Physical Properties of
Solution
Types of Solutions
We can distinguish six types of solutions:
Solute
Gas
Gas
Gas
Liquid
Solid
Solid
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Solvent
Gas
Liquid
Solid
Liquid
Liquid
Solid
Solution
Gas
Liquid
Solid
Liquid
Liquid
Solid
Example
Air
Soda water
H2 in Pd
Vinegar
NaCl soln
Alloys
A saturated solution: A solution that contains the
maximum amount of a solute that will dissolve in
a given solvent, at a specific temperature.
An unsaturated solution: A solution that contains
less solute than the solvent has the capacity to
dissolve.
A supersaturated solution: A solution that
contains more solute than is present in a
saturated solution.
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Molecular view of solution process
When a solute dissolves in a solvent,
particles of the solute disperse
throughout the solvent. The ease with
which a solute particle replaces a solvent
molecule depends on the strength of:
1. solvent - solvent interaction.
2. solute - solute interaction.
3. solvent - solute interaction
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Intermolecular Attractions and
Solubility
(((LIKE DISSOLVES LIKE)))
Benzene does not dissolve in water while
methanol is freely soluble in water. Why
is this behavior? To answer this and
similar questions, we should consult
our knowledge about intermolecular
forces. First, let us have a close look at
the solubility process:
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§
Solvent molecules should be
expanded so that a space for solutes is
created. In order to expand solvent
molecules, we need to input energy in
order to overcome the intermolecular
forces
§
Solute molecules should also be
expanded and we need to input energy
to overcome intermolecular forces
between solute molecules
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§ The solubility of expanded solute
molecules in solvent will release
energy. The released energy should, at
least, be of the same magnitude as the
input energy for expanding both solute
and solvent molecules so that the
solution process is an ideal one.
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Now, consider the case of benzene and why it
does not dissolve in water. A high input
energy is needed to expand water molecules
since there are relatively strong hydrogen
bonds. These hydrogen bonds will be broken
so that dispersion forces are formed between
water and benzene molecules. This is an
unfavorable process since we need to input
much higher energy for solute expansion but
will gain very little energy as dispersion
forces are formed. Therefore, benzene is not
expected to dissolve in water and the two
solutions are said to be immiscible.
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On the other hand, methanol dissolves
freely in water since water and
methanol have same type of
intermolecular attractions (hydrogen
bonding and dipole-dipole) and when
hydrogen bonds in the solvent are
broken, new hydrogen bonds with the
solute are formed. This is a feasible
process and such solutions are very
stable.
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When the hydrocarbon chain becomes longer
and longer, the alcohol becomes less soluble
(miscible) in water since the alcohol size
becomes larger which requires breaking
several hydrogen bonds to accommodate
one solute molecule; an unfavorable
process. Methanol, ethanol and propanol are
all completely miscible with water while
octanol is not. On the other hand miscibility
decreases from butanol to heptanol
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It could be fairly concluded that polar solutes
will dissolve in polar solvents and vice versa.
In another statement, one can say that the
intermolecular forces in both solute and
solvent should be alike for the solute to
dissolve in the solvent. A non polar solute
like I2 has only dispersion forces as the
intermolecular forces between I2 molecules
and will thus dissolve in a non polar solvent
(has dispersion forces) like CCl4 but will not
dissolve appreciably in water.
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Solubility of ionic compounds
Ionic solids are highly polar and will only
dissolve in very polar solvents like
water. Even polar solvents like ethanol
do not have enough polarity to dissolve
most ionic solutes. Ions in water
become hydrated where water
molecules form a layer of water around
each ion; thus decreasing the available
charge, which adjacent ions can feel.
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Heats of Solutions (DHsoln)
When solutes dissolve in solvents, the
solution process either releases or absorbs
energy. The amount of heat that is absorbed
or released when a solute is dissolved in a
solvent is called the heat of solution (DHsoln).
DHsoln is negative when the solution process
releases heat (exothermic, like solubility of
LiCl, LiI, AlCl3, and Al2(SO4)3 in water) and is
a positive value when the solution absorbs
heat (as for the solution of KCl, KBr, NH4Cl,
and NH4NO3 in water).
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DHsoln = 0 in the case where no heat is
absorbed or released like the case of
solubility of benzene in carbon
tetrachloride). The magnitude of DHsoln
provides information about relative
intermolecular forces of solute, solvent,
and solution. When DHsoln = 0, the
solution is referred to as an ideal
solution.
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The solubility process of liquids or nonionized
solids in liquid solvents involves the
following steps:
§
Expanding solute molecules requires an
input of energy equals DHsolute to overcome
potential energy.
