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30 January 2012

- Objective: You will be able to:
- review and discuss answers to the midterm exam.

Agenda

- Objective and Agenda
- Opportunities
- Midterm exam answers and discussion

Homework: Read p. 174-194

Tomorrow: Gases!

Field Trip Opportunity

- Novartis is a biotech company in Cambridge that aims to discover, develop and successfully market innovative products to prevent and cure diseases, to ease suffering and to enhance the quality of life.
- Tour, learn about what scientists at Novartis does, learn about biotech careers
- Wednesday, February 15 periods 1-4

Harvard Museum of Natural History

- Exhibit: The Language of Color
- Why do we see the colors we see?
- Why does a blue butterfly look blue?
- Why does a blue fish look blue?

31 January 2012

- Objective: You will be able to:
- describe and convert to standard units for temperature and pressure
- use the Gas Laws to make calculations for pressure, temperature, volume and number of moles of a gas.

Gases Topics

- Definitions
- The Gas Laws
- The Ideal Gas Equation
- Gas Mixtures and Partial Pressures
- Kinetic Molecular Theory of Gases
- Effusion and Diffusion
- Real Gases Deviate from Ideal Behavior

Definitions

- Pressure: the force that acts on a given area.
- Gases exert a pressure on any surface they contact.

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/gasesv6.swf

- Standard pressure (at sea level):

1.01x105Pascals = 101 kPa = 1 atm= 760 mm Hg = 760 torr

The Gas Laws

- Relate the pressure (P, atm), volume (V, liters) and temperature (T, Kelvin) of a sample of gas
- http://phet.colorado.edu/en/simulation/gas-properties

If you hold T constant…

Boyle’s Law: P1V1=P2V2

Boyle’s Law

- If the pressure of a fixed amount of gas doubles at a constant temperature, what happens to its volume?

If you hold P constant…

- Charles’ Law:

Charles’ Law

- When the temperature of a fixed amount of gas doubles from 20oC to 40oC, what happens to the volume at constant pressure?

Avogadro’s Hypothesis

- Equal volumes of gases under the same conditions of temperature and pressure contain equal numbers of molecules
- a mole of gas at STP has a volume of 22.4 L

Avogadro’s Law

- The volume of a gas at constant temperature and pressure is directly proportional to the number of moles of the gas
- The coefficients in a balanced equation can be taken as ratios of moles or liters at constant T and P

Example 1

- How many liters of water vapor can be obtained from the complete combustion of 24 liters of methane gas? Assume constant temperature and pressure and water in the gas phase.

Example 2

- A weather balloon on the ground contains 25.8 L of He at 29.0oC and 741 torr. What is the volume of the balloon when it rises to an altitude where the temperature is 11.0oC and the pressure is 535 torr?

Problem 1

- The gas pressure in an aerosol can is 1.5 atm at 25oC. Assuming that the gas inside is an ideal gas, what would the pressure be if the can were heated to 450oC.

Problem 2

- A large natural gas storage tank is arranged so that the pressure is maintained at 2.20 atm. On a cold day in December when the temperature is -15oC (4oF), the volume of gas in the tank is 3.25x103 m3. What is the volume of the same quantity of gas on a warm July day when the temperature is 31oC (88oF)?

ideal gas: a hypothetical gas whose pressure, volume and temperature behavior is completely described by the ideal gas equation

- no gas is an ideal gas
- all gases are “real” gases
- but most gases, above 25oC and below 1 atm, behave like ideal gases

1 February 2012

- Take Out Homework
- Objective: You will be able to:
- use the ideal gas equation to solve for the pressure, volume, temperature or number of moles of a gas.
- Homework Quiz: The volume of a gas is 5.80 L, measured at 1.00 atm. What is the pressure of the gas in mmHg if the volume is changed to 9.65 L? Temperature remains constant.

