Chapter 11: Gases

1. Chapter 11:Gases Coach Kelsoe Chemistry Pages 360–389

2. Section 11–1:Gases and Pressure Coach Kelsoe Chemistry Pages 361–367

3. Section 11–1 Objectives • Define pressure and relate it to force. • Describe how pressure is measured. • Convert units of pressure. • State the standard conditions of temperature and pressure.

4. Pressure • If you have a one-liter bottle of air, there’s really no way to know how much air you have in there unless you know the conditions inside the bottle. • To describe a gas fully, you need to state four measurable quantities: • Volume • Temperature • Number of molecules • Pressure

5. Pressure and Force • If you blow air into a rubber balloon, it will increase in size. The increase in volume is caused by the collisions of molecules of air with the inside walls of the balloon. The collisions cause an outward push, or force, against the inside walls. • Pressure (P) is defined as the force per area on a surface, or P = F/A.

6. Pressure and Force • The SI unit for force is the newton, abbreviated N. It is the force that will increase the speed of a one kilogram mass by one meter per second each second it is applied. • At Earth’s surface, each kilogram of mass exerts 9.8 N of force, due to gravity. • Force on Earth can be found by multiplying an object’s mass by 9.8.

7. Pressure and Force • For example, if a person’s mass is 51 kg, then the amount of force they apply on the Earth’s surface is 500 N. No matter how a person stands, he or she will exert 500 N of force on Earth. • However, a person can change the amount of pressure they apply to the earth’s surface by changing the area in contact with the ground – by standing on their tiptoes.

8. Pressure and Force • Let’s say this 500 N person stands flat-footed. The area of his feet is equal to 100 cm2. The pressure he applies to the ground is 5 N/cm2. • If this person stood on his tiptoes, and the area of his tiptoes was 10 cm2, then the pressure he applies to the ground is 50 N/cm2. • This is how the “bed of nails” trick works, and the secret of karate.

9. Pressure and Force • Gas molecules exert pressure on any surface with which they collide. • The pressure exerted by a gas depends on volume, temperature, and the number of molecules present. • Even the atmosphere exerts pressure. At sea level, the pressure is 10.1 N/cm2. This pressure can be thought of as caused by the weight of the gases that compose the atmosphere.

10. Pressure and Force • To understand gas pressure and it’s magnitude, let’s think about an empty can. The can does contain a small amount of air. Let’s say the can has a total area of 1700 cm2. If the can is not expanding or contracting, then the pressure inside must be the same as the outside. If Earth’s pressure is 10.1 N/cm2, then the can’s pressure is 10.1 N/cm2. You can find the force of the air inside the can mathematically.

11. Measuring Pressure • A barometer is a device used to measure atmospheric pressure. • The first barometer was introduced by Evangelista Torricelli in the early 17th century. • Through Torricelli’s experiments, it is known that at sea level at 0°C, the height of mercury in a barometer will be 760 millimeters high.

12. Units of Pressure • A number of different units are used to measure pressure: • Pascal • mm Hg • Since atmospheric pressure is often measured by a mercury barometer, a common unit of measure is millimeters of mercury (mm Hg). • Torr • Atmosphere

13. Units of Measure • In honor of Torricelli, 1 mm Hg is now called 1 torr. The average atmospheric pressure at sea level at 0°C is 760 torr. • Pressures are often measured in units of atmospheres. One atmosphere of pressure (atm) is defined as being exactly equivalent to 760 torr.

14. Units of Measure • In SI, pressure is expressed in derived units called pascals, after Blaise Pascal, a French mathematician and philosopher who studied pressure during the 1600s. • One pascal (Pa) is defined as the pressure exerted by a force of one newton acting on an area of one square meter. It is often more convenient to express pressure in terms of kilopascals. • The standard atmosphere is equal to 1.01325 x 105 Pa, or 101.325 kPa.

15. Units of Measure

16. Standard Temperature and Pressure • To compare volumes of gases, it is necessary to know the temperature and pressure at which the volumes are measured. • For purposes of comparison, scientists have agreed on standard conditions of exactly 1 atm pressure and 0°C. These conditions are called standard temperature and pressure and commonly abbreviated as STP.

17. Sample Problem 10-1 • The average atmospheric pressure in Denver, Colorado, is 0.830 atm. Express this pressure (a) in mm Hg and (b) in kPa. • Given: P of atmosphere = 0.830 atm, 760 mm Hg = 1 atm, 101.325 kPa = 1 atm • 0.830 atm x 760 mm Hg/atm = 631 mm Hg • 0.830 atm x 101.325 kPa/atm = 84.1 kPa

18. Vocabulary • Atmosphere of pressure • Barometer • Millimeters of mercury • Newton • Pascal • Pressure • Standard temperature and pressure • Torr

19. Conversion Factors • 1 torr = 0.1333 kPa • 1 kPa = 7.5 torr • 1 atm = 101.325 kPa • 1 kPa = 0.00987 atm • 1 atm = 760 torr • 1 torr = 0.001316 atm