NU CHEM 1211 - Chapter 6: Gases
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Announcements/Reminders:Last time: Chapter 5: ThermochemistryThis time:Prof Gilbert LECTURE 15 CHEM 1211 10/14/10Announcements/Reminders:Mid-Term Exam: Average was 74/75If you scored less than a 50 you should plan to see me in Chem Central today at noon or next Tuesday during my office hours. Last time: Chapter 5: Thermochemistry- Calculating H0rxn values from standard enthalpies of formation (H0f) of the products and reactants. - Application of Hess’s law to calculate H0rxn values- Fuel values and the impact of oxygenated additives in gasolineThis time:Chapter 6: GasesBarometric Pressure is the force (F) exerted by the weight of theatmosphere over the surface area (A) of the Earth. That is: P (Pa) = F (N) / A (m2)Where Pa is pascals (the SI unit of pressure) and N is newtons, the SIunit of force. It’s more common to use kPa (kilopascals) where anatmosphere of pressure (atm): 1 atm = 101.325 kPa = 1013.6 mbar = 760 mmHg That 760 mm value is the height of the column of Hg (d = 13.595 g/cm3at 0C) in this Hg-pool barometer For any other liquid, the heightof the column above the pool isinversely proportional to thedensity of the liquid. Why?Working barometers don’t relyon liquid mercury. A popular design for making a recording barometer,or barograph, is shown on the left.Variations in barometric pressure: (1) meteorological. Here, for example is a satellite view of Hurricane Katrina as it intensified to a Category 5 hurricane with maximum sustained winds of 175:mph (280:km/h) and a minimum central pressure of902 mbar. Generally, the lower the central pressure of a hurricane, the higher the wind speeds. Why?(2) Pressure decreases with increasing altitude. Why?The Gas LawsBased on personal experience you know that:(1) The pressure of a volume of gas is proportional to the amount of gas in the volume (as in inflating a bicycle tire): P  n(2) The pressure of a volume of gas increases with increasing temperature: P  T(3) The volume of a quantity of gas is inversely proportional tothe applied pressure: V  1/ P Combining these observations we have: or PV  nTTurning this expression into an equation by installing a constant of proportionality (R) we have the ideal gas law: PV = nRTKeep in mind that this equation works only if temperature values are on an absolute temperature (e.g., Kelvin) scale. Also, the value of R, called the gas constant, depends on the units. Most of the time we use 0.0821 L·atm / mol·K.Calculations(1) Use PV = nRT when 3 of the 4 variables are known and you seek the 4th. Oftenthe key variable is the number of moles (n) of a gas produced or consumed in achemical reaction. You may first need to convert a mass of gas into an equivalentnumber of moles by dividing by its molar mass. (2) Inquiry 1: The volume of helium in this balloon, which was flown around theworld by the late Steve Fossett in 2002, was about 1.56×107 liters at 20°C and1.00 atm of pressure. What mass of helium was needed to fill the balloon? (3) Inquiry 2: A weather balloon filled with 100.0 liters of He is launched from sea level (T = 20°C, P = 755 torr). What is the volume of the balloon when it rises to an altitude of 10 km, where T = 252°C and P = 195 torr? Solution: When n is a constant, then PV/T is a constant and the following general gas equation may be used:Determining molar mass (M): The number of moles (n) of a substance is equal to its mass (m) divided by its molar mass (M). Substituting this into fraction into the ideal gas law we have: or M = Inquiry 3: A balloon is inflated with 4.62 g of a gas to a volume of 2.46 L at 27°C and 1.05 atm. What is the molar mass of the gas? Gas density and molar mass A mole of any ideal gas occupies the same volume at a given temperature and pressure. But the mass (m) of that volume, and, therefore, the density of the gas, is proportional to the molar mass, M:M = = or d = M Inquiry 4: What is the density of propane (C3H8) at STP? Would propane leaking from a tank stored in the basement of a house tend to collect in the basement or rise into the floors above?Inquiry 5: The tanks used in barbecue grills, such as the one shown here, contain about 20pounds of propane. If this much propane leaked out, what volume would it occupy at 25°Cand 1.00 atm?Mixtures of gasesRule: ideal gases behave independently in a mixture, each contributing its own partial pressure so that the total pressure is the sum of the partial pressures: Ptotal = P1 + P2 + P3 +…This relationship is called Dalton’s law of partial pressures.One application: collecting a gas by displacing a liquid with a significant vapor pressure, e.g., water from a jar inverted in a pneumatic trough. In this example PO2 = Pbar - PH2O where Pbar is barometric (atmospheric) pressure. At 25C PH2O = 23.8 mm.Note: vapor pressure of a substance increases with increasing temperature.Inquiry 6: Suppose you are setting up an O2generator like the one on the right. You add 5.00 g ofKClO3 to the test tube. If it all decomposed, how big abottle would you need to catch all the O2 at 25C? Partial pressure (Px) and the mole fraction (Xx) ofgas “x” in a mixture Px = Xx PtotalEx: at 1.00 atm, the values of PN2 and PO2 are 0.79and 0.21 atm, because the mole fractions XN2 and XO2 are 0.79 and 0.21.Inquiry 7: A high-performance “100” stainless steel scuba tank weighs 7.4 pounds more when it is filled with air at 3500 psi, than when it is empty (and its internal pressure is only 1.00 atm or 14.7 psi). If the temperature of the tank is 25C, what is its

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# NU CHEM 1211 - Chapter 6: Gases

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