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## Gases

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**Gases**Chapter 12**Gases**• Properties: • Gases are fluids because their molecules/atoms can flow • Gases have low density • Highly compressible – their volume can be reduced • Gases will completely fill their container**Gases**• Gas Pressure • In the atmosphere, gas molecules collide with Earth’s surface to create atmospheric pressure.**Gases**• As you descend toward earth, the atmosphere is denser and the pressure is higher • When flying your ears may “Pop” due to the change in pressure**Gases**• Standard Temperature and Pressure • Short form: STP • Scientists have specified a set of standard conditions when studying the effects of changing temperature and pressure • Stands for:: • 0 degrees C • 1 atm**Gases**• How to convert between pressure units Use dimensional analysis Ex: Convert the pressure of 1.000 atm to mm of mercury Conversion factor: 101325 Pa and 1 mm Hg 1 atm 133.322 Pa Calculation: 1.000 atm x 101325 Pa x 1 mm Hg = 760 mm Hg 1 atm 133.322 Pa**Gases**• Practice • The critical pressure of carbon dioxide is 72.7 atm. What is this value in units of pascals? • The vapor pressure of water at 50 deg C is 12.33kPa. What is this value in mm of Hg?**Gases – Kinetic Molecular Theory**• A model used to predict gas behavior • Using the following animation, we will determine the main points of the theory • http://www2.biglobe.ne.jp/~norimari/science/JavaApp/Mole/e-gas.html • http://www.chm.davidson.edu/ChemistryApplets/KineticMolecularTheory/PT.html**Gases**• Pressure-Volume Relationships Boyle’s Law Based on the following facts: 1. Gases can be compressed 2. Gases exert pressure Boyle found that: As volume decreases, the concentration, and therefore the pressure, increases**Gases**As volume decreases, the concentration, and therefore the pressure, increases**Gases**• Boyle’s Law States: • For a fixed amount of gas at a constant temperature: as the volume of the gas decreases the pressure increases • We can use the following equation to calculate changes in pressure or volume**Gases**• A graph of this equation looks like:**Boyle’s Law**http://www.grc.nasa.gov/WWW/K-12/airplane/aboyle.html Animation of Boyle’s Law**Gases**• Boyle’s Law • Example: • A given sample of gas occupies 523 mL at 1.00 atm. The pressure is increased to 1.97 atm, while the temperature remains the same. What is the new volume of gas?**Gases**• Temperature-Volume Relationships • Charles’ Law • For a fixed amount of gas at a constant pressure, the volume of a gas increases as the temperature of the gas increases**For a fixed amount of gas at a constant pressure, the volume**of a gas increases as the temperature of the gas increases**Charles’ Law Formula:**Remember! ALWAYS USE KELVIN when dealing with temperature in gas laws! All °C temperatures MUST be converted to Kelvin!!!**Charles’**• http://wright.nasa.gov/airplane/aglussac.html Animation of Charles’ Law**Gases**• Example: • A balloon is inflated to 665 mL volume at 27 deg C. It is immersed in a dry-ice bath at -78.5 deg C. What is its new volume, assuming the pressure remains constant?**Gases**• Temperature-Pressure Relationships • Gay-Lussac’s Law: • The pressure of a gas at a constant volume is directly proportional to the absolute temperature (temperature in Kelvin)**Gay-Lussac’s Law Formula:**At constant volume: P1 = P2 T1 T2**Gay-Lussac’s Law**• http://www.marymount.k12.ny.us/marynet/06stwbwrk/06gas/1amcslussac/amcsgaylussac.html • Animation of Gay-Lussac’s Law**Gases – Avogadro’s Law**• In 1811, Avogadro proposed that equal volumes of all gases, under the same conditions, have the same number of particles. • http://www.marymount.k12.ny.us/marynet/06stwbwrk/06gas/2lpimavogdro/2lpimavogadro.html**Gases – Avogadro’s Law**• We know volume of a gas can change with temperature and pressure, but what about the number of molecules? • Through Avogadro’s observations, the following has been defined: • 1 mole of any gas at STP (0°C and 1 atm) occupies 22.41 L • The mass of 22.41L at STP is the Molecular Mass of the gas**Gases - Combined Gas Law**• When you take Boyles, Charles’, Gay-Lussac’s, and Avogadro’s Laws and combine them, you get the COMBINED GAS LAW This law is used to solve problems where pressure, volume and temperature of a gas vary with a constant molar quantity of the gas**Gases – Combined Gas Law**• Example: • A sample of hydrogen gas has a volume of 65.0 mL at a pressure of 0.992 atm and a temperature of 16 deg C. What volume will be hydrogen occurpy at 0.984 atm and 25 deg C?**Gases – Dalton’s Law of Partial Pressures**• John Dalton showed that in a mixture of gases, each gas exerts a certain pressure as if it were alone with no other gases with it. • This is called “partial pressure”