CHEM1312 Gas Behavior Pressure Laws PDF

Preview

Summary

CHEM 1312 homework handout Lamar University Fall 2015 semester...

Description

CHEM 1312

Fall 2015

Page 1

Activity 2. Gas Behavior, Gas Pressure, and Gas Laws Why? When making any calculations regarding gases, the ideal gas law can make things easy to figure out, it is the "Swiss Army Knife" of gas calculations. Its application is critical to anyone who works with gases, that would be many scientists, most engineers, and anyone planning on taking the MCAT, DAT, PCAT, etc. Team Members (Each team member should print and sign that person's own name.) Captain:_____________________________________________________________________(first to sign-in) Contributes by: actively participating as a learner, keeping team on task, assuring that all team members participate and understand, making certain each team member is performing within their role. Member 2:________________________________________________________________(second to sign-in) Contributes by: actively participating as a learner, assessing the performance and interactions among team members, asking and answering questions for the team. Member 3:__________________________________________________________________(third to sign-in) Contributes by: actively participating as a learner, identifies strategies and methods for problem solving, identifies what the team is doing well and what needs improvement in consultation with the others. Member 4:________________________________________________________________(fourth to sign-in) Contributes by: actively participating as a learner, questioning and checking the assumptions made by the team, being constructive in helping the team improve its performance. The recorder is responsible for completing this document. Learning Objectives ● To describe gas pressure. ● To describe how gas pressure is measured. ● To describe and apply the named gas laws. ● To explain the ideal gas constant and its units. ● To describe and use the ideal gas law. Resources ● Read section(s) in your textbook about gas behavior, gas pressure measurement, the named gas laws, and the ideal gas law. Planning Ahead ● Read section(s) in your textbook about gas property calculations, Dalton's Law, and calculations using partial pressures. Expectations ● Problems should show all work with units. ● Questions should be answered in complete sentences.

CHEM 1312

Fall 2015

Page 2

Information - Gas Pressure The English unit for gas pressure is pound per square inch (psi). That measure is a reading of how much force a gas exerts on each square inch of surface area of its container. At sea level standard air pressure is 14.7 psi, meaning each square inch of surface has 14.7 pounds of force exerted upon it by the atmospheric gases. This occurs because the gas molecules are held near the earth by gravity. Considering a column of air above the square, the air in that one inch square column has a total weight of 14.7 pounds, since we are essentially at sea level. Key Question 1. Considering a column of air above one square inch of a surface at sea level, the air in that one inch square column must have total weight of how many pounds, kilograms? (1 lb = 0.4535 kg)

Problem 2. What is the total weight in pounds of air resting on this 8.5 in x 11 in rectangle of paper (assuming it is at sea level)?

Key Question 3. The height of the atmosphere is fairly consistent measured from the center of the earth, r. If the mass of a 1 in2 column of air is 14.7 pounds from sea level to the top of the atmosphere (A), what should happen to the mass is air in a 1 in2 column as you go higher elevations (B)? What effect should that have on the measured atmospheric pressure?

Information - Measurement of Atmospheric Pressure The device used to measure atmospheric pressure is called a barometer. There is a simple diagram of such a device to the right. The important point about a barometer, is that air pressure can support a column of a liquid in a closed tube which contains no gas (a vacuum). The height of the column of liquid is usually measured in inches or millimeters mercury that the air pressure will support. At standard pressure air supports 760.0 mm Hg. In chemistry we define 760.0 mm Hg = 1 atmosphere (atm). Evangelista Torricelli discovered that air pressure could be measured this way and is honored with the unit torr where, 1 torr = 1 mm Hg. Key Questions 4. a. What should happen to the height, H, of the column of mercury if the atmospheric pressure decreases?

b. Consider a less dense liquid than mercury is used to make a barometer. Compared to the column height of mercury at the same pressure, the less dense liquid column height should?

Problem 5. If the atmospheric pressure in Denver is 550 mm Hg, what is that pressure in atmospheres?

CHEM 1312

Fall 2015

Page 3

Information - Torr Unit of Pressure Evangelista Torricelli discovered that air pressure could be measured using a mercury barometer and is honored with the unit torr which is defined as 1 torr = 1 mm Hg. Problem 6. What is the pressure in torr if the measured pressure is 0.458 atm?

Information - Measurement of Gas Pressure The device is used to measure the pressure of a trapped gas is called a manometer. The pressure of the trapped gas can be determined by adding or subtracting the level difference, H, to or from the atmospheric pressure, Pa as appropriate. A diagram of such a device shown to the right. Key Question 7. a. When the trapped gas pressure, Pg, is greater than the atmospheric pressure, the equation for the trapped gas pressure will be?

b. When the trapped gas pressure is less than the atmospheric pressure, the equation for the trapped gas pressure will be?

c. When the trapped gas pressure is equal to the atmospheric pressure, the equation for the trapped gas pressure will be?

Information - Gas Laws There are several gas laws that describe the observations your team made in the examples above. 1. Boyle's Law states that as gas pressure changes, the gas volume changes inversely - if the container is occupied by a constant number of moles of gas at a constant temperature. (PV= k1) 2. Charles's Law states that as gas temperature changes the gas volume will change proportionally - if the gas pressure and number of moles of gas remains constant. (V/T = k2) 3. Avogadro's Law states that as number of moles of gas changes the gas volume proportionally - if the gas pressure and the gas temperature are constant. (V/n = k3) 4. Gay-Lussac's law state that the pressure exerted by a gas on its container is proportional to the absolute temperature of the gas when the gas volume and number of moles of gas are held constant . (P/T = k4) NOTE: The subscripts are added to this information to denote that the constants are different values. Key Question 8. a. A rigid container is held at a constant temperature while gas molecules are added to it, what happens to the gas pressure inside of that container? Which of the gas laws above applies?

CHEM 1312

Fall 2015

Page 4

b. A rigid container holding a fixed number of molecules is heated, what happens to the gas pressure inside of that container? Which of the gas laws above applies?

c. An expandable container (a piston in a cylinder) holds a fixed number of gas molecules at a constant temperature. What happens to the gas pressure inside of the container when the volume of the container is increased (by pulling the piston out)? Which of the gas laws above applies?

d. An expandable container (a piston in a cylinder) holds a fixed number of gas molecules at a constant pressure. What happens to the volume of the gas as its temperature is reduced? Which of the gas laws above applies?

e. What happens to the volume of an expandable container (a piston in a cylinder) when more molecules of gas are added at fixed temperature and pressure? Which of the gas laws above applies?

9. What do all of the gas laws have in common in addition to the four variables: pressure, volume, number of moles, and temperature?

Information - Ideal Gas Law Boyle's, Charles's, Avogadro's, and Gay-Lussac's laws can be combined in one neat, easy to use equation called the ideal gas law: PV = nRT. NOTE: All gas calculations must be made using temperature in Kelvin. Problem 10. At standard temperature and pressure of 1.0000 atm and 0.0000 ºC, 1.0000 mol of ideal gas occupies 22.4141 L volume, what is the value of the gas constant, R? Don't leave out the units!

Key Question 11. Why must the temperature used in all gas calculations in units of Kelvin rather than Celsius or Fahrenheit? (Hint: would R be the same above and below the freezing point temperature of water?)

CHEM 1312

Fall 2015

Page 5

Problems 12. What number of moles of gas occupies a volume of 10.0 L, at 1.21 atm pressure, and 25.4 ºC temperature? Standard values of R can be looked up in a textbook or online. (Hint: PV = nRT)

13. What is the temperature of 23.4 moles of a gas occupying 2.5 L at 100.0 atm pressure? (Hint: PV = nRT)...