13.1
Types of Solutions
Saturated Solutions and Solubility
The opposite of the solution
process (dissolution) is
called crystallization.
There is an dynamic
equilibrium between the solution
and solute
+ solvent; example: a liquid at its boiling
point
is where the gas and
liquid phases are in
dynamic
equilibrium).
For solutions, this situation
occurs when no more
solute can
be dissolved
in
the solvent.
At this point, we say the
solution
is saturated.
Definition:
amount of solute required to form a
saturated solution
for a given amount of solvent is
called the solubility of
that solute in the solvent in
that experiment.
Supersaturated
solutions are also possible
in
which
more solute is dissolved than
normally would be
at equilibrium (think about
supercooling).
13.2
Molecular View of Solution Process
Factors Affecting Solubility:
Solute-Solvent
Interactions:
Polar liquids
tend
to dissolve in polar solvents;
non-polar liquids dissolve in non-polar solvents.
LIKE
DISSOLVES LIKE!!!
Miscible liquids:
mix in any proportions.
Ex.:
water and alcohol;
water and ethylene glycol
(antifreeze).
Immiscible
liquids: do not mix at all.
Ex.: water and olive oil;
water and gasoline.
Intermolecular
forces are important: water and
ethanol are miscible
because the broken hydrogen
bonds in both pure
liquids
are re-established
(albeit in a different way) in the mixture
(solution).
The
number of carbon atoms in a chain can affect
solubility:
the more C atoms in a compound, the less soluble
that compound is
in water.
The
number
of -OH groups within a molecule increases
its solubility
in water; sugars have many OH groups,
and are soluble in water.
(REMEMBER::::LIKE
DISSOLVES LIKE!)
13.3
CONCENTRATION UNITS
1) PERCENT BY MASS: % = pph; ppt; ppm
2)
Molarity M:
CALCULATE M OF 23.5 g NaOH DISSOLVED
IN ENOUGH WATER TO MAKE 450 mL OF SOLUTION.
3)
molality m:
CALCULATE m OF 23.5 g NaOH DISSOLVED
IN 450 mL OF WATER.
NOTICE THE DIFFERENCE
BETWEEN THESE TWO!!!
Examples:
1 -
CALCULATE M OF 25.5 g BENZENE (MW = 78.0 g/mol)
DISSOLVED IN ENOUGH CARBON TETRACHLORIDE
TO MAKE 550. mL OF SOLUTION.
M = 25.5g (1 mol/78.0g) / 0.550 L = 0.594 M
2
- CALCULATE m OF 25.5 g BENZENE (MW = 78.0 g/mol)
DISSOLVED IN 550. mL OF CARBON TETRACHLORIDE
(DENSITY OF CCl4
= 1.60 g/mL).
m =
25.5g (1 mol/78.0g)/550. mL(1.60 g/mL)(1kg/1000g)
m = 0.32602mol/ 0.880 kg = 0.370 m
13.4
Effect of Temperature on Solubility
For solids
in liquids.....we note that sugar dissolves
more easily in warm
water than in cold; the sugar is
more soluble in hot tea
than in iced tea.
Therefore,
as
temperature increases, the solubility of
solids
in
solvents generally increases.
For gases
in liquids....experience tells us that
carbonated beverages
go flat as they get warm.
Gases
are less soluble at higher temperatures.
Thermal
pollution: if lakes get too warm, CO2
and
O2
become less
soluble and are not available for
plants or animals.
13.5
Effect
of Pressure on the Solubility of Gases
Solubility of a gas in a liquid is a function of the
pressure of the gas.
The higher
the pressure of a gas above the solution,
the more molecules of gas
are close
to
the surface
of the solvent and the
greater
the chance of
a gas
molecule striking
the
surface
and entering the
solution.
Therefore,
the higher the
pressure of the gas, the
greater the
solubility of
the gas in the liquid.
The lower
the pressure of the gas above the solution,
fewer molecules of gas
are close
to the
solvent,
and the lower the
solubility.
Henry's law: kPgas = Cgas
where Pgas
is the partial pressure of gas above the
solution, in atm.
Cgas is the molar
concentration of dissolved gas and
k = Henry's law constant for a particular gas and a
particular solvent
at some T.
Calculate
the solubility of CO2 in water at
atmospheric
conditions if the solubility at 25oC and 1 atm is
0.034 mol/L. The partial pressure of CO2 in air is
0.00030
atm.
kPgas
= Cgas
Find k: k = C/P = 0.034 mol/L /1atm = 0.034 mol/L-atm
For
solubility in water: C = kP
C =
0.034mol/L-atm(0.00030atm)
C = 1.0 x 10-5 mol/L =0.00044 g CO2/L
Carbonated
beverages are bottled under Pgas > 1 atm.
As the bottle
is opened, Pgas decreases
and the
solubility of CO2 decreases.
Therefore, bubbles of
CO2 escape
from solution.
IMPORTANT
EXCEPTIONS: IF THE DISSOLVED GAS
REACTS
WITH WATER, HIGHER SOLUBILITIES RESULT.
EX:
AMMONIA + WATER GIVES THE AMMONIUM ION
AND
HYDROXIDE
ION. CARBON DIOXIDE REACTS
WITH
WATER
TO MAKE CARBONIC ACID.
13.6
Colligative
Properties
These
properties
depend on the quantity (numbers)
of individual solute particles
(molecules):
Vapor Pressure Lowering
Boiling-Point Elevation
Freezing-Point Depression
Osmotic Pressure
Vapor
Pressure Lowering:
1)
SOLUTE = non-volatile: solutes reduce the ability
of the surface solventmolecules
to escape the liquid.
Therefore, vapor
pressure of
solvent is lowered and
the amount
of vapor pressure lowering depends on
the amount
of solute. EX: sugar dissolved in water.
Raoult's Law: P1 = X1 P1o
P1
is the vapor pressure of the solution, P1o
is the
vapor pressure
of the pure solvent, and X1 is the
mole
fraction of the
solvent in the solution.
NOTE: the decrease
in vapor pressure, DP, is directly
proportional
to the concentration (mole fraction) of
the solute
present.
LOWERING!!!
2) BOTH
COMPONENTS ARE VOLATILE: THE VAPOR
PRESSURE OF THE SOLUTION IS THE SUM OF THE
INDIVIDUAL PARTIAL PRESSURES OF THE
COMPONENTS.
PA=XA
PAo
PA AND PB ARE
PARTIAL PRESSURES
PB=XB
PBo
PAo AND
PBo ARE
VAPOR PRESSURES
OF
THE PURE SUBSTANCES.
AND PTOTAL= PA + PB
IS VAPOR PRESSURE OF
THE SOLUTION
REMEMBER: DALTON'S LAW OF PARTIAL PRESSURE