Transcript Ions in Solution

Ions in Solution
Chapter 14
I. Ionic Compounds in Aqueous Solution
 (Aqueous - water is solvent)
 A. Theory of Ionization

1. Faraday - current causes ions to form



a. Electrolytes
b. Nonelctrolytes
2. Arrhenius - ionization of molecules
in water produces ions
B. Dissolving Ionic Compounds
1. The solution process for ionic
compounds
 a. Hydration - solution process with
water as solvent
 b. Factors affect # of water molecules
needed for hydration:

1) size of ion

2) charge of ion
2. Heat of solution for ionic compounds

heat of hydration - energy released
when ions become surrounded by
water

a) Exothermic - releases heat ;
negative heat of solution

b) Endothermic - absorbs heat ;
positive heat of solution
3. Dissociation - separation of ions when
an ionic compound dissolves

NaCl ---> Na+(aq) +Cl-(aq)

1 mol
1mol
1 mol


CaCl2 ---> Ca +2(aq) + 2Cl -(aq)

1 mol
1mol
2 mol
C. Ionic Equations and Precipitation
Reactions
1. Reactions in Solution
 a. Precipitate (ppt) - insoluble
substance formed through a
chemical reaction in a
solution
 b. Some double replacement
reactions produce ppt; others
form a gas or water.
c. Solubility Table

1. i - insoluble - forms a ppt

2. ss - slightly soluble - formation
of a slight ppt

3. s - soluble - no ppt forms
2. Writing Ionic Equations
 a. Write formula for compound.
sodium chloride = NaCl
 b. Write the compound as ions:
NaCl becomes Na+ + Cl c. Check solubility table to determine if a
forms
 d. If all combinations give ‘s’ - reaction is
 e. If one combination gives either ‘i’ or ‘ss’
then a reaction takes place
ppt
NR
-
e. Overall ionic equation includes all ions
those that form a ppt and those that are
referred to as ‘spectator ions’ because
they do not form a ppt
f. Net ionic equation includes only those
ions that form a ppt; cancel out the
spectator ions on both sides of the
equation.
Examples:
1. Write the overall ionic equation and the
net ionic equation that occurs when
aqueous solutions of zinc nitrate and
ammonium sulfide are combined.
2. A solution of sodium sulfide is combined
with a solution of iron(II) nitrate. Write
the net ionic equation for any reaction
that occurs.
II. Molecular Electrolytes
(Polar covalent molecules can form electrolytes)
A. The solution process for molecular
electrolytes

1. Polar molecules in water - opposite
dipoles attract - if strong enough bond
breaks and the molecule is separated into
simpler charged parts
2. Ionization - formation of ions from
solute molecules by the action of the
solvent
[Dissociation: ionic compounds ---Ionization: polar compounds]
B. The Hydronium Ion
1. H+ is only a proton, smaller than any
other ion - it is attracted to others so
strongly it does not have any independent
existence
2. H + + H2O --->
H3O +
hydrogen ion
water
hydronium ion
C. Strong and Weak Electrolytes
1. Strong - 100% ions
2. Weak - low concentration of ions
III. Properties of Electrolyte Solutions
A. Conductivity of Solutions
 1. Strong-weak: degree of ionization
 2. Concentrated-dilute; amount of
solute-solvent
 3. Ionization of H2O

2H2O ---> H3O+ + OH-
B. Colligative Properties of Electrolyte
Solutions
1. Electrolytes affect colligative properties
more than nonelectrolytes
Example: Compute the bp and fp for a
solution made by adding21.6 g of NiSO4
to100 g of water.
 2. Theory vs Reality

a)Theory - electrolytes reduce fp by 2,3
times - depending on # of ions

b) Reality - reduces more than
nonelectrolytes , but not as much as predicted

c) Reason - because ions are attracted to
each other in water - more concentrated
solutions have higher attraction for each other
because they are closer together
3. “Ideal Solution” - dilute enough that the
ions have the expected activity
IV. Colligative Properties of Solutions
A. Definition - a property that depends
on the number of solute particles but is
independent of their nature
1. Nonelectrolytes - 1 solute particle
2. Electrolytes - # of solute particles
dependent on # ions
• NaCl: 2
• MgCl2: 3
AgNO3: 2
K3PO4: 4
1. Vapor Pressure Lowering - the
tendency for molecules to escape from
a liquid to a gas is less in a solution
than a pure solvent
2. Freezing Point Depression - solution
has a lower fp than solvent
∆ tf = Kfm
∆ tf- freezing point change
Kf- molal freezing point constant
m - molality of the solution
Example: What is the fp of water in a
solution of 17.12 g C12H22O11 and 200 g of
water?
3. Boiling Point Elevation - solution has
a higher bp than solvent
∆ tb = Kbm
∆ tb - change bp
Kb - molal boiling pt constant
m - molality
Example: What is the bp of a solution
that is made by adding 20 g C12H22O11 in
500g H20?
C. Determination of Molar Mass of a
Solute
1. Determine Δtf(Δ tb)
2. Determine m
Δ t = Km
3. If ionic divide by number of particles
4. Calculate moles of solute
m X kg of solvent
5. Molar mass = mass of solute
moles of solute
Example: When 1.56 g of an unknown ,
nonelectrolyte solute is dissolved in 200 g
H2O, the ∆ tf = -0.453 Co. Determine the
molar mass.