SOLUTION INTERMOLECULAR FORCES

Review of Solutions:

A solution is a mixture of two or more substances in a single phase. At least two substances must be mixed in order to have a solution. The substance in the smallest amount and the one that dissolves or disperses is called the SOLUTE. The substance in the larger amount is called the SOLVENT. In most common instances water is the solvent. The gases, liquids, or solids dissolved in water are the solutes.

Introduction:

The dissolving process involves a consideration of the relative strength of three intermolecular attractive forces. The type of forces between solute-solute molecules and solvent-solvent molecules must be considered. These intermolecular attractions must be broken before new solute-solvent attractive forces can become effective. Perhaps the bond breaking and bond forming processes take place simultaneously. A solute will dissolve in a solvent if the solute-solvent forces of attraction are great enough to overcome the solute-solute and solvent-solvent forces of attraction. A solute will not dissolve if the solute-solvent forces of attraction are weaker than individual solute and solvent intermolecular attractions. Generally, if all three of the intermolecular forces of attraction are roughly equal, the substances will be soluble in each other.

Solubility Rule and Summary

LIKES DISSOLVE LIKES.

This means that ionic or polar solutes dissolve in polar solvents.
Non-polar solutes dissolve in non-polar solvents.

Polar and ionic solutes DO NOT dissolve in non-polar solvents and vice versa.

Remember that when applying the solubility rule: "Likes Dissolve Likes", that there are no absolutes and there are exceptions with a small amount of solubility possible. The rule is most useful when making comparisons between a series of compounds.

IONIC SOLUTE - POLAR SOLVENT:

When an ionic crystal such as NaCl is placed in water, a dissolving reaction will occur. Initially, the positive and negative ion are only attracted to each other. The water molecules are hydrogen bonded to each other. If the crystal is to dissolve, these bonds must be broken.

Negative chloride ions on the surface are attracted by neighboring positive sodium ions and by the partially positive hydrogen atom in the polar water molecule (See the graphic on the right).

Similarly, the positive sodium ions are attracted by both chloride ions and the partially negative oxygen atom in the polar water molecule. (See the graphic on the right).

A "tug-of-war" occurs for the positive and negative ions between the other ions in the crystal and the water molecules. Whether the crystal dissolves is determined by which attractive force is stronger. If the internal ionic forces in the crystal are the strongest, the crystal does not dissolve. This is the situation in reactions where precipitates form. If the attractions for the ions by the polar water molecules are the strongest, the crystal will dissolve. This is the situation in sodium chloride.

Once the ions are released from the crystals, the ions are completely surrounded by water molecules. Note that the proper atom in the water molecule must "point" toward the correct ion. The charge principle and the partial charges in the polar molecule determine the correct orientation.

POLAR SOLUTE - POLAR SOLVENT:

Polar ammonia molecules dissolve in polar water molecules. These molecules mix readily because both types of molecules engage in hydrogen bonding. Since the intermolecular attractions are roughly equal, the molecules can break away from each other and form new solute (NH3), solvent (H2O) hydrogen bonds.

A wide variety of solutions are in this category such as sugar in water, alcohol in water, acetic and hydrochloric acids.

NON-POLAR SOLUTE - NON-POLAR SOLVENT:

In all types of non-polar compounds, about the only intermolecular attractions are the very weak induced dipole forces. The weak attractive forces formed by the solute-solvent molecules compensate for breaking those weak bonds in the two pure non-polar substances. An example is solid iodine (I2) dissolved in liquid bromine (Br2).

Figure 21. NON-POLAR SOLUTE - SOLVENT INTERACTIONS

In addition to diatomic molecules with identical atoms, the most common type of non-polar compounds are the hydrocarbons. Many C-C and C-H non-polar bonds are present. Hydrocarbons are present in oils, grease, fats, dry cleaning solvents, turpentine, gasoline, etc.

Demonstrations:
Disappearing Cup
Packing Peanut Race
Revealing Fingerprints
Deep Purple Magic

 NON-POLAR SOLUTE - POLAR SOLVENT:

Non-polar Iodine is not very soluble in water. An intermolecular bond between an induced dipole (I2) and a polar bond in water is not very strong compared to the hydrogen bonds in water. The water molecules would rather remain hydrogen bonded to each other, then to allow an iodine molecule come between them. The water molecules effectively "squeeze" out the non-polar iodine. The intermolecular forces are not roughly equal, therefore, the "unlike" substances are not soluble in each other.

Various gases such as O2, N2, H2, CO2 are not very soluble because the gases are essentially non-polar. Of course you may say that oxygen must be dissolved in water to sustain fish life -- true, but the solubility is very low. Carbon dioxide is soluble in water such as carbonated beverages -- again this is true but why does it fizz when opened or lose the bubbles on standing? Carbon dioxide is not very soluble in water.

Demonstrations:
Flaming Wood Splint
Deep Purple Magic
Lava Lamp
Dancing Raisins