Chapter 13: Properties of
Solutions
Recall: From Ch. 4, a solution is a homogeneous mixture
of two or more substances
We dealt specifically with aqueous solutions in Ch 4
Defined solvent and solute
From Ch 11: we dealt with intermolecular forces and
physical properties of pure substances (e.g., H2O(l))
Now we ask: how do intermolecular forces affect the
properties of solutions?
The Solution Process
What determines whether or not a given solute will
dissolve in a given solvent?
E.g., ethanol (C2H5OH) is infinitely soluble in water,
but hexane is not soluble in any proportion in water
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In general: solutions form when the attractive forces
between solvent and solute are comparable to those
between solvent particles alone and solute particles alone
What types of intermolecular forces operate between
solute and solvent particles?
The intermolecular forces are the same as discussed
in chapter 11......
Dipole-dipole forces operate between neutral,
polar molecules
How do we recognize hydrogen bonding?
London dispersion forces operate between
neutral, nonpolar molecules (and polar ones also)
Ion-dipole forces operate between permanent ions
and a polar solvent molecule
E.g., how does NaCl(s) dissolve in H2O? (review)
The ion-dipole interactions between Na+, Cl-, & H2O are
strong enough pull the ions from their lattice positions
This process is called solvation (hydration if H2O is the
solvent)
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The important difference now is this: to determine
whether a solution will form, we look at the intermolecular
forces within the solvent and solute separately and
compare these interactions to that between the solvent
and solute molecules.....
In solution formation we classify the interactions as
follows:
solute - solute interactions
attractive forces between solute particles only
solvent - solvent interactions
attractive forces between solvent particles only
solvent - solute interactions
attractive forces between solvent and solute
E.g., what are the interactions involved in dissolving
NaCl(s) in water?
E.g., what are the interactions involved in dissolving
CH3OH in water?
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Easiest to think about the energetics of solution formation
in terms of a series of steps:
Step 1: overcome solute-solute interaction
Always endothermic
Requires energy Hsolute
Step 2: overcome solvent-solvent interaction
Always endothermic
Requires energy Hsolvent
Step 3: let separated solvent and solute particles interact
This is the solvent-solute interaction
Associated energy called Hmix
If the solvent-solute interaction is very strong, Hmix
will be very exothermic
If the solvent-solute interaction is weak, Hmix will be
only slightly exothermic
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The net enthalpy change in the formation of a solution is
called Hsoln and is given by
Hsoln = Hsolute + Hsolvent + Hmix
Hsoln = sum of enthalpy changes associated with each of
the interactions
What is the sign of H for the first 2 steps in solution
formation?
A solution will not form if Hsoln is too endothermic –
spontaneous processes tend to be exothermic
Solvent - solute interactions must be strong enough to
make Hmix  Hsolute+ Hsolvent
E.g., NaCl dissolves in H2O; would NaCl dissolve in CCl4?
Why or why not?
Would H2O dissolve in CCl4? Why or why not?
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Is energy the only factor involved in solution formation?
e.g., NH4NO3 spontaneously dissolves in H2O even though
Hsoln is endothermic (26.4 kJ)
e.g., CCl4 and C6H14 are soluble in all proportions; what are
the attractive forces at work?
Two factors at work in solution formation:
- Energy
Processes in which the energy content of the
system decreases tend to occur spontaneously
Spontaneous processes tend to be exothermic
How to explain the NH4NO3 case?
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Disorder (entropy)
Processes which increase the amount of disorder in the
system tend to occur spontaneously
Solution formation is favored by the increase in disorder
which accompanies mixing
In general: A solution will form unless solute - solute or
solvent - solvent interactions are too strong relative to
solute-solvent interactions
E.g., when two nonpolar organic liquids, such as heptane
and hexane, are mixed, Hsoln is generally quite small.
Why is this?
Given that Hsoln  0, why does the solution form?
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Problems du Jour
Indicate the type of solvent-solute interaction that should
be most important in each of the following solutions:
CCl4 in C6H6
CaCl2 in water
CH3CH2CH2OH in water
HCl in CH3CN
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