Pressure – Volume Relationship A4 – A8
Pressure Force applied to one unit of surface area Pressure = Force Area
Which hurts more? High heeled shoe, why?
More thoughts… • When on thin ice should you lay out and crawl or run like mad? • More pressure – a woman in high heels or an elephant? • How many different ways could you increase tire pressure? • Which way will the book exert the most pressure flat or up and down? • Which is the best tool for chopping wood – a hammer or an ax of the same weight? • Mud tires – Skinny or Wide? • Bed of nails – 200 or 20 more comfortable?
Caused by particles colliding with the sides of the container. In a gas, pressure is… The more collisions particles have the greater the pressure.
Units of Pressure psi (pounds per square inch) atm (atmospheres) mmHg (mm of Mercury) kPa (kiloPascal) Pa (Pascal) torr
Volume The amount of space occupied by a substance If a gas is a non-rigid container then its volume can change. Ex of non-rigid: balloon If a gas is in a rigid container then its volume cannot change. Rigid container: steel canister
Units of Volume mL (milliliters) L (Liters) cm3 or cc (cubic centimeters)
Boyle’s Law At constant temperature, the product of the pressure and the volume of a gas sample is a constant value.
Boyle’s Lawexplained using kinetic molecular theory Relates pressure to volume When you make the volume of a container smaller: Particles get closer together Therefore having more collisions Which results in higher pressure. So when V decreases, Pressure increases.
Boyle’s Law An example of this: Think about the marshmallows in the syringe. You pushed down on the syringe what happened to the marshmallows? They got smaller – why? By pushing down on the syringe you were increasing pressure, forcing the air particles in the marshmallow to get closer together.
An inverse relationship is when one variable increases and the other variable decreases. They aren’t doing the same thing.
Boyle’s Law In Equation form: P1V1 = P2V2 Temperature must be held constant! We also assume that the number of gas molecules remains constant.
Practice with Boyle’s Law Given the following data find the missing variable. P1 = 500mmHg V1 = 250mL P2 = 1000 mmHg V2 = ?
Practice with Boyle’s Law A certain steel gas tank in a chemistry laboratory has a volume of 2.5L and it contains oxygen gas at a pressure of 45atm. What volume would the gas from such a tank occupy at the classroom’s pressure, 1 atm? Assume the temperature remains constant.
Review - Volume The amount of space occupied by a substance Units: mL (milliliters) L (Liters) cm3 or cc (cubic centimeters)
Temperature Extent of “hotness” or “coldness” of a sample, which is related to the kinetic energy of the particles Increase in temperature means more movement and greater kinetic energy Units ˚F, ˚C, and K
WORD! Lord Kelvin 0K is absolute zero Scientists believe this is the temp were all particle motion stops and the volume of the gas is 0. 0K = -273˚C Therefore to convert from ˚C to K K = ˚C + 273
Charles’ Law The volume of a gas sample at constant pressure is directly proportional to its kelvin temperature
Charles’ Lawexplained using kinetic molecular theory As you increase the temperature: Particles move faster In order for pressure to remain constant those particles will have to move further away Which results in larger volume So when Temperature increases, Volume increases.
Chuck’s Law pv simulaiton 0 Kelvin's = -273 C sterling engine
Charles’ Law Think about the balloon you put in the hot pot of water. What happened to it? It got bigger – why? When the balloon is placed in the hot water, the air inside the balloon warms. As it warms the particles move faster causing it to expand.
Charles’ Law In Equation form: V1 = V2 T1 T2 Pressure must be held constant! Number of molecules must be the same. Temperature MUST be in Kelvin!
Practice with Charles’ Law In the party store a balloon contains 3200mL of helium gas at 18C (65 F). What is the new volume of the balloon when you take it outside on a hot summer day 38C (100 F)? Assume that the pressure remains constant.
Practice with Charles’ Law If a sample of gas has a volume of 1000 mL when the pressure is 720 mmHg, what is its pressure when the volume has been reduced to 500mL? Assume that the pressure remains constant.
Forced exerted by a gas which is caused by particle colliding with the sides of the container. Units psi (pounds per square inch) atm (atmospheres) mmHg (mm of Mercury) kPa (kiloPascal) Pa (Pascal) Pressure
Temperature Extent of “hotness” or “coldness” of a sample. Temperature MUST be in Kelvin! K = ˚C + 273
Gay Lussac’s Law The pressure of a gas sample at constant volume is directly proportional to its kelvin temperature
Gay Lussac’s Lawexplained using kinetic molecular theory As temperature increases Gas particles move faster If in a rigid container, volume CANNOT change Resulting in more collisions Creating a higher pressure
What can happen to a car tire on a hot summer day, especially if you have traveled for many hundreds of miles?
Gay Lussac’s Law In Equation form: P1 = P2 T1 T2 Volume must be held constant! Number of molecules cannot change. Temperature MUST be in Kelvin!
Practice Gay Lussac’s Law The gas in an aerosol can is at a pressure of 3.0 atm and 25 C. Directions on the can warn the user NOT to keep the can in a place where the temperature can exceed 52C (125F). What would the gas pressure be at 52C that causes the can to explode?
Practice with Boyle’s Law What pressure is required to reduce the volume of a sample of air from 1.0L to .25 L? The original pressure on the sample is 1.5 atm. Assume the temperature of the sample remains constant.