Ummm…
Solutions
Solutions
Solutions
• Solutions are homogeneous mixtures of two
or more pure substances.
• In a solution, the solute is dispersed
uniformly throughout the solvent.
How Does a Solution Form?
• As a solution forms, the solvent pulls solute
particles apart and surrounds, or solvates,
them.
• The intermolecular
forces between
solute and solvent
particles must be
strong enough to
compete with those
between solute
particles and those
between solvent
particles.
• For aqueous solutions, the interactions of
solvation is called hydration.
How Does a Solution Form
• If an ionic salt is
soluble in water,
it is because the
ion-dipole
interactions are
strong enough
to overcome the
lattice energy of
the salt crystal.
Energy Changes in Solution
• Simply put, three
processes affect the
energetics of the
process:
 Separation of solute
particles
 Separation of solvent
particles
 New interactions
between solute and
solvent
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Energy Changes in Solution
The enthalpy
change of the
overall
process
depends on
H for each of
these steps.
Therefore:
ΔHsoln = ΔH1 + ΔH2 + ΔH3
If established
interactions release
more energy than
required to separate
solute/solvent
particles, the
solvation process is
exothermic
Some compounds form solutions
in a given solvent and others do not;
why?
Polar compounds ΔH is high
If established
interactions release
less energy than
required to separate
solute/solvent
particles, the
hydration (aq sol)
process is
endothermic
Factors Affecting Solubility
• Chemists use the axiom “like dissolves like”:
 Polar substances tend to dissolve in polar solvents.
 Nonpolar substances tend to dissolve in nonpolar
solvents.
Non-polar compounds ΔH is low
ΔH for polar-polar interactions is high
ΔH for nonpolar-polar interactions is low
ΔH for non-nonpolar interactions is low
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Factors Affecting Solubility
• The more similar the intermolecular
attractions, the more likely one
substance is to be soluble in another.
“Like Dissolves Like”
So:
Solvent
Solute
Outcome
Polar
Polar
Polar
Non-polar
Non-polar
Non-polar
Non-polar
Polar
Solution
formed
No Solution
formed
Solution
formed
No Solution
formed
Why Do Endothermic
Processes Occur?
• Yet we know
that in some
processes, like
the dissolution
of NH4NO3 in
water, heat is
absorbed, not
released.
Factors Affecting Solubility
• Example:
– Vitamin A is soluble in nonpolar compounds
(like fats).
– Vitamin C is soluble in water.
Why Do Endothermic
Processes Occur?
• Things do not tend
to occur
spontaneously (i.e.,
without outside
intervention)
unless the energy
of the system is
lowered.
Enthalpy Is Only Part of the Picture
The reason is that
increasing the
disorder or
randomness
(known as entropy)
of a system tends
to lower the energy
of the system.
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Enthalpy Is Only Part of the Picture
Guiding principle in the solution
process:
1. Processes in which the energy
content of a system decreases tend to
occur spontaneously, without extra
input of energy from outside the
system
So even though
enthalpy may
increase, the
overall energy of
the system can
still decrease if the
system becomes
more disordered.
2. Processes in which the disorder of
the system increases tend to occur
spontaneously
Types of Solutions
• Saturated
Solvent holds as
much solute as is
possible at that
temperature.
Dissolved solute is
in dynamic
equilibrium with
solid solute
particles.
Types of Solutions
• Supersaturated
Solvent holds more solute than is normally
possible at that temperature.
These solutions are unstable; crystallization
can usually be stimulated by adding a “seed
crystal” or scratching the side of the flask.
Types of Solutions
• Unsaturated
Less than the
maximum
amount of
solute for that
temperature is
dissolved in
the solvent.
Solubility is defined as the
amount of a solute required to form a
saturated solution in a given volume
of solvent at a given temperature.
(usually expressed in g/L)
Saturated
Vs.
Unsaturated and Supersaturated
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Beware!
Beware!
• Dissolution is a physical change—you can
get back the original solute by evaporating
the solvent.
