The Nature of Gases Kinetic Energy – the energy an object has because of its motion. Kinetic – means motion Kinetic Theory – all matter consists of tiny particles that are in constant motion.

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

The Kinetic Theory of Gases Makes three descriptions of gas particles A gas is composed of particles molecules or atoms Considered to be hard spheres far enough apart that we can ignore their volume. Between the molecules is empty space.

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

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

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

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

Barometer At one atmosphere pressure a column of mercury 760 mm high. A second unit of pressure is mm Hg 1 atm = 760 mm Hg Third unit is the Pascal 1 atm = 101.3 kPa 1 atm Pressure 760 mm

Pressure units Convert kilopascals – kPa 1 atm = 760 mm Hg = 101.3 kPa 743 mm Hg to atm 895 kPa to mm Hg

Gas Pressure Gas pressure is the result of simultaneous collisions of billions of rapidly moving particles in a gas with an object. e.g. – a helium-filled balloon maintains its shape because of the pressure of the gas within it. Vacuum – an empty space with no particles and no pressure. (no particles, no collisions)

Atmospheric Pressure Atmospheric pressure results from the collisions of atoms and molecules in air with objects. Atmospheric pressure decreases as you climb a mountain because the density of Earth’s atmosphere decreases as elevation increases. less particles, less pressure

The Nature of Liquids Particles are in motion. Tends to pull them apart Attractive forces between molecules keep them close together. liquids are much more dense than gases. These are called intermolecular forces. Inter = between

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

Evaporation Molecules at the surface break away and become gas. Only those with enough KE escape It requires energy.

Solids Intermolecular forces are strong Molecules still move Can only vibrate and revolve in place. Particles are locked in place - don’t flow. Melting point is the temperature where a solid turns into a liquid. The melting point is the same as the freezing point.

Phase Changes Solid Gas Liquid Melting Vaporization Freezing Condensation

Sublimation Sublimation – the change of a substance from a solid to a vapor without passing through the liquid state. Sublimation can occur because solids, like liquids, have vapor pressure. Sublimation occurs in solids with vapor pressures that exceed atmospheric pressure at or near room temperature.

For example: Solid carbon dioxide (dry ice) sublimes at atmospheric pressure. Used as a coolant. It does not produce a liquid as ordinary ice does when it melts.

Solid Gas Liquid Require energy Sublimation Melting Vaporization Deposition Freezing Condensation Release energy

End of Chapter 13