States of Matter Chapter 13

What You Need to Master How to use the kinetic-molecular theory to explain the physical properties of gasses, liquids, and solids Compare types of intermolecular forces Explain how kinetic energy and intermolecular forces combine to determine the state of a substance Describe the role of energy in phase changes

Gases Chapter 13.1

Kinetic-Molecular Theory Jan Baptista Van Helmont (Flemish) used the word Chaos (without order) to describe products of reactions that had no fixed shape or volume. By 18 th century, scientists knew how to collect gasses by displacing water –They could now measure properties of gasses 1860 Ludwig Boltzmann and James Maxwell proposed models to explain the properties of gasses –Kinetic-molecular theory

Kinetic-Molecular Theory Kinetic – Greek word meaning “to move” –any object in motion has “kinetic energy” Kinetic-Molecular Theory describes the behavior of gasses in terms of particles in motion. –Model makes several assumptions about size, motion, and energy of particles

Kinetic-Molecular Theory Assumptions: –Particle Size : particles are so small and so far apart that there no significant forces of attraction –Particle motion: particles are in constant, random motion. Particles move in a straight line until they collide with another particle or the container wall Particle collisions are elastic (no energy is lost, but may be transferred)

Kinetic-Molecular Theory Assumptions: –Particle Energy: 2 factors, mass & velocity KE = ½ mv 2 All particles do NOT have the same energy Temperature is a measure of the average kinetic energy of the particles

How K-M Theory Explains Gas Behavior Low Density –Density is mass/volume –Cl 2 density at 20°C is 2.95 x g/mL –Au density is 19.3 g/mL –Because Chlorine is a gas, K-M theory says there must be a lot of empty space between molecules. Thus, fewer molecules in the same volume

How K-M Theory Explains Gas Behavior Compression and Expansion –Because there is a lot of empty space between molecules/atoms, you can compress or squeeze together the atoms/molecules Removes some of the empty space –Allowing the atoms to return to normal is expansion

How K-M Theory Explains Gas Behavior Diffusion and Effusion –Diffusion – describes the movement of one material through another –Effusion – gas escaping through a tiny opening Like puncturing a balloon or tire –Graham’s Law of Diffusion Rate of effusion 1/Ã molar mass means ‘is proportional to’ Larger particles diffuse slower than smaller particles

How K-M Theory Explains Gas Behavior Rate Relationships: Rate a Rate b = Ã Molar mass a Molar mass b

Gas Pressure Pressure = force/area –An elephant has less pressure on its foot than a human. Why? Gasses exert pressure when they collide with walls of the container. –Small mass means small pressure –But there are about particles in one liter Pressure can be substantial –Earth’s air pressure varies – gets lower with altitude

Measuring Air Pressure Evangelista Torricelli (Italian – before 1650) first demonstrated air pressure –Designed equipment to measure air pressure using mercury in a tube. –Now called a barometer

Measuring Air Pressure A ‘manometer’ measures air pressure in a u-shaped tube, in a closed container.

Units of Pressure SI unit is ‘Pascal’ –1 Pascal = 1 newton/m 2 –PSI (pounds per square inch) is still used –Barometers and manometers still report millimeters of mercury (mm Hg) At sea level, air pressure is 760 mm Hg at 0°C –Air pressure is frequently reported as 1 atmosphere (atm) = 760 mm Hg or 760 torr or kilopascals

Units of Pressure The units 1 atm, 760 mm Hg, 760 torr, and kilopascals are considered to be defined units –They have as many significant digits as necessary

Dalton’s Law of Partial Pressure Dalton studied mixtures of gasses: –Found that each gas exerts pressure independently. Law of Partial Pressures: –The total pressure is equal to the sum of pressures by each gas in the mixture –Partial pressure depends on the moles of gas, size of container and temperature. –Does NOT depend on the identity of the gas

Dalton’s Law of Partial Pressure At a given temperature and pressure, P TOTAL = P A + P B + P C + P D + …. P N Dalton’s Law can be used for: –Determine amount of gas produced by a reaction Partial pressures of gasses at same temperature are related to concentrations. –Look up values in reference tables –At 20° C, partial pressure of water is 2.3 kPa