NaCl + H2O Net eqn H+ + OH-  H2O
Determine # moles OH- required to reach end point
Use equation to determine # moles of H+
Determine [H+] by dividing moles H+ by volume of acid solution