GASES

Chapter Thirteen: The Behavior of Gases 13.1 Gases, Pressure, and the Atmosphere 13.2 The Gas Laws

Chapter 13.1 Learning Goals Describe the composition of Earth’s atmosphere. Make comparisons between Earth’s atmosphere and the atmospheres of other planets. Explain the meaning of atmospheric pressure and describe how it is measured.

Pressure Temperature Relationship Investigation 13A Pressure Temperature Relationship Key Question: How are temperature and pressure of a gas related?

13.1 What’s in Earth’s atmosphere? Nitrogen (N2) gas makes up about 78 percent of Earth’s atmosphere. Nitrogen is released into the air by volcanoes and decaying organisms and is a vital element for living things.

13.1 Comparing atmospheres An atmosphere is a layer of gases surrounding a planet or other body in space.

13.1 Life changed Earth’s atmosphere Over time, photosynthesis breaks down carbon dioxide, uses carbon to build the organism, and releases oxygen into the air.

13.1 Life changed Earth’s atmosphere Earth stores carbon as calcium carbonate so it doesn’t return to the atmosphere. “Fossil fuels” (oil, coal, and natural gas) are carbon from decaying plants and animals in the ground.

13.1 Atmospheric pressure Atmospheric pressure is a measurement of the force of air molecules in the atmosphere at a given altitude. Your ear drum is one way you can detect changes in pressure.

13.1 Pressure in the atmosphere At sea level, the weight of the column of air above a person is about 9,800 newtons (2,200 pounds)! This is equal to the weight of a small car. Why aren’t we crushed by this pressure?

13.1 Measuring Pressure A barometer is an instrument that measures atmospheric pressure. Mercury barometers were common until we discovered their vapors were harmful.

13.1 Measuring Pressure Today we use aneroid barometers. They have an airtight cylinder made of thin metal. The walls of the cylinder respond to changes in pressure.

13.1 Pressure in the atmosphere The gas molecules closest to Earth’s surface are packed together very closely. This means pressure is lower the higher up you go into the atmosphere.

13.1 Pressure changes with altitude

13.1 Units of pressure

Chapter Thirteen: The Behavior of Gases 13.1 Gases, Pressure, and the Atmosphere 13.2 The Gas Laws

Chapter 13.2 Learning Goals Explain how pressure, temperature, volume, and the number of molecules in a gas are related when one or more of these factors in held constant. Apply the gas laws to solve problems. Analyze graphs of data to explain how factors like pressure and volume are related.

Boyle’s Law Investigation 13B Key Question: How are pressure and volume of a gas related?

13.2 Boyle’s Law When you squeeze a fixed quantity of gas into a smaller volume the pressure goes up. This rule is known as Boyle’s law.

Solving Problems A kit used to fix flat tires consists of an aerosol can containing compressed air and a patch to seal the hole in the tire. Suppose 5 liters of air at atmospheric pressure (1 atm) is compressed into a 0.5 liter aerosol can. What is the pressure of the compressed air in the can? Assume no change in temperature or mass.

Looking for: Given Relationships: Solution Solving Problems …final pressure in atmospheres (P2) Given …V1 = 5 L , P1= 1 atm, V2 = .5 L Relationships: Boyle’s Law: P1V1 = P2V2 Solution Rearrange equation so P2 = P1V1 / V2 P2 = 1atm x 5.0 L/ 0.5 L = 10 atm.

13.2 Pressure and Temperature The pressure of a gas is affected by temperature changes. If the mass and volume are kept constant, the pressure goes up when the temperature goes up, and down when the temperature goes down.

13.2 Gay-Lussac’s Law

13.2 Pressure and Temperature Any time we apply gas laws, the the temperature must be in Kelvins. Only the Kelvin scale starts from absolute zero, when energy of particles is theoretically zero.

13.2 Charles’ Law According to Charles’ law, the volume of a gas increases with increasing temperature. Volume decreases with decreasing temperature.

13.2 Charles’ Law A hot-air balloon floats because the air inside is less dense than the air outside.

Solving Problems A can of hair spray has a pressure of 300 psi at room temperature 21°C. The can is accidentally moved too close to a fire and its temperature increases to 295°C. What is the final pressure in the can? (Round answer to nearest whole number.)

Solving Problems Looking for: Given Relationships: Solution …final pressure in atmospheres (P2) Given …P1= 300 atm, T1 = 21 C , T2 = 295 C Relationships: Convert temps using K = C + 273 Charles’ Law: P1/T1 = P2/T2 Solution Rearrange equation so P2 = P1xT2 / T1 P2 = 300 atm. x 568K / 294K = 580 atm.

Charles’ Law Investigation 13C Key Question: What is the relationship between the volume and temperature of a gas?

Up, Up and Away Airplanes use air pressure in many different ways. Air pressure is used to keep an aircraft in the air as well as to propel it forward. Flight instruments use air pressure to measure altitude, air speed, and vertical speed. Finally, a pressurized airplane uses air pressure to maintain appropriate pressure for human survival in its cabin.