Unit 10: Gases Section 1: Kinetic Molecular Theory and the Combined Gas Law
Introduction Molecules in a gas behave uniquely Gas molecules move rapidly and expand to fill their space Kinetic Molecular Theory: describes the action of gas molecules in a confined space Kinetic means moving, so the theory focuses on how gas molecules behave as a result of their motion As molecules collide with each other and their environment, they affect the pressure and temperature in the system (container)
Pressure Force exerted per unit area of the surroundings Atmospheric Pressure = pressure pushing down by the atmosphere Air is matter and matter has weight that exerts pressure on everything We don t notice this pressure much because it s the same inside and outside our homes, the same inside and outside our body, etc. We notice pressure change when riding on an airplane, etc. Units of pressure 1 atm = 760 torr = 760 mm = 14.7 inches = 101,325 pascals
Temperature Measure of the speed at which molecules and atoms are moving = amount of energy Higher Temperature = Faster Speed = More Energy Units of temperature Celsius (ºC) and Kelvin (K) Gases use the Kelvin temperature scale, since it s based on the idea of absolute zero Absolute zero (0 K) is where all energy and MOTION stops Remember: Celsius to Kelvin = Add 273
Volume Describes an amount of space Volume units include gallons, quarts, liters, cm 3, ounces, etc. Can be either fixed or elastic Fixed: volume is contained and has a definite shape Drink can, box, Tupperware Elastic: volume can change Balloon, raft, basketball
Relationship between Variables There is a relationship that exists between Pressure, Volume, and Temperature Volume Temperature Pressure DECREASES (Compressed) Kept Constant INCREASES Kept Constant INCREASES INCREASES INCREASES (Expands) INCREASES Kept Constant
Mathematical Approach to Gases Combined gas law is used to compare situations where one or more variables change P = pressure V = volume T = temperature (in kelvins)
Boyle s Law Describes the relationship between pressure and volume as temperature is kept constant As volume decreases, pressure increases P 1 Example: How many liters in size will a balloon become at a pressure of 7 atm if it has a volume of 4 liters at a pressure of 6 atm? (6 atm)(4 L) = (7 atm)( ) (6)(4) = 7 = 3.43 L 3 L
Charles Law Describes the relationship between volume and temperature as pressure is kept constant. As temperature increases, volume increases (expands) = Example: What is the volume of a helium-filled balloon at 200 K if it has a volume of 8 L at 100 K? = 8 L = 100 K 200 K (8)(200) = (100) 1,600 = 100 = 16 L 20 L
Gay-Lussac s Law Describes the relationship between pressure and temperature as volume is kept constant. Pressure builds up as gas molecules move faster and strike the walls of the container more often and with more force As temperature increases, pressure increases P 1 Example: Calculate the pressure that results when the temperature becomes 100ºC if the pressure at 25ºC is 750 torr. 750 298 373 P 2 = (750)(373) 298 P 2 = 938.8 torr 900 torr Convert ºC to K first!!!
Combined Gas Law NOT IN NOTES! Since the equation has pressure, volume, and temperature, it can be used as a calculation of any combination of these three = Combined Gas Law!!!
Combined Gas Law Most real applications of gases involve varying the temperature, pressure, AND volume P 1 Example: What pressure is expected to develop at a volume of 10 L and a temperature of 25ºC if the volume is 5 L, the temperature is 50ºC, and the pressure is 30 atm? (30 atm)(5 L) = (P 2 )(10 L) 50 +273 25 + 273 (30)(5) = (10)(P 2 ) 323 298 (30)(5)(298) = (10)(P 2 )(323) (30)(5)(298) (10)(323) P 2 = 13.84 atm 10 atm