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Plan for Mon, 6 Oct 08

Plan for Mon, 6 Oct 08. New schedule and practice problems for Ch 5, 6, 10 Exp 1 post-lab Typo in Exp 2 pre-lab Lecture Pressure (5.1) Gas laws of Boyle, Charles, and Avogadro (5.2) The ideal gas law ( 5.3) Exam 1 returned. Pressure. Pressure is force per unit area:

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Plan for Mon, 6 Oct 08

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  1. Plan for Mon, 6 Oct 08 • New schedule and practice problems for Ch 5, 6, 10 • Exp 1 post-lab • Typo in Exp 2 pre-lab • Lecture • Pressure (5.1) • Gas laws of Boyle, Charles, and Avogadro (5.2) • The ideal gas law ( 5.3) • Exam 1 returned

  2. Pressure • Pressure is force per unit area: • In SI units, pressure is expressed in newtons per square meter, N/m2, the pascal (Pa). 1 Pa = 1 N/m2

  3. Pressure Example • What is the pressure that one penny exerts on a table? The penny has a radius of 9.3 mm, and weighs 2.5 g. • We need to find the force that the penny exerts, and the area over which it exerts that force. • Force due to the penny: F = (mass) x (accel. due to gravity) = (2.5 x 10-3 kg) x (9.81 m/s2) = 2.5 x 10-2 kg m/s2 = 2.5 x 10-2 N

  4. Pressure Example (cont) • Cross-sectional area of the penny: A = p(radius)2 = p(9.3 x 10-3 m)2 = 2.7 x 10-4 m2 • Therefore the pressure due to the penny is: • The pascal is actually a pretty puny unit, so more often the kilopascal (kPa) is used.

  5. How do we measure the force of a gas?

  6. Barometer Closed Open to atmosphere Vacuum 1 atm 1 atm 1 atm 1 atm Petrucci pg. 177

  7. Making a Mercury Barometer A dish is filled with mercury. A long glass tube – closed at one end – is also filled with mercury.

  8. Making a Mercury Barometer Air bubbles are removed from the mercury in the tube. Mercury is added until the tube is completely filled.

  9. Making a Mercury Barometer Vacuum The tube is carefully upended and placed in the dish of mercury, so that none of the Hg in the tube falls out. Some portion of the mercury will then flow out of the tube, until the pressure of the Hg is equal to the atmospheric pressure on the Hg in the dish.

  10. Making a Mercury Barometer Measuring the height of the Hg column will tell you what the atmospheric pressure is. Units: “mm Hg”(milimeters of mercury) Also known as “torr” after Evangelista Torricelli, inventer of the barometer.

  11. h h h Manometers operate on the same principle as barometers, but they measure the pressure of an isolated gas sample rather than the whole atmosphere. Manometers Pgas = PHg = h Pgas = Patm – h Pgas = Patm + h Note: Patm is obtained from a barometer.

  12. Empirical Gas Laws • Gases are relatively easy to measure and observe in a laboratory. • This made the physical properties of gases a popular object of study in the 17th, 18th, and 19th centuries. • Boyle, Charles, and Avogadro (yes, that Avogadro) determined fundamental connections between P, V, T and n for gases.

  13. Boyle’s Law (c. 1650) Boyle studied the connection between P and V of gases. or… PV = constant E&G pg. 179

  14. Boyle’s Law (cont) A gas for which PV = const is called “ideal.”

  15. Charles’s Law (c. 1800) Charles studied the connection between T and V of gases. Cooling down the blue balloon decreases its volume dramatically. Liquid nitrogen, T = 77 K Blue balloon is placed in the liquid N2…

  16. The volume of an ideal gas is directly proportional to its temperature measured in Kelvin Charles’s Law V = bT T = t + 273.15 V = bt + a t Vol vs. temp data for several gases. When these lines are extrapolated, they all intersect at -273.15oC. Same plot, except temperatures are given in Kelvin instead of Celcius.

  17. V = an Avogadro’s Law Volumes of gases that react do so in small whole number ratios: 2 vol H2 + 1 vol O2 = 2 vol H2O Avogadro said this was so because the same volume of two different gases at the same T and P will have the same number of particles.

  18. Empirical Gas Law Summary • Boyle’s Law: • Charles’s Law: • Avogadro’s Law: • Since V is directly proportional to 1/P, T, and n, V must also be directly proportional to the product: add proportionality constant

  19. Ideal Gas Law Common R values: P = pressure V = volume n = number of moles R = “gas constant” T = temperature in Kelvin CHE161 CHE162

  20. What is Ideality? • Molecules of an ideal gas do not attract or repel one another • The volume of an ideal gas molecule is negligible with respect to the container • Essentially, an ideal gas is a collection of non-interacting point particles. Is this a reasonable approximation? Under what conditions would you expect ideality to fail? high P  molecules get too close, start interacting low T  same thing • Over moderate T and P ranges, gases are fairly dilute, so the ideal gas law is a reasonable approximation. DON’T FORGET YOU MUST NEVER USE CELCIUS TEMPRATURES! KELVIN ONLY!!

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