Chapter 19: Chemical Interactions

Four types of chemical interactions: (strongest to weakest)

ion - dipole

dipole - dipole

dipole - induced dipole

induced dipole - induced dipole

These are all much weaker (over 100 times) than covalent or ionic bonds.

Ion - dipole

In many molecules, electrons are distributed closer to one side than the other. This causes one side to be slightly more negative than the other. We call this type of molecule a dipole.

ionic - dipole interaction: sodium chloride is an ionic compound

Dipole - dipole

When dipoles interact between themselves we have a dipole - dipole interaction. The hydrogen bond relatively strong.

Dipole - induced dipole

When a molecule with no dipole (such as O₂) is placed near a molecule that has a dipole (H₂O) it may create an induced dipole in a molecule that has no dipole. The induced dipole is temporary, existing only as long as the dipole is close to the other molecule.

Induced dipole - induced dipole

The random motion of the electrons surrounding a nonpolar element or molecule makes it possible for more electrons to gather on one side than the other. This creates a temporary dipole, which can induce a dipole in other nearby nonpolar molecules which can in turn induce yet more dipoles ...

The different types of interactions help explain some of the physical properties of a substance. A solid is formed when the chemical interactions between atoms or molecules is strong enough to hold them in a fixed arrangement. As the interactions between molecules are become unfixed, molecules keep shifting around to interact with other molecules, the compound becomes a liquid. As more shifting takes place, the compound becomes a gas.

Special properties of water

As a solid (ice) its molecules form a crystal, taking up more volume than an equivalent mass of liquid water just above the freezing temperature. Water has a high heat capacity due to the strength of the hydrogen bond.

In a liquid, molecules at the surface are attracted to one another. A force must be applied in order for a object to penetrate the surface. The energy need to do this is referred to as the surface tension of the liquid. Surface tension also explains why liquids form spherical drops and why a liquid spilled onto a flat surface will hold together.

Adhesive force: chemical interactions that arise between two different substances.

Cohesive force: chemical interactions that arise between like substances.

Solution: a homogeneous mixture consisting of only one phase. i.e. sugar water and air.

Solvent: the component of the solution with the greatest quantity.

Solute: all the other components in the solution.

Example: mix 50 grams of sugar in 100 grams of water. Solvent is water and solute is sugar.

The quantity of solute dissolved in a solution is often referred to by its concentration:

concentration = (amount of solute)/(volume of solution)

Example: mix 50 grams of sugar in 100 grams of water and you get a concentration of 0.5 grams of sugar per liter.

Chemists are usually more interested in the number of molecules of solute rather than the number of grams. The unit of use is the mole.

Solubility: ability of a solute to dissolve in a solvent. Temperature affects solubility. In general, the higher the temperature of the solvent, the easier it is to dissolve the solute. Also with higher temperatures, more of the solute can be dissolved.