Solution Process

Synopsis

In the South people know how to make really sweet tea! Everyone knows that the higher the temperature the greater the solubility of a solid in a liquid. So if people put sugar in the tea just after it has brewed and while it is still very hot; they can prepare a saturated solution of sugar in the tea. If you wait until the tea has cooled and ice is added, you will dissolve considerably less sugar per mL of tea. Of course you may see some sugar precipitate out of the saturated sugar/tea solution when ice is added. Intuitively we all understand that the solubility of a solid in a liquid is temperature dependent. Table 14.9 shows the variation of solubility with temperature for a few solids. The solubility of most solids increases with a temperature increase, but a few solids show a decrease in solubility with an increase in temperature. To understand this phenomenon, we must develop a better understanding of the solution process.

Probably you also intuitively understand that "like dissolves like", meaning that substances with the same type of polarity will dissolve substances of a similar type of polarity. Water and ethanol are both polar so we expect them to be mutually miscible. Water is polar and benzene is nonpolar so we expect these two substances to be immiscible. Octane and carbon tetrachloride are both nonpolar so we expect them to be miscible. For two substances to be miscible, the intermolecular forces between pairs of the molecules must be about the same so that an A-A stable pair is about as likely to occur as an A-B stable pair. Because the energetics of the pair formation (the intermolecular attraction) are about the same the driving force for solution to occur must be something other than energy. We will understand later that the driving force for solution is predominantly the increase in disorder of the solution over the separate components. Entropy is a measure of the disorder of a system and so we say that increasing entropy is the driving force for solution to occur.

"Like dissolves like" also applies to solids. Solid I₂ is nonpolar so we would expect it to be soluble in nonpolar CCl₄ much more than in water. Sugar is a large molecule with polar groups so we would then expect it to be soluble in water.

Ionic compounds will be soluble in polar solvents like water, as you will remember from our Chapter 5 discussions.

Heats of solution can be either positive or negative. Let's again consider that we are dissolving A into B. The A molecules are attracted to each other in their original state with intermolecular forces A-A. The B molecules are attracted to each other in their original state with intermolecular forces B-B. For the solution process to occur the A-A and B-B intermolecular bonds must be broken and new A-B intermolecular bonds must be formed. The breaking of A-A and B-B bonds requires the expenditure of energy and the making of A-B bonds evolves energy. So, we need to consider the sum of these two energies to determine if the overall process is exothermic or endothermic. Dissolving NaOH in water is a strongly exothermic process and dissolving NH₄NO₃ (used in cold packs) in water is endothermic. If we represent the solution as a reaction then we can calculate the enthalpy of solution:

NaCl(s) --> NaCl(aq)

H_solution = H_f NaCl(aq) - H_f NaCl(s)

= -407.3 kJ/mol - (-411.2 kJ/mol) = 3.9 kJ/mol (data obtained from Table 14.1) so this dissolution is slightly endothermic.

When you open a can of soda, gas is released. Why? What you are experimentally observing is that the solubility of gases in liquids is a function of pressure. The gas CO2 is dissolved in the soda and sealed at a higher pressure than atmospheric pressure. When you open the can, you are exposing the contents to atmospheric pressure, and the gas is not as soluble at this pressure so gas is released from solution. When divers go to sufficient depths, the nitrogen gas in the nitrogen-oxygen gas source (SCUBA) will have an increased solubility. Then as the divers surface the nitrogen will be released into the blood as bubbles of nitrogen that can lead to severe pain and even death. Henry's law is a mathematical expression of the solubility of a gas in a solute:

S_g = k_H P_g

Where S_g is the gas solubility, k_H is the Henry's law constant, and P_g is the partial pressure of the gaseous solute. Table 14.2 contains a few Henry's law constants.

We talked at the beginning of this section about the temperature effects on solubility and now we can expand upon that discussion. The solubility of gases decreases as the temperature is increased so surface water has less dissolved oxygen than the lower depths. Why does the solubility of a gas decrease with increasing temperature? Of help here is Le Chatelier's principle, which states that a change in any of the factors determining an equilibrium causes the system to adjust to reduce or counteract the effect of the change. An application of this principle to the solubility of gases in liquids is that if the solution is heated the equilibrium will shift to absorb some of the added heat energy. The dissolving of gases in liquids is usually an exothermic process (toward the product side), so if we add energy the system will shift back toward the undissolved gas and the liquid (the reactant side):

Gas + liquid solvent saturated solution, exothermic

Review Questions

Work problem 28, 29, and 32 at the end of the chapter.
Why is a helium-oxygen mixture preferred over a nitrogen-oxygen mixture for deep diving? Explain on a molecular level
Explain how a cold pack works.
Pick a solute and a solvent, each with hydrogen bonding, and sketch the hydrogen bonds before and after dissolution occurs
Here is a site descussing the Solution Process.
Go to this site for some information on solutions and an example using Henry's law: Henry's law.
Here is an extensive table of Henry's Law Constants