# Chapter 13 Gases Kinetic-Molecular Theory of Gases.

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Chapter 13

Gases Kinetic-Molecular Theory of Gases

§All particles are in constant motion. §As temperature increases kinetic energy ? increases §As gas particles move apart the volume ? increases

Point Masses §Gas particles are treated as a point with no volume and no mutual attraction -this is because they’re so small compared to the distances between them.

Ideal Gases §A theoretical gas with no volume and no attraction. §A series of theories will be studied about ideal gases -standard pressure of 101.32 kPa -standard temperature of 0°C or 273K -standard conditions are abbreviated STP

Therefore: §Kinetic theory explains properties of gases based on a molecular view. §The assumptions are: l The molecules are in continuous, random motion. l A molecule has negligible volume. l The forces between molecules are negligible. l The average kinetic energy depends on the temperature.

Four Gas Law Variables Are: §V = volume §P = pressure §T = temperature §n = number of particles §(Upper and Lower Case is important)

Behavior of Gases §Compression §Expansion §Diffusion – movement of material from high to low concentration - lighter particles diffuse faster than heavier particles §Effusion- gas escapes through a tiny opening

Gas Pressure §Pressure = force/area §Snowshoes in the snow – force is spread out over a larger area §Gas particles exert pressure as they collide with the walls of their container §More particles in a given space, greater pressure §Barometer is tool used to measure atmospheric pressure – mercury rises or falls

Units of pressure §SI unit is Pascal (Pa) §1Pa = 1 N/m 2 derived from force §1atm = 760mm Hg = 760 torr = 101.3 kPa =14.7 psi §1torr = 1 mm Hg

Dalton’s Law of Partial Pressure §Total pressure of a mixture of gases is equal to the sum of all the pressures of the individual gases §Pg. 392 practice problems

Intermolecular Forces (between molecules) §Dispersion Forces – weak forces between temporary dipoles – electron cloud is in constant motion (sometimes called London Forces). §Dipole – Dipole Forces forces between permanent dipoles (polar molecules). §Hydrogen Bonds - a type of dipole-dipole attraction that occurs between molecules containing a hydrogen atom bonded to a small highly electronegative atom (oxygen, nitrogen, or fluorine)

Liquids §Denser than gases. §Can’t be easily compressed. §Fluidity – ability to flow (gases and liquids). §Viscosity – measure of the resistance to flow. Decreases with temperature. §Surface Tension – measure of the inward pull by particles in the interior (molecules at the surface have no attractions from above to balance the attractions from below).

Liquids (continued) §Capillary Action – liquids in a narrow container form meniscus. This is caused by two forces: l Cohesion – attraction between identical molecules. l Adhesion – attraction between different molecules. l Example – Adhesion between glass and water in a graduated cylinder is greater than cohesion between water molecules. Therefore water rises along the inner walls of the cylinder.

Solids §Density is greater than liquids §Crystals – orderly, geometric, 3-D structure. l Atomic l Molecular l Covalent l Ionic l Metallic §Amorphous – not arranged in regular, repeating pattern

Phase Changes §That require energy l Melting l Vaporization (Evaporation) l Sublimation §That release energy l Freezing l Condensation l Deposition