Chemistry - A Molecular Science

Chapter 10 Solutions

Ionic substances and polar molecules are more soluble in polar solvents, while nonpolar molecules are more soluble in nonpolar solvents. Figure 10.1 demonstrates the solution process

for dissolving a polar solute in water.

ΔH

solute

0 for a solid or liquid composed of polar molecules because energy is required to

overcome the dipolar and dispersion forces.

ΔH

solvent

0 because dipolar and H-bonding

forces must be overcome to separate water

molecules and create cavities for the solute

molecules. Energy is released (

ΔH

mixing

< 0) when regions of opposite charge on solute and

solvent interact. If the solute is not polar,

ΔH

mixing

~ 0, the entire process is endothermic,

and the solute is insoluble.

The combined process of creating a cavity in

the solvent and placing the solute into

the cavity (steps 2 and 3 above) is known as

solvation

. Thus, we can write

HΔ

solvation

=

HΔ

solvent

+

HΔ

mixing

Eq. 10.2

The solute is said to be

solvated

, because it is surrounded by an

d interacting with solvent

molecules. In the special case where the solvent is water, the process is called

hydration

and the solute is said to be

hydrated

.

Example 10.4 a) Which has the greater solubility in water, HBr or Br

? 2

HBr is a polar molecule, and Br

is a nonpolar molecule, so HBr is more soluble in polar 2

substances such as water.

b) Which is the better solvent for I

, CS 2

(l) or H 2

O(l)? 2

Iodine is a nonpolar molecule and dissolves best in nonpolar solvents. Because water is polar, it is a poor solvent for nonpolar molecules such as I

. Carbon disulfide, on the other 2

hand, is nonpolar (CS

is linear, and both electron regions around the carbon are 2

identical), and is a much better solvent for nonpolar molecules such as I

. (In fact, I 2

is 2

almost 1,000 times more soluble in CS

than in H 2

O.) 2

c) In which solvent would KCl be more soluble, CCl

or water? 4

KCl is ionic, and ionic substances are most

soluble in polar solvents. Thus, KCl is more

soluble in water than in a nonpolar solvent such as CCl

. 4

Solvent (water)

molecules

Soluteparticlesa) b) c)

DH

solute D

Hsolvent D

(^1) Hmixing
2
3
4
(a) (b) (c) (d)
Figure 10.1 The Solution Process a) A polar solid solute (blue end is positive and red end
is negative) and a liquid solvent (H
O in the figure). 2
b) The solute particles are separated. c) Solvent molecules are separated to produce cavities
(labeled 1 through 4).
d) The solute particles enter the cavities and interact
with the solvent to produce a solution.
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