Agenda

- Homework Quiz
- Homework answers
- Using the Ideal Gas Equation
- Partial Pressure
- Molar volume of gas lab introduction

Homework: p. 217 #35, 37, 39, 41, 51, 53 – Mon., read lab and do pre-lab questions in lab notebook - tomorrow

Using the Ideal Gas Equation

Example 1

Gasoline is a mixture of many hydrocarbon compounds but its chemical formula can be approximated as C8H18. How many liters of carbon dioxide gas are formed at 25.0oC and 712 torr when 1.00 gallon of liquid gasoline is burned in excess air? Liquid gasoline has a density of 0.690 g/mL. One gallon = 3.80 L

Problem 3

- Calcium carbonate, CaCO3(s), decomposes upon heating to give CaO(s) and CO2(g). A sample of CaCO3 is decomposed, and the carbon dioxide is collected in a 250-mL flask. After the decomposition is complete, the gas has a pressure of 1.3 atm at a temperature of 31oC. How many moles of CO2 were generated?

Problem 4

- Tennis balls are usually filled with air or N2 gas to a pressure above atmospheric pressure to increase their “bounce.” If a particular tennis ball has a volume of 144 cm3 and contains 0.33 g of N2 gas, what is the pressure inside the ball at 24oC?

6 February 2012

- Objective: You will be able to:
- calculate the density of a gas
- describe kinetic molecular theory
- define effusion and diffusion
- Homework Quiz: Methane, CH4, the principal component of natural gas, is used for heating and cooking. If 10.0 moles of methane are reacted, what is the volume of CO2, in liters, produced at 23.0oC and 0.985 atm?

Agenda

- Homework Quiz
- Homework answers
- Density calculations
- Partial pressures
- Kinetic-Molecular theory
- Effusion and Diffusion

Homework: p. 218 #63, 65, 67, 69, 71, 93, 95

Example

What is the density of sulfur dioxide gas at 35oC and 1270 torr?

Problem 5

- What is the density of carbon tetrachloride vapor at 714 torr and 125oC?

Problem 6

A series of measurements are made to determine the molar mass of an unknown gas. First, a large flask is evacuated and found to weight 134.567 g. It is then filled with the gas to a pressure of 735 torr at 31oC and reweighed. Its mass is now 137.456 g. Finally, the flask is filled with water at 31oC and found to weigh 1067.9 g. Calculate the molar mass of the unknown gas.

(The density of the water at this temperature is 0.997 g/mL)

Gas Mixtures and Partial Pressures

- partial pressure: the pressure exerted by an individual gas in a mixture of gases
- Dalton’s Law of Partial Pressures: the total pressure of a mixture of gases is the sum of the pressures that each gas would exert if it were present alone.

Ptotal = P1 + P2 + P3 + …

mole fraction: the ratio of moles of one gas to the total moles of gas in a mixture

X1 = n1/nt

Each gas in a mixture behaves independently of all the other gases in the mixture and obeys the ideal gas law.

Example 1

A mixture of 9.00 g oxygen, 18.0 g argon and 25.0 g of carbon dioxide exerts a pressure of 2.54 atm. What is the partial pressure of argon in the mixture?

2 February 2012

- Objective: You will be able to:
- determine the molar volume of a gas by generating and measuring hydrogen gas, and using the gas laws.
- Do now: Grab a pair of goggles

Agenda

- Objective
- Pre-lab questions
- Lab directions
- Do the lab!
- Collect and analyze data
- Post lab questions

Homework: Book work assigned yesterday

Lab notebook: due Monday

Molar Volume of a Gas Lab

- Weight your 3 cm piece of magnesium instead of calculating the mass.
- Follow the directions carefully!
- Leave the HCl at the front – bring your graduated cylinder.
- Use the 1000 mL graduated cylinder for step 15. We only have one. Take turns.
- Barometric pressure = 30.03 in Hg now

p. 7 #7:

- graduated cylinder eudiometer

Your lab notebook

- Name, etc. on top.
- Purpose
- Pre-lab questions
- Procedure summary
- Data organized into a table or along with procedure
- Calculations, including units and labels describing what the calculation is!
- Answer to post lab questions
- One more source of error and specifically how it affects your data. How would you correct it?

Problem 7

- A gaseous mixture made from 6.00 g O2 and 9.00 g CH4 is placed in a 15.0 L vessel at 0oC. What is the partial pressure of each gas, and what is the total pressure in the vessel?

7 February 2012

- Objective: You will be able to:
- describe the Kinetic Molecular theory of gases
- describe effusion and diffusion and calculate effusion rate
- Homework Quiz:

a. What is the mass of the solid NH4Cl formed when 73.0 g of NH3 gas are mixed with an equal mass of HCl gas?

b. What is the volume of the gas remaining, measured at 14.0oC and 752 mmHg? Which gas is it (NH3 or HCl)?

