The Behavior of Gases

Properties of Gases (Review) No definite shape No definite shape No definite volume No definite volume compressible compressible

Kinetic Molecular Theory movingmolecules well supported ideas

Basic Kinetic Theory of Gases 1. Composed of particles like atoms (ex: He) or molecules like (O 2 and CO 2 ) There are no attractive/repulsive forces. Lots of empty space!!

Basic Kinetic Theory of Gases 2. Particles move in random, constant, straight-line motion. Move independently of each other.

Basic Kinetic Theory of Gases 3. All collisions are elastic meaning that KE is transferred without loss of energy. Gases tend to diffuse towards areas of lower concentration.

Gas Pressure Pressure- force exerted on container walls by particles in a gas Pressure- force exerted on container walls by particles in a gas Units used- kPa, atm, Torr, mmHg Units used- kPa, atm, Torr, mmHg STP (Standard Temperature and Pressure) Table A STP (Standard Temperature and Pressure) Table A 273 K or 0°C and 273 K or 0°C and kPa = 1 atm = 760 Torr (mmHg)

Factors Affecting Pressure Amount of Gas (number of moles) Increasing amount will increase P (and vice versa) Ex: bicycle tires, car tires Temperature Increasing temp. will increase P (and vice versa) Ex: Tires deflate in winter Volume Decreasing volume will increase P, increasing volume decreases P Ex: press down on a balloon and it pops

Pressure and volume have an inverse relationship, if temperature remains constant. Pressure and volume have an inverse relationship, if temperature remains constant. If volume is increased, pressure is decreased by the same factor. If volume is increased, pressure is decreased by the same factor.

Mathematically, the product of PV is constant or PV = k (where k is some constant). Boyle’ Law P 1 V 1 = P 2 V 2 = P 3 V 3 …

Summary

Volume and temperature have a direct relationship, if pressure is held constant. Volume and temperature have a direct relationship, if pressure is held constant. If temperature (K) is increased, volume is increased by the same factor. If temperature (K) is increased, volume is increased by the same factor.

Mathematically, the relationship of volume divided by Kelvin temperature is constant or V/T = k. Charles’ Law V 1 /T 1 = V 2 /T 2 = V 3 /T 3 …

Summary

Pressure and temperature have a direct relationship, if volume remains constant. Pressure and temperature have a direct relationship, if volume remains constant. If temperature (K) is increased, pressure will be increased by the same factor. If temperature (K) is increased, pressure will be increased by the same factor.

Mathematically, the relationship of volume divided by Kelvin temperature is constant or P/T = k. PressurePressure Gay-Lussac’s Law P 1 /T 1 = P 2 /T 2 = P 3 /T 3 …

Combined Gas Law Equation P 1 V 1 = P 2 V 2 P 1 V 1 = P 2 V 2 T 1 T 2 T 1 T 2

Combined Gas Law Equation Steps: Steps: Determine which variable is kept constant (if any) Determine which variable is kept constant (if any) Cancel those terms and remove them from the equation Cancel those terms and remove them from the equation Plug in values that are given. Plug in values that are given. Solve for the unknown. Solve for the unknown. Be sure to always use temperature in Kelvins. Be sure to always use temperature in Kelvins.

Ideal Gases vs. Real Gases “Ideal gases” behave as predicted by Kinetic Molecular Theory. “Ideal gases” behave as predicted by Kinetic Molecular Theory. Examples: H 2 and He Examples: H 2 and He Gases are most ideal at high temperature and low pressure. Gases are most ideal at high temperature and low pressure.

“Real gases” deviate from ideal behavior. “Real gases” deviate from ideal behavior. Why? Why? At low temps, gas particles become attracted to each other (KMT says they are not). At low temps, gas particles become attracted to each other (KMT says they are not). Under high pressure, gases occupy a specific volume (KMT says they don’t). Under high pressure, gases occupy a specific volume (KMT says they don’t).

Avogadro’s Law Avogadro’s number: 6.02 x Avogadro’s number: 6.02 x Simply refers to the quantity of particles found in a mole. Simply refers to the quantity of particles found in a mole. At STP, 6.02 x particles of a gas occupies 22.4 L. At STP, 6.02 x particles of a gas occupies 22.4 L.

Vapor Pressure In a sealed container, vapor pressure can be measured above a liquid. In a sealed container, vapor pressure can be measured above a liquid. Evaporation occurs when some particles from the surface of a liquid escape causing pressure to build up above the liquid Evaporation occurs when some particles from the surface of a liquid escape causing pressure to build up above the liquid

Factors that Increase the Rate of Evaporation Heating a liquid (not to boiling point) Heating a liquid (not to boiling point) Increasing surface area Increasing surface area

Liquid-Vapor Equilibrium Some of the gas particles condense and then we find both evaporating and condensing occurs at the same rate. Some of the gas particles condense and then we find both evaporating and condensing occurs at the same rate. Rate of Evaporation = Rate of Condensation Rate of Evaporation = Rate of Condensation

Table H Notice, increasing temperature increases vapor pressure. Line drawn at kPa corresponds to normal boiling point.