Solutions I
Characteristics
CHM 1010
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Barbara A. Gage
Solutions
• Homogeneous mixture : has no visible
boundaries because the components are
mixed as individual atoms, ions, and
molecules.
• Solution : A homogeneous mixture is also
called a solution. Solutions in water are
called aqueous solutions. Although we
normally think of solutions as liquids, they
can exist in all three physical states.
CHM 1010
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Barbara A. Gage
Components…
• Solute – substance that is
dispersed or dissolved
• Solvent – substance that
facilitates the dispersing or
dissolving
• Entropy will generally make
solution formation favorable!
CHM 1010
PGCC
Barbara A. Gage
Solubility
• Maximum amount of solute that
dissolves in a given amount of solvent at
a set temperature
NH4Cl 37.2 g/100g H20 @20oC
CHM 1010
PGCC
Barbara A. Gage
Factors Affecting Solubility
• Nature of solute and solvent…IMFs
“Like dissolves like”
If the forces are similar the components
will mix…
CHM 1010
PGCC
Barbara A. Gage
Figure 13.1
The major types of intermolecular forces in solutions.
CHM 1010
PGCC
Barbara A. Gage
Figure 13.3
Like dissolves like: solubility of methanol in water.
water
methanol
A solution of
methanol in water
CHM 1010
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Barbara A. Gage
Figure 4.2
The dissolution of an ionic compound…
Solvation
CHM 1010
PGCC
Barbara A. Gage
Dissolution of a polar
compound…Solvation
The slightly charged
ends of polar solute
molecules are
attracted to the
polar ends of polar
solvents. The
molecules are
surrounded by solute
particles. The solute
particles stay intact.
CHM 1010
PGCC
Barbara A. Gage
To determine solubility, decide what the
IMFs of the solute and solvent are first.
Solute IMFs
Solvent IMFs
Ion-ion
Dipole-dipole
Dissolve or
Mix?
yes
Ion-ion
Dispersion
no
Dipole-dipole
Dipole-dipole
yes
Dispersion
Dipole-dipole
no
Dispersion
Dispersion
yes
CHM 1010
PGCC
Barbara A. Gage
SAMPLE PROBLEM 13.1 Predicting Relative Solubilities of Substances
PROBLEM:
Predict which solvent will dissolve more of the given solute:
(a) Sodium chloride in methanol (CH3OH) or in propanol (CH3CH2CH2OH)
(b) Ethylene glycol (HOCH2CH2OH) in hexane (CH3CH2CH2CH2CH2CH3)
or in water.
(c) Diethyl ether (CH3CH2OCH2CH3) in water or in ethanol (CH3CH2OH)
PLAN:
Consider the intermolecular forces which can exist between
solute molecules and consider whether the solvent can provide
such interactions and thereby substitute.
SOLUTION:
(a) Methanol - NaCl is ionic and will form ion-dipoles with the -OH groups
of both methanol and propanol. However, propanol is subject to the
dispersion forces to a greater extent.
(b) Water - Hexane has no dipoles to interact with the -OH groups in
ethylene glycol. Water can H bond to the ethylene glycol.
(c) Ethanol - Diethyl ether can interact through a dipole and dispersion
forces. Ethanol can provide both while water would like to H bond.
CHM 1010
PGCC
Barbara A. Gage
Factors Affecting Solubility
• Nature of solute and solvent…IMFs
• Temperature
As temperature increases, particle
motions and interactions increase.
CHM 1010
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Barbara A. Gage
Solubility Curves
What is the
general trend for
solids?
For gases?
Why are they
different?
CHM 1010
PGCC
Barbara A. Gage
Heats of solution and solution cycles
1. Solute particles separate from each other - endothermic
solute (aggregated) + heat
solute (separated)
DHsolute > 0
2. Solvent particles separate from each other - endothermic
solvent (aggregated) + heat
solvent (separated)
DHsolvent > 0
3. Solute and solvent particles mix - exothermic
solute (separated) + solvent (separated)
solution + heat
DHsoln = DHsolute + DHsolvent + DHmix
CHM 1010
PGCC
Barbara A. Gage
DHmix < 0
Figure 13.4
Solution cycles and the enthalpy components of the heat of solution.
Exothermic solution process
Endothermic solution process
CHM 1010
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Barbara A. Gage
Endo- and Exothermic Dissolvers
• Endothermic dissolvers can be used to
form chemical cold packs because the
solvent (water) becomes cold as it
provides energy for the dissolving
process.
• Exothermic dissolvers provide the basis
for chemical hot packs since the energy
released heats the solvent.
CHM 1010
PGCC
Barbara A. Gage
Factors Affecting Solubility
• Nature of solute and solvent…IMFs
• Temperature
• Molar Mass
The larger the solute molecule the more
solvent molecules are required to keep
the solute in solution.
Solvation shell of
water molecules for
Na+ (left) and a
larger peptide (right)
CHM 1010
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Barbara
A. Gage
http://www.theochem.rub.de/~axel.kohlmeyer/research.html
Factors Affecting Solubility
• Temperature
• Nature of solute and solvent…IMFs
• Molar Mass
The larger the molecule the more
solvent molecules are required to keep
the solute in solution.
• Pressure (for gases only)
CHM 1010
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Barbara A. Gage
The higher the pressure over the solution, the
greater the solubility of the gas (Henry’s Law).
The “bends”…
CHM 1010
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Barbara A. Gage
Electrolytes
• If a solute conducts electricity when
dissolved it is called an electrolyte.
• Solutes that dissolve but do not form
ions form non-conducting solutions and
are called non-electrolytes.
• A solute that dissociates partially and
conducts electricity on a limited basis is
called a weak electrolyte.
CHM 1010
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Barbara A. Gage
Figure 4.3
The electrical conductivity of ionic solutions.
CHM 1010
PGCC
Barbara A. Gage
Colloids and Suspensions
• When a solute is suspended in a solvent
the result is a heterogeneous mixture that
may appear homogeneous. Such mixtures
are called colloids or colloidal suspensions.
These generally do not settle in the short
term.
• Suspensions are heterogeneous mixtures
that do settle in the short term. They
require shaking before dispensing.
CHM 1010
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Barbara A. Gage
Colloids and Suspensions
• Both colloids and suspensions scatter
light. A true solution will not scatter
light because the dispersed solute
particles are too small to affect the
path of the light.
CHM 1010
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Colloid
Barbara A. Gage
True solution
Types of Colloidal Suspensions
Dispersed Material
Dispersing material
Gas
Liquid
Solid
Liquid aerosol
Solid aerosol
Cloud, fog
Smog, smoke
Foam
Emulsion
Sol
Shaving cream
Milk, plasma,
mayonnaise
blood
Solid foam
Gel
Solid sol
Styrofoam
Gelatin, opal
Colored glass
Gas
Liquid
Solid
CHM 1010
PGCC
Barbara A. Gage