Problem 8

- What is the total pressure exerted by a mixture of 2.00 g of H2 and 8.00 g of N2 at 273 K in a 10.0 L vessel?

Problem 9

- A study of the effects of certain gases on plant growth requires a synthetic atmosphere composed of 1.5 mol percent CO2, 18.0 mol percent O2 and 80.5 mol percent Ar.
- Calculate the partial pressure of O2 in the mixture if the total pressure of the atmosphere is 745 torr.
- If this atmosphere is to be held in a 121 L space at 295 K, how many moles of O2 are needed?

Kinetic Molecular Theory

- a model that explains the macroscopic behavior of gases at the atomic and molecular level.
- Gases consist of atoms or molecules in continuous, random motion
- The volume of the gas particles is negligible relative to the volume of their container
- Attractive and repulsive forces between gas particles are negligible.
- Collisions between gas particles and between gas particles and the walls of their container are perfectly elastic (no energy is lost)
- The average kinetic energy of the particles is proportional to the absolute temperature.

Problem 10

- Use kinetic molecular theory to explain why, when the temperature of a fixed volume of gas increases, the pressure increases.

Problem 11

- A sample of O2 gas initially at STP is compressed into a smaller volume at constant temperature. What effect does this change have on:
- the average kinetic energy of O2 molecules?
- the average speed of O2 molecules?
- the total number of collisions of O2 molecules with the container walls in a unit time?
- the number of collisions of O2 molecules with a unit area of container wall per unit time?

8 February 2012

- Objective: You will be able to:
- describe effusion and diffusion and calculate effusion rate.
- Homework Quiz:
- A mixture of gases contains 4.46 moles of neon and 0.74 moles of argon. Calculate the partial pressures of the gases if the total pressure is 2.00 atm at a certain temperature.
- Calculate the volume occupied by the argon gas at 0oC.

Agenda

- Homework quiz
- Homework answers
- Effusion and Diffusion
- Diffusion demo and video

Homework: Gases problem set: Mon.

Effusion and Diffusion

- Effusion: the escape of gas molecules through a tiny hole into an evacuated space.
- Diffusion: the spread of one substance through space or through another substance.
- Graham’s Law of Effusion: the effusion rate of a gas is inversely proportional to the square root of its molar mass.
- the equation relates the ratio of effusion rates of two gases:

Example

- At a particular temperature and pressure, neon gas effuses at a rate of 16.0 mol/s.
- What is the rate at which argon effuses under the same conditions?
- Under a different set of conditions, 3.0 mol of argon effuse in 49.0 seconds. How long will it take an equal amount of helium to effuse?

9 February 2012

- Objective: You will be able to practice gas unit objectives and calculations.
- Homework Quiz: A gas of unknown molecular mass was allowed to effuse through a small opening under 2.0 atm and 20oC. It required 105 s for 1.0 L of the gas to effuse. Under identical conditions, it required 31 s for the same amount of O2 gas to effuse. Calculate the molar mass of the unknown gas.

Problem 12

- An unknown gas composed of homonuclear diatomic molecules effuses at a rate that is only 0.355 times that of O2 at the same temperature. Calculate the molar mass of the unknown and identify it.

Real Gases: Deviations from Ideal Behavior

- Real gas particles have volume!
- Real gas particles have attractive and repulsive forces between them!
- Especially at high pressure and low temperature, real gases do not be have ideally.
- Deviation from ideal behavior increases with increasing complexity and mass of a molecule of a gas.

Problem 13

- Which of the noble gases will deviate most from ideal behavior? Why?
- Arrange the following in order of increasing deviation from ideality:

H2O(g), CH4(g), Ne

Justify your answer

Problem Set Work Time

- 30 minutes: independently
- Then, with a partner
- Why?
- If you’ve never tried problems independently before a test…
- The fewer brains, the more yours works

Homework

- Problem Set: due Monday
- Gases Unit Test Tuesday
- 10 MC
- 1 multi-part free response question
- Review session: tomorrow 3:20-4:15 in 205

14 February 2012

- Take Out: Periodic table and calculator
- Objective: You will be able to:
- show what you know about gases!

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