Properties of Gases

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Properties of Gases

  1. 1. Chemistry Unit 10:The Gas Laws
  2. 2. The Atmosphere is all around us  an “ocean” of gases mixed together Compositionnitrogen (N2)………….. ~78%oxygen (O2)…………… ~21%argon (Ar)……………... ~1%carbon dioxide (CO2)… ~0.04% Trace amounts of: He, Ne, Rn, SO2,water vapor (H2O)……. ~0.1% CH4, NxOx, etc.
  3. 3. Elements that exist as gases at 250C and 1 atmosphere
  4. 4. Gases have mass.• Gases seem to be weightless, but they are classified as matter, which means they have mass.• The density of a gas – the mass per unit of volume – is much less than the density of a liquid or solid, however.
  5. 5. Gases have mass. It’s this very low density that allowsus to be able to walk through theroom without concerning ourselveswith air resistance. Since it is so easy to “swim” acrossthe room we don’t put much thoughtinto the mass of a gas. Really it is only noticeable if wehave a large collection of gas in acontainer.
  6. 6. Physical Characteristics of Gases• Gases assume the volume and shape of their containers.• Gases are the most compressible state of matter.• Gases will mix evenly and completely when confined to the same container.• Gases have much lower densities than liquids and solids.
  7. 7. CompressibilityGases can expand to fill its container, unlike solids or liquidsThe reverse is also true: They are easily compressed, or squeezed into a smaller volume Compressibility is a measure of how much the volume of matter decreases under pressure
  8. 8. Variables that describe a GasThere are FOUR variables used to describe gases: 1. pressure (P) 2. volume (V) 3. temperature (T) 4. amount (n)
  9. 9. 1. Pressure of GasPressure results from the collisions between gas molecules and the walls of the container they are in. More molecules means more collisions which means more pressure.Gases naturally move from areas of high pressure to low pressure, because there is empty space to move into – a spray can is example.
  10. 10. Units of PressureThe standard unit for pressure is ATMOSPHERE or atm Other units include:  kPa = kilopascal  mmHg = millimeter of Mercury  torr = Torrricelli  psi = pounds per square inch
  11. 11. Convert between Pressure units 1 atm equals... 101.325 kPa 760 mm Hg 760 torr 14.69 psi
  12. 12. 2. Volume of GasIn a smaller container, the molecules have less room to move.The particles hit the sides of the container more often.As volume decreases, pressure increases. (think of a syringe) Thus, volume and pressure are inversely related to each other
  13. 13. Units of VolumeThe standard unit forvolume is the... LITER or L*There are 1,000 mL in 1 L
  14. 14. 3. Temperature of Gas Temperature is a measurement of the amount of Kinetic Energy the gas molecules contain Raising the temperature of a gas increases the pressure, if the volume is held constant. (Temp. and Pres. are directly related) The molecules hit the walls harder, and more frequently! Should you throw an aerosol can into a fire? What could happen?
  15. 15. Units of TemperatureThe standard unit for temperatureis... Kelvin or KOther units include: Degrees Celsius = oC Degrees Fahrenheit = oF
  16. 16. Absolute ZeroThe theoretical temperature atwhich all kinetic motioncompletely stops. Equal to 0 Kor -273 oC Conversions K = °C + 273 °C = K – 273
  17. 17. 4. Amount of GasWhen we inflate a balloon, we are adding gas molecules.Increasing the number of gas particles increases the number of collisions thus, the pressure increases The standard unit for the amount of gas molecules is the: MOLE (mol)
  18. 18. And now, we pause for this commercial message from STP OK, so it’s really not THIS kind of STP… STP in chemistry stands for Standard Temperature and PressureStandard Pressure = STP allows us to compare 1 atm amounts of gases between different pressures andStandard Temperature temperatures = 0 oC or 273 K
  19. 19. Kinetic Molecular Theory The theory states that the tinyparticles in all forms of matter inall forms of matter are inconstant motion. This theory is used to explainthe behaviors common amonggases There are 3 basic assumptionsof the KMT as it applies to gases.
  20. 20. Kinetic Molecular Theory of GasesThree basic assumptions of the kinetic theory as it applies to gases: #1. A gas is composed of small, particles that have mass- usually molecules or atoms. They have... Insignificant volume; relatively far apart from each other No attraction or repulsion between particles
  21. 21. Kinetic Molecular Theory of Gases#2. Particles in a gas move rapidlyin constant random motionMove in straight paths, changing direction only when colliding with one another or other objectsAverage speed of O2 in air at 20 oC is an amazing 1700 km/h!
  22. 22. Kinetic Molecular Theory of Gases#3. Collisions are perfectlyelastic- meaning kinetic energy istransferred without loss from oneparticle to another- the totalkinetic energy remains constant
  23. 23. Summary of The Kinetic Molecular Theory -- explains why gases behave as they do1. …are so small that they are assumed to have zero volume 2. …are in constant, straight-line motion 3. …experience elastic collisions in which no energy is lost 4. …have no attractive or repulsive forces toward each other 5.…have an average kinetic energy (KE) that is proportional to the absolute temp. of gas (i.e., Kelvin temp.) as Temp. , KE
  24. 24. Kinetic Molecular Theory of GasesGas Pressure – defined as the forceexerted by a gas per unit surfacearea of an object Due to: a) force of collisions, and b) number of collisions No particles present? Then there cannot be any collisions, and thus no pressure – called a vacuum
  25. 25. Kinetic Molecular Theory of GasesAtmospheric pressure results fromthe collisions of air molecules withobjects Decreases as you climb a mountain because the air layer thins out, meaning less particles, as elevation increasesBarometer is the measuring devicefor atmospheric pressure, which isdependent upon weather & altitude
  26. 26. Atmospheric pressure changes with altitude: As altitude , pressure . barometer: device to measure air pressure vacuum mercury air (Hg) pressure
  27. 27. Measuring PressureThe first device formeasuring atmosphericpressure was developedby Evangelista Torricelliduring the 17th century.The device was called a“barometer” Baro = weight Meter = measure Torricelli
  28. 28. An Early BarometerThe normal pressuredue to the atmosphereat sea level cansupport a column ofmercury that is 760 mmhigh.
  29. 29. Kinetic Molecular Theory of GasesWhathappens when a substance isheated? Particles absorb energy! Some of the energy is stored within the particles- this is potential energy, and does not raise the temperature Remaining energy speeds up the particles (increases average kinetic energy)- thus increases temperature
  30. 30. Kinetic Molecular Theory of GasesAnincrease in the average kineticenergy of particles causes thetemperature to rise. As it cools, the particles tend to move more slowly, and the average K.E. declines. Is there a point where they slow down enough to stop moving?
  31. 31. Kinetic Molecular Theory of GasesTheparticles would have no kineticenergy at that point, because theywould have no motion Absolute zero (0 K, or –273 oC) is the temperature at which the motion of particles theoretically ceases This has never been reached, but about 0.5 x 10 -9 K has been achieved
  32. 32. •Diffusion:describes the mixingof gases. The rate ofdiffusion is the rate ofgas mixing.•Molecules movefrom areas of highconcentration to lowconcentration.
  33. 33. Effusion: a gas escapes through a tinyhole in its container -Think of a nail in your car tire…

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