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CHAPTER 15 (MOORE)
PHYSICAL PROPERTIES OF SOLUTIONS
This chapter deals aqueous solutions and their physical properties. We will look at some of the
various types of solutions, but will focus on gases in liquids and solids in liquids. We will begin by
reviewing solution concentration units and calculations, including some new units: mass percent,
ppm and ppb, mole fractions and molality.
We will also look at the thermodynamics of solution formation and at solubilities of gases and
solids in liquids – and how solute concentration is affected by temperature and pressure. Finally, we
will see how “colligative properties,” including vapor pressure, freezing point and boiling point, and
osmotic pressure are affected by solution concentrations.
Question:
“Why do some substances dissolve in a given solvent but others do not? …” (Moore, p. 722)
Some Types of Solutions (see Table 15.1, p. 722, Moore)
Solution: a solute dispersed in a solvent.
Visualizing Enthalpy of Solution – in order for a solute to dissolve in a solvent, the magnitudes of
ΔH1 + ΔH2 and of ΔH3 must be approximately the same.
Intermolecular Forces in Solution Formation
An ideal solution exists when all intermolecular forces are of comparable strength, ΔH(soln) = 0.
When solute–solvent intermolecular forces are somewhat stronger than other intermolecular forces,
ΔH(soln) < 0. When solute–solvent intermolecular forces are somewhat weaker than other
intermolecular forces, ΔH(soln) > 0.
When solute–solvent intermolecular forces are much weaker than other intermolecular forces, the
solute does not dissolve in the solvent. Energy released by solute–solvent interactions is insufficient
to separate solute particles or solvent particles.
Intermolecular Forces in Solution
For a solute to dissolve, the strength of solvent–solvent forces and solute–solute forces must be
comparable to solute–solvent forces.
Non-Ideal Solutions
When 50 mL of ethanol and 50 mL of water are mixed, the total is less than 100 mL of solution.
In this solution, forces between ethanol and water are _____ (more, less) than other intermolecular
forces.
Aqueous Solutions of Ionic Compounds
The forces causing an ionic solid to dissolve in water are ion–dipole forces, the attraction of water
dipoles for cations and anions. The attractions of water dipoles for ions “pulls” the ions out of the
crystalline lattice and into aqueous solution. The extent to which an ionic solid dissolves in water is
determined largely by the competition between: interionic attractions that hold ions in a crystal, and
ion–dipole attractions that pull ions into solution.
Ion–Dipole Forces in Dissolution
Here, the negative ends of dipoles attracted to cations, and the positive ends of dipoles are attracted
to anions.
See Examples, pp. 725-727
Some Solubility Terms
Liquids that mix in all proportions are called miscible. When there is a dynamic equilibrium
between an undissolved solute and a solution, the solution is saturated. The concentration of the
solute in a saturated solution is the solubility of the solute. A solution which contains less solute
than can be held at equilibrium is unsaturated.
Formation of a Saturated Solution
Initially, a solid begins to dissolve. As solid dissolves, some dissolved solute begins to crystallize.
Eventually, the rates of dissolving and of crystallization are equal; no more solute appears to
dissolve, and longer standing does not change the amount of dissolved solute.
Solubility as a Function of Temperature
Most ionic compounds have aqueous solubilities that increase significantly with increasing
temperature. A few have solubilities that change little with temperature. A very few have
solubilities that decrease with increasing temperature. If solubility increases with temperature, a hot,
saturated solution may be cooled (but carefully!) without precipitation of the excess solute. This
creates a supersaturated solution. Note that supersaturated solutions ordinarily are unstable …
Supersaturated Solutions
For supersaturated solutions, if a single “seed crystal” of solute is added, solute immediately begins
to crystallize until all of the excess solute has precipitated.
Some Solubility Curves
Solubility curves are plots of saturated solute concentration (y-axis) versus temperature (x-axis).
Solubilities of Gases
Most gases become less soluble in liquids as the temperature increases. Why? At a constant
temperature, the solubility (S) of a gas is directly proportional to the pressure of the gas (Pgas) in
equilibrium with the solution.
S = k Pgas
where the value of k depends on the particular gas and the solvent.
This effect (equation for) of pressure on the solubility of a gas is known as Henry’s law.
Effect of Temperature on Solubility of Gases
In general, the solubility of gases in liquids decreases with increasing temperature.
Pressure and Solubility of Gases (Explanation)
Higher partial pressure means more molecules of gas per unit volume, thus more frequent collisions
of gas molecules with the surface, giving a higher concentration of dissolved gas.
See Example 12.9 (Hill, pp. 500-501)
Colligative Properties of Solutions
Colligative properties of a solution depend only on the concentration of solute particles, and not on
the nature of the solute. Non-colligative properties include: color, odor, density, viscosity, toxicity,
and reactivity.
