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## Pressure – Volume Relationship

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**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)**How are pressure and volume related when temperature is held**constant?**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**How are volume and temperature related when pressure is held**constant?**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**Why does the label on this can warn the user to not put it**in a fire?**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.