Gas Laws Modified
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Gas Laws Modified

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Gas Laws Modified Gas Laws Modified Presentation Transcript

  • Got Gas?
  • What gases are important for each of the following: O 2 , CO 2 and/or He? A. B. C. D.
  • What gases are important for each of the following: O 2 , CO 2 and/or He? A. CO 2 B. O 2 /CO 2 C. O 2 D. He
  • Gases are made up of molecules. Air consists of a mixture of gases, including nitrogen N 2 , Oxygen O 2 , and carbon dioxide CO 2 . Gases exert pressure on any surface with which they come in to contact. The gases in our atmosphere are subject to the Earth’s gravitational pull therefore the atmosphere is much denser nearer the Earths surface and less dense as we travel away from the planet.
  • Some Gases in Our Lives oxygen O 2 nitrogen N 2 ozone O 3 argon Ar carbon dioxide CO 2 water H 2 O Noble gases : helium He neon Ne krypton Kr xenon Xe Other gases: fluorine F 2 chlorine Cl 2 ammonia NH 3 methane CH 4 carbon monoxide CO nitrogen dioxide NO 2 sulfur dioxide SO 2
  • Pressure is one of the most easily measured properties of a gas. We define pressure as the force per unit area. P= Force/Area.
  • Pascal: The SI unit of pressure. 1 Pascal = 1 N /m 2 = 1 kg x m/s 2 x 1/m 2 Named after Blaise Pascal
  • Ice skates are familiar examples of the effects of pressure. The area of the blades of a skate are much smaller than, say, the soles of your feet. So if you strap on ice skates, your weight will act on an area much smaller than it would if you were wearing normal shoes. Since A decreases while F stays the same, the pressure you exert on the ice will be much greater if you're wearing skates. This pressure is often enough to melt a layer of ice, which allows your skate to glide smoothly across an ice rink. If you try the same maneuver with normal shoes, you will not generate enough pressure to melt the ice and won't get anywhere fast.
  • Air pressure is the force exerted on you by the weight of tiny particles of air (air molecules). Although air molecules are invisible, they still have weight and take up space. We are usually unaware of the air pressure around us, probably because we are used to it. Since there's a lot of "empty" space between air molecules, air can be compressed to fit in a smaller volume.
  • How much pressure are you under? Earth's atmosphere is pressing against each square inch of you with a force of 1 kilogram per square centimeter (14.7 pounds per square inch). The force on 1,000 square centimeters (a little larger than a square foot) is about a ton!
  • Why doesn't all that pressure squash me? Remember that you have air inside your body too, that air balances out the pressure outside so you stay nice and firm and not squishy. Well…some of us are more squishy than others
  • Run for Cover! Air pressure can tell us about what kind of weather to expect as well. When it's compressed, air is said to be "under high pressure". If a high pressure system is on its way, often you can expect cooler temperatures and clear skies. If a low pressure system is coming, then look for warmer weather, storms and rain.
  • Some important pressure terms
    • Atmosphere - A unit of measurement defined as 101,325 Pascals. The typical pressure at sea level varies around one standard atmosphere (atm). Atmospheric pressure P can be calculated via the following equation:
    • P = ghρ
    • Where g = acceleration due to gravity
    • h = height of liquid in barometer (usually Hg)
    • ρ = density of liquid in barometer
    • WRITE THIS IN YOUR NOTES NOW
  • Look a barometer!!
  • Barometer? Weather forecasters measure air pressure with a barometer . Barometers are used to measure the current air pressure at a particular location in "inches of mercury" or in "millibars" (mb). A measurement of 29.92 inches of mercury is equivalent to 1013.25 millibars. Crazy huh? Bar - A unit of measurement equivalent to 1×10 5 Pascals.
  • More pressure terminology
    • mm Hg- A unit of pressure commonly used with the barometer. It corresponds to 1 torr and 1/760 atm at 0 o Celsius only.
    • Torr- A unit of pressure closely related to mm Hg, but more convenient and absolute. 1 Torr = 1 mm Hg at 0 0 Celsius. 1 Torr always equals 1/760 atm, irrespective of the temperature.
    • Pounds per Square Inch- A unit of pressure commonly used in the United States. 1 psi = 1 l b in -2 . 14.6960 psi corresponds to one atmosphere.
  • Why do my ears pop as I increase altitude? As the number of molecules of air around you decreases, the air pressure decreases. This causes your ears to pop in order to balance the pressure between the outside and inside of your ear. Since you are breathing fewer molecules of oxygen, you need to breathe faster to bring the few molecules there are into your lungs to make up for the deficit. Air pressure on Mt. Everest is 670 mb less than on the Cayman Islands.
  • 1. If you were on a mountain, would the weight of the air above you (air pressure) be greater than or less than it is now?
  • What do you think causes wind? Don’t say beans!!!
  • If you lived in Kansas and observed a sudden drop in air pressure, what kind of weather would you expect to see soon?
  •  
  • Why do hot air balloons rise?
  • Air weighs less than water, would you expect the pressure exerted by water to be greater or less than the pressure exerted by the same volume of air?
  • The Gas Laws:
      • The volume of a gas means nothing unless the conditions under which it was collected are known.
      • A temperature and pressure are needed to describe any volume of gas
  • Temperature - Temperature changes cause particle motion changes which cause a volume change. Pressure - Gases can be compressed, or squeezed, causing a change in the gas volume.
