Phases of Matter and Intermolecular Forces Adapted from: Wilbraham, Anthony. Chemistry, Addison-Wesley. Upper Saddle River, NJ: Prentice Hall, Inc.,2002.

1.Review the organization of particles in the three phases of matter: gases, liquids and solids. 2.Name and describe the weak attractive forces that hold groups of molecules together. 3.Determine which type of intermolecular force is important to overcome in converting a substance from a liquid to a gas. Objectives

States of Matter - Gases Gases can be compressed because the particles are spaced far apart. Gases fill their container uniformly and completely, and they flow because there are only weak forces attracting molecules to one another. The particles move randomly in a gas.

States of Matter - Liquids Liquids are condensed states, their particles are in close contact with each other. Liquids take the shape of the container and they can flow because the particles are not rigidly packed and the attraction between particles is weak. The particles of a liquid move randomly.

States of Matter – Solids Solids are condensed because the particles are packed tightly together. Solids have a definite shape and volume.

The three states of matter Solids Liquids Gases

Intermolecular Forces Intermolecular forces – occur between molecules Intramolecular forces – occur inside the molecules

Three types of Intermolecular Forces (IMF) in order of increasing strength: 1. Dispersion forces 2. Dipole Interactions 3. Hydrogen bonding Dispersion forces and dipole interactions are the weakest and are sometimes referred to a van der Waals forces.

Dispersion Forces Dispersion forces are the weakest of all molecular interactions. Important in nonpolar substances, such as F 2, N 2, and CO 2 They are also called London forces and induced dipole forces.

About dispersion forces… Dispersion forces are caused by the motion of electrons. When two nonpolar molecules encounter one another, attractions and repulsions lead to distortions in their electron clouds, inducing “momentary dipoles”. The strength of dispersion forces increases as the number of electrons in a molecule increases.

Question…… Why are F 2 and Cl 2 gases at room temperature, while Br 2 is a liquid and I 2 is a solid? Answer: F 2 has 18 electrons and Cl 2 has 34 electrons, while Br 2 has 70 electrons and I 2 has 106 electrons. More electrons causes a greater force of attraction.

Dipole Interactions Dipole interactions occur when polar molecules are attracted to one another. They are stronger than dispersion forces. The oppositely charged ends of polar molecules create electrostatic attractions, and these hold polar molecules in the liquid or solid state. They are much weaker than ionic bonds.

Hydrogen Bonding Hydrogen bonds are attractive forces in which a hydrogen atom that is covalently bonded to a very electronegative atom is attracted to an unshared pair of another very electronegative atom. Hydrogen bonding is an extreme case of dipole interaction. This occurs only when H is directly bonded to either N, O, or F.

Question: Which has the higher boiling point? (…consider which has the stronger IMF) 1. O 2 or N 2 ? 2. SO 2 or CO 2 ? 3. HF or HI? 4. SiH 4 or GeH 4 ?

Answers: 1.O 2 – both N 2 and O 2 are nonpolar, both have dispersion forces, but O 2 has more electrons 2. SO 2 – CO 2 is nonpolar (has dispersion forces), while SO 2 is polar (has stronger dipole interactions) 3. HF – HI has dipole interactions, HF has stronger hydrogen bonding 4. GeH 4 – both SiH 4 and GeH 4 are nonpolar, but GeH 4 has more electrons

Challenge: Place these in order of increasing strength of intermolecular forces: SCl 2, NH 3, CH 4, Cl 2

Answer: CH 4 (nonpolar, dispersion, few electrons), Cl 2 (nonpolar, dispersion, more electrons), SCl 2 (polar, dipole interactions), NH 3 (H bonded to N, O, or F, hydrogen bonding)

Question: Which has the higher boiling point? dimethyl ether (H 3 C-O-CH 3 ) or ethyl alcohol (H 3 C-CH 2 -OH)? (both have the general formula C 2 H 6 O.)

Answer: Ethyl alcohol. (Dimethyl ether is slightly polar and has dipole interactions, while ethyl alcohol has hydrogen bonding.)