WebLecture: Gases in Chemical Reactions
Kotz and Triechel, Chemistry and Chemical Reactivity Chapter 10: Sections 4-8
- 10.4: We can use gas laws to predict the outcome of chemical reactions, for example, the volume of gas released if known amounts of reactances are combined in a gas-producing reaction.
- 10.5: In an ideal world, individual gas samples each contribute to the pressure of mixture without affecting each other. We can figure out partial pressures (the pressure exerting by one gas) by assigning to each gas a fraction of pressure based on the fraction of its molecules to the total sample. If we have 2 moles of hydrogen gas mixed with 1 mole of oxygen gas, hydrogen will be responsible for 2/3 of the pressure exerted by the gas sample.
- 10.6: We can relate the temperature of a gas to the average kinetic energy of the molecules of gas using the average velocity of all of the molecules. The value û is the average velocity (this is usually u with a bar over it; a circumflex is as close as I can get in unicode). Average Kinetic Energy then is 1/2 the average mass (for a gas, this will be the exact mass for all molecules) times the square of the average velocity:
KE = 1/2 mû2 = 3/2 RT
The quantity û2 is the mean square speed (we use speed rather than the vector quantity velocity to emphasize that we are not concerned with direction).
The root-mean-square of the average velocity (now independent of direction) can be related to the temperature of the gas as
√û2 = √3RT/M.
While all gases have the same KE at the same temperature, because the molar mass of the gas M varies, the average velocity will vary. The heavier the gas, the lower the average velocity at the same temperature.
- 10.7: Diffusion is the dispersal of gases over time to uniformly fill a volume. Effusion is the dispersal of a gas from one container through a narrow opening into another. Since diffusion rates depend on root mean square speed, which in turn is dependent on the mass of the molecules, differences in rates of effusion for two different gases at the same temperature and pressure depends only on the molar mass, a relationship known as Graham's Law:
|Rate of effusion of gas 1
Rate of effusion of gas 2
- 10.8: We have to compensate the "ideal" situation described by PV = nRT for real gases. We recognize that pressure will be affected by intermolecular forces -- the fact that mass attracts mass and electrical fields can attract or repel, depending on charges. This means that collisions are not perfectly elastic, so our observed pressure will be somewhat less than the predicted ideal temperature. We also realize that gas particles do occupy volume in reality, so the space available to a gas has to be slightly diminised by the space occupied by the invidividual molecules. We compensate for deviations from the ideal gas law by using the van der Waals equation:
|( P + a [n/V]2)||(V - bn) ||= ||nRT
So in general, we need to increase observed pressure slightly and decrease observed volume slightly to match the values predicted by the ideal gas law.
Videos for Chapter 10: Gases and their Properties
Review the Videos at Thinkwell Video Lessons.
- Under "Gases: The Ideal Gas Law and Kinetic-Molecular Theory of Gases"
- The Ideal Gas Law
- Partial Pressure and Dalton's Law
- Applications of the Gas Laws
- The Kinetic-Molecular Theory of Gases
- Under "Gases: Molecular Motion of Gases
- Molecular Speeds
- Effusion and Diffusion
- Under "Gases: Behavior of Real Gases
- Comparing Real and Ideal Gases
Homework problems: See your Moodle assignment!
AP #8 GUIDED INQUIRY — Measuring the deviation of real gases from the ideal gas law — Phase II
Carry out your procedure to collect data and formulate a prediction for gas behavior, including behavior near absolute zero. Identify test cases to use for validating your predictions.
- IGHCE Lab 14.1 OR HSCKM VIII-1: Volume-Pressure relationships (Boyle's Law)
- IGHCE Lab 14.2 OR HSCKM VIII-2: Volume-Temperature relationships (Charles' Law)
- IGHCE Lab 14.3 Pressure-Temperature relationship (Gay-Lussac's Law)
- Alternate Labs (two): Gas Volumes and Gas Generation
OnLine Quiz: Take the Chapter 10 Quiz from the Moodle Chemistry page within the next two weeks.
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