  • The Four Gas Law Variables:
    • Temperature,
    • Pressure,
    • Volume,
    • Moles
  • Volume All gases must be enclosed in a container that, if there are openings, can be sealed with no leaks. The three-dimensional space enclosed by the container walls is called volume. When the generalized variable of volume is discussed, the symbol V is used.
  • Volume in chemistry is usually measured in liters (symbol = L) or milliliters (symbol = mL). A mL is also called a cubic centimeter cm 3 A liter is also called a cubic decimeter dm 3
  • Temperature All gases have a temperature, usually measured in degrees Celsius (symbol = °C). Note that Celsius is capitalized since this was the name of a person (Anders Celsius). When the generalized variable of temperature is discussed, the symbol T is used.
  • There is another temperature scale which is very important in gas behavior. It is called the Kelvin scale (symbol = K). Note that K does not have a degree sign and Kelvin is captalized because this was a person's title (Lord Kelvin, his real name was William Thomson).
  • All gas law problems will be done with Kelvin temperatures. You can convert between Celsius and Kelvin like this: Kelvin = Celsius + 273 For example, 25 °C = 298 K, because 25 + 273 = 298.
  • Standard temperature is defined as zero degrees Celsius or 273 K.
  • Pressure Gas pressure is created by the molecules of gas hitting the walls of the container. This concept is very important in helping you to understand gas behavior. Keep it solidly in mind. I will use this idea of gas molecules hitting the wall often. When the generalized variable of pressure is discussed, the symbol P is used.
  • Three different units of pressure used in chemistry .
    • atmospheres (symbol = atm)
    • millimeters of mercury (symbol = mm Hg)
    • Pascals (symbol = Pa) or, more commonly, kiloPascals (symbol = kPa)
  • When working with gas laws, all pressure units must be the same in any calculation. 1 atm = 760 mm Hg = 101 kPa = 101,300 Pascals = 1013 mb = 29.92 in Hg If needed, be able to convert from one pressure unit to another.
    • Do the following conversions as practice:
        • 1. 27 K to Celcius
        • 2. 1450 0 C to K
        • 3. 1.25 atmospheres to inches of Hg
        • 4. 500 millimeters of Hg to kPa
  • Standard temperature and pressure =STP:
  • Standard temperature is 0 0 Celcius. But All gas calculations must use Kelvin temperatures. K = 0 C + 273
  • Standard pressure is 1 ATM (at sea-level). mm Hg is millimeters of mercury. This describes the height of a vertical column of mercury that the pressure will support against gravity. The unit Torr can be used to indicate mm Hg. The barometric pressure reported in U.S. weather reports is usually expressed as inches of mercury. There are 25.4 millimeters in one inch. kPa is kiloPascals. A Pascal is a unit of force equal to a N / m 2 . It is describing the pressure exerted by the molecules of the gas striking a surface. mb is millibars. One millibar is equal to 100 N / m 2 . Crazy huh?
  • The last gas law variable Amount of Gas
    • Don’t “freak out” but the amount of gas present is measured in moles (symbol = mol) or in grams (symbol = g or gm). Typically, if grams are used, you will need to convert to moles at some point. When the generalized variable of amount in moles is discussed, the letter "n" is used as the symbol (note: the letter is in lowercase. The others discussed above are all caps.).
  • To be successful working gas law problems, do the following:
        • First read the question to see what question is being asked.
  • What do you know?
        • Write and label all variables given in the problem.
  • What are you trying to find?
        • Label the variable the question is asking you to find.
  • Reread the question to see what conditions change.
        • Label all measurements before the change as P 1 ,V 1 , and T 1 .
        • Label all measurements after the change as P 2 , V 2 , and T 2 .
  • Find the gas equation that connects the variables used in the problem . Boyles Charles Gay/Lussac
  • Plug the variables into the gas equation you’ve chosen
  • Rearrange the gas equation to solve for the “Find” variable. V T P
  • Plug and Chug
    • Plug numbers and units into the equation
    • Pickup your calculator and punch buttons.
    • Solve.
    • Check your units, do they make sense?
    • If so,
    • Write the answer to the problem and circle it.
  • Avagadro’s Law Equal volumes of gasses, at the same temperature and pressure, have equal numbers of particles.
  • Important points to note….
    • All gasses have the same physical behavior
    • Increase volume of gas = increase number of particles (if temp and pressure are constant)
    • Volume is directly proportional to number of particles
    • V = n (n = number of moles)
  • Molar volume = volume of 1 mole of gas At STP, (273k, 1atm) 1 mole of any gas = 22.4 Liters
  • What is the molar mass of… 22.40 Liters of H 2 (g)
  • What is the molar mass of….. 22.40 Liters of N 2 (g)
  • What is the molar mass of….. 22. 40 liters of O 2 (g)
  • Calculate the number of liters of occupied by 2.5 moles of N 2 gas
  • Calculate the number of liters of occupied by 0.350 moles O 2 gas
  • The universal gas constant = R This is a constant that lets us solve “ideal gas” equation problems Think of it as a correction factor
  • Boyles law Pressure and volume are inversely proportional P = V PV = ______
  • Charles Law Volume & Temperature are directly proportional V = T V =____ T
  • Avagadro’s Law Volume & moles are directly proportional V = n V = _______ n
  • If we rearrange….. PV = R nT At STP what is the gas constant ??? (watch your units)