Chapter 15: Solutions - Weironline.net

Figure 15-4
The solvation of a sugar cube
(sucrose) can be seen in the
above photo. Polar sucrose
(C 12 H 22 O 11 ) molecules contain
eight O–H bonds. Forces
between polar water molecules
and polar sucrose molecules
break these O–H bonds and the
sucrose dissolves.
Figure 15-5
Nonpolar oil molecules do not
mix with polar water molecules.
Because oil is less dense than
water, it floats on the water’s
surface. For this reason, oil spills
at sea often wash ashore.
456 Chapter 15 Solutions
H
O — H H — O
H — O
CH2 O — H
H
H C O H CH2 O
O — C H
O — C
H
C
C
O
C
H
C
H
O
C
H
H O
O H
H — O H
O — H
H
H H — O
H
H
—
—
—
—
H — O
—
—
H
—
Aqueous solutions of molecular compounds Water also is a good solvent
for many molecular compounds. You know that table sugar dissolves in
water. Table sugar is the molecular compound sucrose. See Figure 15-4.
Note that its structure has a number of O–H bonds. Sucrose molecules are
polar. When water is added to sucrose, each O–H bond becomes a site for
hydrogen bonding with water. As soon as the sugar crystals contact the water,
water molecules collide with the outer surface of the crystal. The attractive
forces among sucrose molecules are overcome by the attractive forces
between polar water molecules and polar sucrose molecules. Sugar molecules
leave the crystal and become solvated by water molecules.
The oil in Figure 15-5 is a substance made up of primarily carbon and
hydrogen. It does not form a solution with water. Why are oil and water immiscible?
There is little attraction between the polar water molecules and the nonpolar
oil molecules. However, oil spills can be cleaned up with a nonpolar
solvent. Nonpolar solutes are more readily dissolved in nonpolar solvents.
Factors that affect rate of solvation As you have just learned, solvation
occurs only when and where the solute and solvent particles come in contact
with each other. There are three common ways to increase the collisions
between solute and solvent particles, and thus increase the rate at which the
solute dissolves: agitating the mixture, increasing the surface area of the
solute, and increasing the temperature of the solvent.
Agitating the mixture by stirring and shaking moves dissolved solute particles
away from the contact surfaces more quickly and thereby allows new
collisions between solute and solvent particles to occur. Without stirring or
shaking, solvated particles move away from the contact areas slowly. Breaking
the solute into small pieces increases its surface area. A greater surface area
allows more collisions to occur. This is why a teaspoon of granulated sugar
dissolves more quickly than an equal amount of sugar in a cube. Raising the
temperature of the solvent increases the kinetic energy of its particles, resulting
in more frequent collisions and collisions with greater energy than those
that occur at lower temperatures. Can you think of an example of how an
increase in temperature affects the rate of dissolving?
—
—
—
H
—
—
H
—
C
CH 2 — O
H
H — O
H
H — O
—
H