§
Expanding solvent molecules requires
an input of energy equals DHsolvent to
overcome potential energy.
§
Combining expanded solute and solvent
molecules will release energy equals
DHcombination, due to decrease in potential
energy (attraction occurs)
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Three points could be made:
1. The solution process will be exothermic
when the intermolecular forces between
solute-solvent molecules are stronger than
solute-solute or solvent-solvent molecules.
2. An endothermic solution process is
observed when the intermolecular forces
between solute-solute and solvent-solvent
molecules are stronger than solute-solvent
molecules.
3. DHsoln = 0 for ideal solutions
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Solubility and Temperature
For most solids and liquids dissolved in
water, the solubility increases as the
temperature is increased. This means
that the equilibrium shifts towards
forming more concentrated solutions at
higher temperatures. At any
temperature, a solute dissolves to the
extent where the equilibrium at that
definite temperature is reached.
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In the case of the solubility of gas solutes
in liquids, usually the solubility
decreases as the temperature is
increased. This is because the
solubility of gases in liquids is almost
always exothermic. According to Le
Chatelier principle solubility will thus
be inversely related to temperature.
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Concentration Units
Molarity is defined as the number of moles of solute
dissolved in one liter of solution
Molality
the number of moles of solute dissolved in one kg of
solvent
Mole Fraction
The number of moles of a particular component to the
overall number of moles of everything in solution is
called the mole fraction of that component.
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For example, a solution having 1.0 mole
of acetone, 2.0 moles of ethanol, 3.0
moles of methanol, and 15.0 moles of
water will have:
Xacetone = 1.0/(1.0 + 2.0 + 3.0 + 14.0) = 0.05
Xethanol = 2.0/(1.0 + 2.0 + 3.0 + 14.0) = 0.10
Xmethanol = 3.0/(1.0 + 2.0 + 3.0 + 14.0) = 0.15
Xwater = 14.0/(1.0 + 2.0 + 3.0 + 15.0) = 0.70
XT = 0.05 + 0.10 + 0.15 + 0.70 = 1.0
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Mole Percent
The mole fraction multiplied by 100 is
called the mole percent. In the above
example:
Mol % acetone = 0.05 x 100% = 5%
Mol % ethanol = 0.10 x 100% = 10%
Mol % methanol = 0.15 x 100% = 15%
Mol % water = 0.70 x 100% = 70%
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Weight Fraction
The weight of a particular component to
the total number of grams of solution is
called the weight fraction of that
component.
Wcomponent = g component/g solution
For example, a solution containing 12.0 g
methanol and 38.0 g water has:
wmethanol = 12.0/(12.0 + 38.0) = 0.24
wwater = 38.0/(12.0 + 38.0) = 0.76
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Weight Percent
The weight fraction multiplied by 100 is called
the weight percent. In the example above:
Wt% Methanol = 0.24 x 100% = 24%
Wt% Water = 0.76 x 100% = 76%
Note that summation of mole or weight percent
of all components will add up to 100%. Also,
summation of mole or weight fractions of all
components will add up to 1.0.
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Molality
Molality is defined as the number of moles of
solute dissolved in one kilogram of solvent.
Therefore, the volume of one molal solution
can be more or less than 1.0 L. Molality is not
concerned with volume. You should be able
to contrast molality and molarity and be able
to use molality in calculations as well as
conversion to other concentration units as
mentioned earlier.
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Example
A certain aqueous solution contains 7% ethanol
(C2H5OH) by mass. Calculate the mole fraction, mole
percent, and the molality
Solution
7% by mass would mean that we have 7g ethanol in a
100g sample (and, therefore, 93g H2O)
Molar mass of ethanol is (2 x 12.0) + (6 x 1.01) + (1 x
16.0) = 46.1g/mole
Molar mass of H2O is (2 x 1.01) + (1 x 16.0) = 18.0g/mole
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Number of moles ethanol = 7/46.1 = 0.152 mol
Number of moles of water = 93/18.0 = 5.17 mol
Total moles in sample is (0.152 + 5.17) = 5.32
Xethanol = 0.152/5.32 = 0.0286
Xwater = 1 – 0.0286 = 0.9714
Mol% ethanol = 0.0286x100% = 2.9%
Mol% water = 0.9714x100% = 97.1%
Molality:
In the same 100g sample above, the ethanol
0.152 mol and the H2O weighs 93g (or 0.093
kg), therefore:
molality = 0.152mol/0.093 kg = 1.63 m
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