Advanced Chem Lecture #8
Sections 4.1-4.5
General Properties of Aqueous Solutions
Molarity
Precipitation Reactions
Acid-Base Reactions
Oxidation-Reduction Reactions
General Properties of Aqueous Solutions
A solution is a homogenous mixture of two or more substances
Solute is substance present in smaller amounts
Solvent is substance present in larger amounts
Solutions can be liquid (seawater), solid (alloy), or gaseous (air)
Here, we will only discuss liquid solutions where the solvent is water, and
the solute is initially a liquid or solid
All solutes that dissolve in water fit into one of two categories:
Electrolyte: when dissolved in H2O, can conduct electricity
Non-electrolyte: does not conduct electricity when dissolved in H2O
Pure H2O is in fact a very poor conductor of electricity
However, add a small amount of NaCl, and current will flow in water
NaCl fully disassociates into Na+ and ClStrong electrolyte (full disassociation into cation and anion)
Also H2SO4, HCl, NaOH, KI
In other words, all H+ separates from all ClAcetic acid and ammonia do not fully disassociate in water
Weak electrolytes (incomplete disassociation)
HF, H2O
That is, not all H+ separates from OHWhat term do we use for the degree of separation?
Non-electrolytes do not disassociate at all in water
Sugars, alcohols
Battery connected to electrodes in (draw on board):
Distilled water
Salt water (NaOH, HCl)
Sugar water
Results:
Ions in salt solution move to electrodes to conduct electricity
Solutions without ions do not conduct
In reality, pure water will not conduct electricity
Still, probably not a good idea to swim in lightning storms
Why is this?
When ionic compounds disassociate, hydration occurs
Process by which ions are surrounded by water molecules
+/- regions of H2O are attracted to opposite charge in solution
Prevents reformation of ionic compound from cation and anion
Ionization of acetic acid represented as: CH3COOH   CH3COO- + H+
COOH is the group with the ionizing proton
Where CH3COO- is the acetate ion
Double arrows show reversible reaction
Some CH3COO- will react with H+ to form CH3COOH
Eventually a state is reached where ionization and
Reionization occur at the same rate: chemical equilibrium
By contrast, HCl has no tendency to re-ionize
Precipitation Reactions
Reaction that results in an insoluble product, or precipitate
Pb(NO3)2 + 2KI  PbI2 + 2KNO3
How can we predict whether a precipitate will form or not?
Depends on the solubility of the solute
Maximum amount of solute that will dissolve in a given quantity
of solute at a given temperature
Molecular Equations, Ionic Equations, Net Ionic Equations
Molecular equations: formulas of compounds are written as though all
species existed as molecules or whole units
Pb(NO3)2 + 2KI  PbI2 + 2KNO3
Useful to recreate the reaction in the laboratory
However, does not accurate describe how ions are behaving
Ionic equations: shows dissolved species as free ions
Pb2+ + 2NO3- + 2K+ + 2I-  PbI2 + 2K+ + 2NO3Precipitate product and spectator ions
K and NO3 simply stay disassociated in solution
Net ionic equation: shows only reacting species
Pb2+ + 2I-  PbI2
More accurate describes reacting species
However, incomplete for the laboratory chemist to carry out rxn
Acid-Base Reactions
Arrhenius definition of an acid: ionizes in water to produce H+ ions
free protons – charged particles
HCl  H+ + ClArrhenius definition of a Base: ionizes in water to form OH-, or hydroxide ions
NaOH  Na+ + OHHowever, it is important to remember that these charged particles
don’t really exist alone as shown… something we’ll address later
Also, this definition only holds for reactions in water!
Bronsted-Lowry definition of an acid: any substance that donates a proton
HCl + H2O  H3O+ + ClStrong evidence that H+ never exists alone
Nature strongly dislikes free charges
Basis for many biological processes - respiration
H+ binds with H2O to give H3O, hydronium ion
Bronsted-Lowry definition of a base: any substance that accepts a proton
NaOH + H2O  Na+ + H2O + OHNH3 + H2O  NH4+ + OHWater donates a proton to ammonia to form ammonium
Lewis acid: electron pair acceptor
BF3 + NH3  H3N:BF3
Lewis base: electron pair donor
NH3 + H2O  NH4 + OHHere, hydrogen ion is acid, and donates e- to ammonia
Note that ammonia is a base under all three definitions
Conjugate Acids and Bases
Compounds formed after reaction of acid and base
Conjugate base formed when acid surrenders H+
H3O  H20 + H+
Conjugate acid formed when base gains H+
H2O + H+  H3O
More practically:
HCl(a) + H2O(b)  H3O(ca) + Cl-(cb)
H3O(a) + Cl-(b)  HCl(ca) + H2O (cb)
A little something on acid-base reactions
Acids and bases tend to react to neutralize one another
Thus, can add more of one to get acidic of basic solution
Acids create more H+ ions
lower pH  ph 4 = 0.01M, pH 2 = 0.0001M
Bases remove H+ ions (by forming OH ions) and raise pH
Acid + Base = Water + Salt
HCl + NaOH  H2O + NaCl
Pure water self-ionizes at 10^-7M, forms trace amounts of H and OH
Thus, neutral is said to be pH 7