Intermolecular Forces

Intra vs. Inter Intermolecular force—holds molecules together. Intramolecular force—holds atoms together in a molecule (any chemical bond).

Recall Draw the 3D Lewis structure of CF4. Draw the 3D Lewis structure for CH3F. What is a dipole? How do they occur? How do we show dipoles?

Intermolecular Forces What are they? The forces of attraction between particles or molecules of a substance. Vary in strength Related to how polar or nonpolar the substance is. Still weaker than actual covalent, ionic, or metallic bond.

Ionic and Metallic Forces Strongest forces of attraction (metallic > ionic) Exist because of charge differences Positively charged ions attracted to negatively charged ions. Greater the charge difference, the stronger the attraction. Metallic forces of attraction have their sea of electrons to hold them together.

Dipole-Dipole Forces Strong force of attraction, but weaker than ionic or metallic forces of attraction. Exists between polar molecules Why so strong? Positively charged regions of one molecule are attracted to negatively charged regions of the other.

Hydrogen Bonding Type of strong dipole-dipole force in which H bonded to a highly electronegative atom is attracted to the unshared electrons of another electronegative atom in a nearby molecule. Force is characterized by compounds having unusually high boiling points. Happens with compounds containing O, F, and N.

Induced-Dipole Forces Charged region of one molecule creates charged region on a nonpolar molecule. Weaker than dipole-dipole forces

London Dispersion Forces Result from the constant motion of electrons and the creation of instantaneous dipoles in a molecule. Weakest force of attraction Exists in all molecules and compounds Only intermolecular force acting among noble gases and nonpolar compounds. Increase with increasing atomic or molecular mass. Why? Greater difference in electron cloud densities.

Relative Strengths

Chapter 10 States of Matter

Section 1 Gases

State of Matter Chemical and Physical properties determined by two things: Composition—what atoms make up the substance. Structure—how the atoms are arranged.

Kinetic Molecular Theory (KMT) What is it? The idea that particles of matter are constantly in motion. Used for describing the properties of solids, liquids, and gases in terms of energy

gases 2 types ideal gas—hypothetical gas that fits all assumptions of the KMT. Real gas—gas that does not behave like KMT assumptions.

For Ideal Gases: 5 assumptions 1.) Gases have large numbers of tiny particles that are far apart relative to their size. Most volume occupied by a gas is empty space Explains why gases have lower densities than liquids or solids. Explains why gases can be compressed easily

4.) There are no forces of attraction between particles. 2.) Collisions between gas particles are elastic (no net loss of energy). Energy is transferred between particles during collisions 3.) Particles are in continuous, rapid motion, and therefore possess kinetic energy. Kinetic energy of particles overcomes the attraction between them. 4.) There are no forces of attraction between particles.

5.) Temperature of a gas is dependent on the average kinetic energy of the gas. KE = ½ mv2 m = mass v = velocity All specific gases contain particles that have the same mass. Therefore, KE only depends on speed of particles. Speed and KE depend on temp. All gases at the same temperature have the same KE. Therefore, gases with different masses have different speeds.

The Nature of Gases Expansion—gases fill their container. KMT explains why 3—gases move rapidly in different directions. 4—have no forces of attraction between them.

The Nature of Gases Fluidity—the ability of gases to behave like liquids Particles flow past one another. Why? Assumption 4—particles have no attractive forces

The Nature of Gases Low Density—particles are spread farther apart than those of solids and liquids. Assumption 1—Particles are spread far apart relative to their size.

The Nature of Gases Compressibility-gas particles are initially far apart, and after compression, get much closer together. Why? Assumption 1—particles are far apart.

The Nature of Gases Diffusion—spontaneous mixing of the particles of two substances caused by random motion. Effusion—gas particles passing through a small opening. Rate of effusion is directly proportional to velocity of particles. Inversely proportional to square root of the molar mass. (the larger the particles, the quicker the rate of effusion).

Deviations from Ideal Behavior All gases have some degree of variation from ideal behavior. When does KMT hold true? Usually for gases whose particles have no attraction (noble gases). Polar gases deviate more than nonpolar gases. NH3 > F2 > Ne High temperature and pressure also make gases deviate from KMT.

Liquids

Liquids Although most abundant, it is the least common state of matter on earth Why? Narrow range of temperature and pressure What makes something a liquid or a gas?

Properties of Liquids Particles are in constant motion Particles are closer together than gases Why? Stronger attractive forces More ordered—strong IMF = lower mobility of particles Particles are not bound together in fixed positions

Properties of Liquids Fluidity—particles flow past one another. Relatively High Density Relative Incompressibility Particles are more packed together Ability to diffuse Much slower than diffusion of gases Why? Attractive forces!!!!

Surface tension A force that tends to pull adjacent parts of a liquid’s surface together, thereby decreasing surface area to the smallest possible size. Results from attraction between particles of a liquid Particles are drawn to body of liquid, creating sphere (smallest surface area).

Capillary Action Attraction of the surface of a liquid to the surface of a solid. Liquid rises high in a narrow tube if strong forces of attraction are present. Attraction pulls liquid against gravity. Process stops when attractive forces are balanced by weight of the liquid.

Evaporation and Boiling Vaporization—process by which a liquid changes to a gas. Evaporation—process by which gas particles escape from the surface of a nonboiling liquid and enter the gas state. Occurs when particles have different KE. High KE overcomes intermolecular force and enters gas state. Boiling—change of a liquid to bubbles of vapor that appear throughout the liquid.

Formation of Solids Liquid is cooled—energy of particles decrease. Energy is low enough to allow attractive forces to take over and pull particles into orderly fashion. Freezing—physical change of a liquid to a solid by removal of energy as heat.