# 27 - Topics for the day Administrative stuff Theories vs....

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Topics for the day Administrative stuff Theories vs. Laws The molecular view of gases Effusion Kinetic Energy Kinetic molecular model of gases Diffusion Administrative stuff Homework ALEKS objective four due now. ALEKS objective Fve due in two weeks time (10 am, 4/12) ALEKS prep homework due midnight tonight. Quest HW3 due ±riday (10 am, 4/2) ALEKS user “Utd 01-15-15-42,Student” who has Fnished (100%) objective #4 already and has O in their gradebook for the Frst three ALEKS objectives. Please let me know who you are so you can get credit for your ALEKS grades. Where were we? We add to our collection of types of problems treatable using the ideal gas law the application to chemical reactions, where reactants and/or products are gaseous. Don’t forget to take the reaction stoichiometry into account. Dalton’s law of partial pressures shows that mixed gases are independent of each other. p Tot = p 1 + p 2 + p 3 + . .. χ i = n i n Tot p Tot = n TOT RT V p i = χ i p Tot The partial pressure of each gas is related to the total pressure by the mole fraction χ . 1 2 3

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If V increases, P increases. B. If T increases, P decreases. C. If n increases, P increases D. All answers are correct. iClicker Time In the ideal gas law PV = nRT, which of the following statements is true? Assume for each answer that the variables not mentioned are held constant. P = P V nRT V 1 P = P T V nRT P = P n V nRT An example of one way science is performed, i.e., the scientifc method , is as follows: Perform experiments, varying some property, keeping all else constant, measuring the effects of that variation. Observe reproducible trends Create a mathematical equation, or law , that Fts the observations Use that law to predict further results Test those predictions ReFne the law if necessary (when results don’t Ft) All the gas laws we’ve spoken about have been based on experimentation. None of these laws explain what actually happens to account for these observations. Theories vs. Laws Although coming up with laws is important, most scientists want to understand WHY nature behaves in this way. To answer this, scientists build models or Theories . In chemistry, theories talk about what happens at the atomic/molecular level ( microscopic ) that could cause the effects we observe via experiment ( macroscopic ). A model is successful if it can explain the observations, derive the laws, and is able to predict correctly the results of future experiments. No model is 100% absolutely perfectly true. We cannot know that. We just Fnd the best model we can, that explains as much of our observations as is possible. The most complete model isn’t always the most useful. Sometimes simple models that use approximations or assumptions are useful to help explain and visualize a process. When we have to make very accurate predictions, then we need to use much more complex models with little to no assumptions or approximations. 4
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## This note was uploaded on 05/02/2010 for the course CH 52365 taught by Professor Mcdevitt during the Spring '09 term at University of Texas.

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27 - Topics for the day Administrative stuff Theories vs....

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