The Foundations of Chemistry

Let us look at the first of these factors. If a solution gets hotter as a substance dissolves,

energy is being released in the form of heat. The energy change that accompanies a disso-

lution process is called the heat of solution, 
Hsolution.It depends mainly on how strongly

solute and solvent particles interact. A negative value of Hsolutiondesignates the release

of heat. More negative (less positive) values of Hsolutionfavor the dissolution process.

In a pure liquid all the intermolecular forces are between like molecules; when the

liquid and a solute are mixed, each molecule then interacts with molecules (or ions) unlike

it as well as with like molecules. The relative strengths of these interactions help to deter-

mine the extent of solubility of a solute in a solvent. The main interactions that affect the

dissolution of a solute in a solvent follow.

a.Weak solute–solute attractions favor solubility.

b.Weak solvent–solvent attractions favor solubility.

c.Strong solvent–solute attractions favor solubility.

Figure 14-1 illustrates the interplay of these factors. The intermolecular or interionic

attractions among solute particles in the pure solute must be overcome (step a) to dissolve

the solute. This part of the process requires an inputof energy (endothermic). Separating

the solvent molecules from one another (step b) to “make room” for the solute particles

also requires the inputof energy (endothermic). Energy is released,however, as the solute

particles and solvent molecules interact in the solution (step c, exothermic). The overall

dissolution process is exothermic (and favored) if the amount of heat absorbed in hypo-

thetical steps a and b is less than the amount of energy released in step c. The process is

endothermic (and disfavored) if the amount of energy absorbed in steps a and b is greater

than the amount of heat released in step c.

Many solids do dissolve in liquids by endothermicprocesses, however. The reason such

processes can occur is that the endothermicity can be outweighed by a large increase in

disorder of the solute during the dissolution process. The solute particles are highly

ordered in a solid crystal, but are free to move about randomly in liquid solutions. Like-

wise, the degree of disorder in the solvent increases as the solution is formed, because

solvent molecules are then in a more random environment. They are surrounded by a

mixture of solvent and solute particles.

We can consider the energy changes
separately, even though the actual
process cannot be carried out in these
separate steps.

544 CHAPTER 14: Solutions

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 14.3, The Solution Process.

Figure 14-1 A diagram
representing the changes in energy
content associated with the
hypothetical three-step sequence in a
dissolution process—in this case, for
a solid solute dissolving in a liquid
solvent. (Similar considerations
would apply to other combinations.)
An exothermicprocess is depicted
here. The amount of energy
absorbed in steps a and b is lessthan
the amount of energy released in
step c, so the energy of the solution
is favorable. In an endothermic
process (not shown), the heat
content of the solution would be
higherthan that of the original
solvent plus solute. Thus, the
amount of energy absorbed in steps
a and b would be greaterthan the
amount of heat released in step c, so
energy (heat of solution) would be
unfavorable.

Solvent

Solute

Potential energy

Step

a

Step

b

Solvent Expanded

solute

Expanded

solvent

Expanded

solute

+ Step

c

Net energy change 
Hsolution (negative value;exothermic)

Solution