Kinetic Molecular Theory

H-ch.13 CP-ch.10 & 12 u Gases indefinite volume and shape, low density. u Liquids definite volume, indefinite shape, and high density. u Solids definite volume and shape, high density u Solids and liquids have high densities because their molecules are close together.

Kinetic Theory are evidence of this. uKinetic theory says that molecules are in constant motion. uPerfume molecules moving across the room

Ê A Gas is composed of particles H usually molecules or atoms H Considered to be hard spheres far enough apart that we can ignore their volume. H Between the molecules is empty space. The Kinetic Theory of Gases Makes three assumptions about gases

Ë The particles are in constant random motion. H Move in straight lines until they bounce off each other or the walls. Ì All collisions are perfectly elastic

u The Average speed of an oxygen molecule is 1656 km/hr at 20ºC u The molecules don’t travel very far without hitting each other so they move in random directions.

Kinetic Energy and Temperature u Temperature is a measure of the Average kinetic energy of the molecules of a substance. u Higher temperature faster molecules. u At absolute zero (0 K) all molecular motion would stop.

u The average kinetic energy is directly proportional to the temperature in Kelvin u If you double the temperature (in Kelvin) you double the average kinetic energy. u If you change the temperature from 300 K to 600 K the kinetic energy doubles. Temperature

u If you change the temperature from 300ºC to 600ºC the Kinetic energy doesn’t double. u 873 K is not twice 573 K

Pressure u Pressure is the result of collisions of the molecules with the sides of a container. u A vacuum is completely empty space - it has no pressure. u Pressure is measured in units of atmospheres (atm). u It is measured with a device called a barometer.

Barometer uAt one atmosphere pressure a column of mercury 760 mm high. Dish of Mercury Column of Mercury 1 atm Pressure

Barometer uAt one atmosphere pressure a column of mercury 760 mm high. uA second unit of pressure is mm Hg u1 atm = 760 mm Hg 760 mm 1 atm Pressure

Avagadro’s Hypothesis u Equal volumes of gas at the same temperature and pressure have equal numbers of molecules. u That means...

Avagadro’s Hypothesis 2 Liters of Helium 2 Liters of Oxygen u Has the same number of particles as..

u Only at STP 0ºC 1 atm u This way we compare gases at the same temperature and pressure. This is where we get the fact that 22.4 L =1 mole

Think of it it terms of pressure. u The same pressure at the same temperature should require that there be the same number of particles. u The smaller particles must have a greater average speed to have the same kinetic energy.

Liquids u Particles are in motion. u Attractive forces between molecules keep them close together. u These are called intermolecular forces. Inter = between Molecular = molecules

Breaking intermolecular forces. u Vaporization - the change from a liquid to a gas below its boiling point. u Evaporation - vaporization of an uncontained liquid ( no lid on the bottle ).

Evaporation u Molecules at the surface break away and become gas. u Only those with enough KE escape u Evaporation is a cooling process. u It requires heat. u Endothermic.

Condensation / Change from gas to liquid / Achieves a dynamic equilibrium with vaporization in a closed system. / What is a closed system? / A closed system means matter can’t go in or out. (put a cork in it)  What the heck is a “dynamic equilibrium? ”

Vaporization n Vaporization is an endothermic process - it requires heat. n Energy is required to overcome intermolecular forces n Responsible for cool earth. n Why we sweat. (Never let them see you.)

Boiling u A liquid boils when the vapor pressure = the external pressure u Normal Boiling point is the temperature a substance boils at 1 atm pressure. u The temperature of a liquid can never rise above it’s boiling point.

Changing the Boiling Point u Lower the pressure (going up into the mountains). u Lower external pressure requires lower vapor pressure. u Lower vapor pressure means lower boiling point. u Food cooks slower.

u Raise the external pressure (Use a pressure cooker). u Raises the vapor pressure needed. u Raises the boiling point. u Food cooks faster. Changing the Boiling Point

Solids u Intermolecular forces are strong u Can only vibrate and revolve in place. u Particles are locked in place - don’t flow. u Melting point is the temperature where a solid turns into a liquid.

u The melting point is the same as the freezing point. u When heated the particles vibrate more rapidly until they shake themselves free of each other. u Ionic solids have strong intermolecular forces so a high mp. u Molecular solids have weak intermolecular forces so a low mp.

Crystals u A regular repeating three dimensional arrangement of atoms in a solid. u Most solids are crystals. u Amorphous solids lack an orderly internal structure. u Think of them as supercooled liquids.

Cubic

Body-Centered Cubic

Face-Centered Cubic

Phase Changes Solid Liquid Gas Melting Vaporization CondensationFreezing

Liquid Sublimation Melting Vaporization Deposition Condensation Solid Freezing Gas

Energy and Phase Change u Heat of fusion energy required to change one gram of a substance from solid to liquid. u Heat of solidification energy released when one gram of a substance changes from liquid to solid. u For water 80 cal/g

Water and Ice Ice Water and Steam Steam Heating Curve for Water

Water and Ice Ice Water and Steam Steam Heating Curve for Water Heat of Vaporization

Water and Ice Ice Water and Steam Steam Heating Curve for Water Heat of Fusion

Water and Ice Ice Water and Steam Steam Heating Curve for Water Slope = Specific Heat Steam Water Ice

Water and Ice Ice Water and Steam Steam Heating Curve for Water Both Water and Steam

Water and Ice Ice Water and Steam Steam Heating Curve for Water Ice and Water

Water and Ice Ice Water and Steam Steam Heating Curve for Water

Phase Diagram of Water