• If you can’t, the substance didn’t dissolve, it
reacted.
Just because a substance disappears
when it comes in contact with a solvent, it
doesn’t mean the substance dissolved.
Factors Affecting
Solubility
1. Solute-Solvent
Interactions
2. Temperature
3. Pressure
Temperature
• Generally, the
solubility of
solid solutes
in liquid
solvents
increases
with
increasing
temperature.
Solute-Solvent Interactions
•
Substances with similar
intermolecular forces tend to be
soluble in one another
“Like dissolves like”
Exception: Network-Covalent solids
Dropping the temperature of a solution is
often used to precipitate a synthesized
compound in organic chemistry.
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Temperature
• The opposite is true
of gases:
Carbonated soft
drinks are more
“bubbly” if stored
in the refrigerator.
Warm lakes have
less O2 dissolved
in them than cool
lakes.
Gases in Solution
• In general, the
solubility of
gases in water
increases with
increasing mass.
• Larger molecules
have stronger
dispersion
forces.
Pressure
• Solubilities of a gas in any
solvent is increased as the
pressure over the solvent is
increased.
• The solubilities of liquids
and solids are not
appreciably affected by
pressure
William Henry (1803)
Henry's Law = the amount
of gas dissolved in a solution is
directly proportional to the
pressure above the solution.
Sg = K P g
(M, atm)
Where:
Sg = solubility of gas in solution (M)
Pg = partial pressure of the gas above
solution
K = Henry's Law constant (different
for every solute-solvent pair)
Henry’s law is accurate
for dilute solutions with no
dissociation or reaction
between gas and solvent.
Also, it is
subject to
partial
pressures
for gas
mixtures.
1. A soft drink is bottled so that a
bottle at 25oC contains carbon
dioxide gas at a pressure of 5.0 atm
over the liquid. Assuming the
partial pressure of carbon dioxide in
the atm of an open system is 4.0 x 104 atm, calculate the concentration of
carbon dioxide gas in the soda both
before and after the bottle is opened
(Henry’s Law constant is 4.48 x 10-5
mol/L atm at 25oC)
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Concentration of Solutions
Ways of
Expressing
Concentrations
of Solutions
Molarity (M)
Qualitative
1. Concentrated = near saturation
2. Dilute = far from saturation
Quantitative
- Defines the amount of solute
present mathematically.
Concentration Units
An IDEAL SOLUTION is one
M=
mol of solute
L of solution
• You will recall this concentration
measure from Chapter 4.
• Because volume is temperature
dependent, molarity can change with
temperature.
Molality (m)
m=
where the properties
depend only on the
concentration of solute.
Need conc. units to tell us
the number of solute
particles per solvent
particle.
The unit “molarity” does not
do this!
Mass Percentage
mol of solute
kg of solvent
Mass % of A =
mass of A in solution
 100
total mass of solution
Because both moles and mass do
not change with temperature,
molality (unlike molarity) is not
temperature dependent.
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Parts per Million and
Parts per Billion
Mole Fraction (X)
Parts per Million (ppm)
ppm =
mass of A in solution
 106
total mass of solution
Parts per Billion (ppb)
ppb =
mass of A in solution
 109
total mass of solution
XA =
moles of A
total moles in solution
• In some applications, one needs the
mole fraction of solvent, not solute—
make sure you find the quantity you
need!
1. A solution is prepared by mixing
1.00 g ethanol with 100.0 g water to
give a final volume of 101 mL.
Calculate the molarity, mass
percent, mole fraction and molality
of the solution.
2. A 2.5 g sample of ground water was
found to contain 5.4 micrograms of
zinc (II) ions. What is the
concentration of zinc in parts per
million? Parts per billion?
Changing Molarity to Molality
3. The electrolyte in automobile lead
storage batteries is a 3.75 molar
sulfuric acid solution that has a
density of 1.230 grams per milliliter.
Calculate the mass percent and
molality of the solution.
If we know the
density of the
solution, we can
calculate the
molality from the
molarity, and
vice versa.
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