1. Unit 10: Gases Section 1: Combined Gas Law

2. 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 increase the pressure and temperature in the system (container)

3. Pressure 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 Units of pressure  1 atm = 760 torr = 760 mm = 14.7 inches = 101,325 pascals

4. 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

5. Volume Describes an amount of space Volume units include gallons, quarts, liters, cm 3, and ounces

6. Relationship between Variables PressureTemperatureVolume As Pressure Increases INCREASESDECREASES As Pressure Decreases DECREASESINCREASES There is a relationship that exists between Pressure, Volume, and Temperature…

7. Mathematical Approach to Gases Combined gas law equation is used to compare situations where one or more components have changed  P 1 V 1 = P 2 V 2 T 1 T 2 P = pressure V = volume T = temperature (in Kelvin)

8. Combined Gas Law Example: What will the pressure be when 10 L of gas is at a temperature of 25ºC if the pressure is 30 atm when the volume is 5 L and the temperature is 50ºC?  P 1 V 1 = P 2 V 2 T 1 T 2  (30 atm)(5 L) = (P 2 )(10 L) CHANGE CELSIUS TO KELVIN! 50 +273 25 + 273  (30)(5) = (10)(P 2 ) CROSS MULTIPLY! 323 298  (30)(5)(298) = (10)(P 2 )(323) SOLVE FOR VARIABLE!  P 2  (30)(5)(298) = P 2 (10)(323)  13.839  10 atm = P 2 SIG FIGS!

9. If a variable is not present, OMIT IT! 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?  Temperature is not listed… So ignore!  P 1 V 1 = P 2 V 2  (7atm)(V 1 ) = (6atm)(4L)  (7atm)(V 1 ) = 24  3.4285  3 LSIG FIGS!!!