Four colligative properties of solutions:
1. Vapor pressure (of the solvent)
2. Freezing point depression
3. Boiling point elevation
4. Osmotic pressure
The Vapor Pressure of a Solution (Raoult’s law)
The vapor pressure of solvent above a solution is less than the vapor pressure above the pure
solvent.
Raoult’s law: the vapor pressure of the solvent above a solution, P(solv) is the product of the vapor
pressure of the pure solvent, P°(solv) and the mole fraction of the solvent in the solution x(solv):
P(solv) = x(solv) ·P°(solv)
The vapor in equilibrium with an ideal liquid solution with two volatile components has a higher
mole fraction of the more volatile component than is found in the liquid.
See Examples 12.10 and 12.12 (Hill, pp. 502-504)
Fractional Distillation
In fractional distillation, the vapor in the distillate (vaporized liquid phase) is richer in the more
volatile component than the original liquid in the distillation flask …so the liquid that condenses in
the collection flask will also be richer in the more volatile component.
See Example 12.13 (Hill, pp. 505-506)
Vapor Pressure Lowering by a Nonvolatile Solute (Raoult’s Law)
The vapor pressure from a solution containing a nonvolatile solute is lower than the vapor pressure
of the pure solvent. Thus, t the boiling point of the solution increases by an amount ΔTb, directly
proportional to the mol fraction of the solute.
Freezing Point Depression and Boiling Point Elevation
ΔTf = –Kf × m
ΔTb = Kb × m
Kf and Kb are molal freezing point and molal boiling point constants, respectively, and are
characteristics of the solvent used (see Table 12.2 (Hill), p. 507).
See Examples 12.14 and 12.15 (Hill, pp. 507-509)
Osmotic Pressure
A semipermeable membrane has microscopic pores, through which small solvent molecules can
pass, but larger solute molecules cannot. During osmosis, we observe a net flow of solvent
molecules that pass through the semipermeable membrane, from a region of lower concentration to
a region of higher concentration. The pressure required to stop osmosis is called the osmotic
pressure (Π) of the solution.
Π = (nRT/V) = (n/V)RT = M RT
Π = M RT (in atmospheres or Torr)
Osmosis and Osmotic Pressure
In osmosis, there is a net flow of water from the “outside” (pure H2O) to the solution (inside) the
membrane. The volume of the solution continues to increase until the height of solution exactly
exerts the osmotic pressure (π) of the solution (in atmospheres or Torr).
See Example 12.16 (Hill, pp. 511)
Practical Applications of Osmosis
Patients are usually given intravenous fluids that are isotonic— these solutions have the same
osmotic pressure as blood. There are three scenarios:
(1) if the external solution is hypertonic, its osmotic pressure > Π(internal), and there is a net flow
of water out of the cell.
(2) if the external solution is isotonic, its osmotic pressure = Π(internal), and red blood cells in the
isotonic solution remain the same size.
(3) if the external solution is hypotonic, its osmotic pressure < Π(internal), and there is a net flow of
water into the cell.
∴ net flow of water is in the direction of greater osmotic pressure
Practical Applications of Osmosis
Reverse osmosis (RO) reverses the normal net flow of solvent molecules through a semipermeable
membrane. Pressure in excess of the osmotic pressure is applied to the solution and water flows
from the more concentrated solution, through the membrane to the less concentrated solution
(“pure”). RO is often used for water purification. Ex. the labs in Henson Hall dispense RO water
from the taps with the white handles.
Solutions of Electrolytes
When an electrolyte dissociates, the number of solute particles is greater than the number of
formula units dissolved. Thus, one mole of NaCl dissolved in water produces more than one mole
(actually about 2 moles) of solute particles. For electrolytes, we expect i to be equal to the number
of ions into which a substance dissociates into in solution. Note: for nonelectrolyte solutes, i = 1.
The van’t Hoff factor (i) is used to modify the colligative-property equations for electrolytes:
where, ΔTf = i × (–Kf) × m ΔTb = i × Kb × m
Π = i × M RT
See *Example 12.17 (Hill, pp. 514)
Colloids
Dispersed particles in solutions may be molecules, atoms, or ions and are roughly 0.1 nm in
diameter. Small solute particles like these don’t “settle out” of solution. However, in suspensions,
such as sand in water, where the dispersed particles are relatively large, the particles will settle out
from the suspension. In colloids, dispersed particles have diameters that are about 1–1000 nm and,
even though they are larger than molecules/atoms/ions, colloidal particles are small enough to
remain dispersed indefinitely.
The Tyndall Effect
Light scattered by larger colloidal particles in a fluid (liquid or a gas) makes the beam visible.