Chapter 12 Worksheet 1 (ws12.1)
Solubility and the Thermodynamics of Dissolving
Solubility
The solubility of a solute in a solvent is the concentration of the solute in a saturated solution.
Solubility is most commonly expressed in units of moles of solute per liter of solution (molarity, M) or
grams of solute per liter of solution. (In this course, the solvent in most solutions will be water.) Here
are some things you probably already know:
Some substances are very soluble in water, others are not.
You can dissolve more sugar in hot water than in cold water.
A soda goes flat if you open it and let it sit around.
In this chapter, you will learn the explanations for these common everyday observations.
The “driving forces” for dissolving (or any chemical process): enthalpy and entropy
Entropy (symbol is “S”) is a measure of the dispersal of energy in a system. It is often described as the
degree of disorder in a system. (More on this in chapter 18.)
The second law of thermodynamics says that a process will occur spontaneously if the process increases
the entropy of the universe. In other words, all spontaneous processes increase the disorder of the
universe. (You never see a shattered glass spontaneously reform! The universe is expanding!)
For a spontaneous process, Suniv = Ssys + Ssurr > 0
For a dissolving process, the system consists of the solvent and solute. When a solute dissolves in a
solvent, Ssys is usually positive. That is, the system becomes more disordered. Why?
A positive Ssys favors dissolving.
A dissolving process can also affect the entropy of the surroundings. If a dissolving process is
exothermic, the entropy of the surroundings increases (Ssurr > 0). If a dissolving process is
endothermic, the entropy of the surroundings decreases (Ssurr < 0). Why?
A negative Hsys favors dissolving.
1. Ammonium nitrate dissolving in water is an endothermic process. Why is ammonium nitrate
pretty soluble in water?
The increase in the entropy of the system must offset the decrease in the entropy of the surroundings!
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The role of intermolecular forces in solubility (Hsys)
The figure below is a molecular view of the solution process portrayed as taking place in three steps:
Step 1: Separate the solvent molecules by disrupting solvent-solvent interactions.
Step 2: Separate the solute molecules by disrupting solute-solute interactions.
Step 3: Mix the solvent and solute molecules resulting in solute-solvent interactions.
1. Dissolving processes can be exothermic or endothermic.
a. Are steps 1, 2, and 3 endothermic or exothermic? (Label the arrows.)
Separating particles that attract each other is endothermic. Bringing such particles together is
exothermic. Therefore, steps 1 and 2 are endothermic and step 3 is exothermic.
b. What must be true for the entire process (steps 1-3) to be exothermic? Explain in terms of the
strengths of the various intermolecular forces (solute-solute, solvent-solvent, and solute-solvent
interactions). Try to write a mathematical equation that illustrates this idea. (An equation is worth
a whole lot of words!)
If the process is exothermic: H = H1 + H2 + H3 < 0
So: │H3│ > H1 + H2
The solute-solvent interactions must be stronger than the sum of the strengths of the solute-solute and
solvent-solvent interactions.
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“Like Dissolves Like”
Last semester you determined the relative solubility of crotonic acid in three solvents: water, toluene and
1-pentanol. You found that crotonic acid was most soluble in 1-pentanol and that its solubility in water
and toluene were very similar. Let’s review the reasons for this.
Crotonic Acid
Water
Toluene
1-pentanol
1. The phrase “like-dissolves-like” means that when the types and strengths of the intermolecular
forces responsible for solute-solute and solvent-solvent interactions are similar, then the solute
will (usually) be very soluble in the solvent. Complete the table below with “yes” or “no” to
indicate the types of solute-solute interactions that occur in crotonic acid and the types of solventsolvent interactions that occur in each of the 3 solvents.
Substance
Crotonic Acid
Water
Toluene
1-Pentanol
Dispersion Forces
Yes
Yes
Yes
Yes
Dipole-Dipole
Yes
Yes
No
Yes
H-bonds
Yes
Yes
No
Yes
a. Crotonic acid, water, and 1-pentanol participate in all three types of interactions. Why is crotonic
acid more soluble in pentanol than in water?
Crotonic acid interacts through stronger dispersion forces with 1-pentanol than with water. This is
because water is smaller than the other two molecules.
b. Crotonic acid and water participate in all three types of interactions. Toluene is non-polar so it
can only participate in dispersion forces. Why, then, does crotonic acid dissolve equally well in
toluene and water?
The answer is the same as for part a. Although water and crotonic acid can H-bond, they interact
through very weak dispersion forces. Toluene and crotonic acid interact through relatively strong
dispersion forces.
Bottom line: Crotonic acid looks a lot like 1-pentanol so their intermolecular forces are similar!
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2. Water is a polar solvent so it is good at dissolving polar solutes. Toluene is a non-polar solvent,
so it is good at dissolving non-polar solutes.
a. Wait a minute! Crotonic acid is polar. Why does it dissolve equally well in both water and
toluene? (I know I asked this already but now I want you to think about it in terms of polarity.)
Crotonic acid has properties of both polar AND non-polar substances. Its carboxylic acid group
(COOH) is polar but it has a non-polar hydrocarbon tail.
Note: Polar molecules are “hydrophilic” (water loving). Non-polar molecules are “hydrophobic”
(afraid of water). Molecules like crotonic acid are “amphiphilic” (The prefix is related to that in
ambidextrous. Recall the term “amphiprotic” which is a substance that is both and acid and a base.)
b. Explain why water is a good solvent for ionic compounds? (Remember what happens when an
ionic compound dissolves in water.)
Ions derived from ionic compounds interact strongly with water through “ion-dipole interactions”.
3. When the solute-solute interactions are sufficiently similar (in type and strength) to the solventsolvent interactions, then the solute and solvent can mix in any proportion. They are said to be
“miscible”. For example, methanol and ethanol are miscible with water (and each other).
a. Why are all substances that are gases at room temperature miscible with each other?
Gases are gases because the intermolecular forces are very weak. Thus, the intermolecular forces
between all gases are very similar.
b. In order to be miscible, the two substances must be in the same phase at the same temperature.
For example, solids are never miscible with liquids. Why?
This is a generalization of part a. The strengths of the intermolecular forces in two substances that
are in the same phase at the same temperature must be very similar. Remember that such substances
will only be miscible if not only the strength but also the types of intermolecular forces are the same.
c. When methanol dissolves in water, what is the approximate value of H for this process?
Approximately zero
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