3 Properties of Gases The volume of a gas is highly variable Gases can be compressedGases can be very “diffuse”, or spread outSolids can only be compressed a littleLiquids can not be compressedGases have low densitiesGases have small amounts of mass per unit of volume
5 Kinetic Molecular Theory Assume: all gases are idealIs this a correct assumption?Kinetic Molecular Theory (KMT) developed in the 1800’s
6 Kinetic Molecular Theory Ideal gas particles are so small that their individual volume is zero (compared to the total volume of the gas)They move constantly, rapidly, and randomly, till they collideThere are no attractive or repulsive forces between ideal gas moleculesCollisions between molecules are 100% elastic
7 KMT (cont’d)Average kinetic energy is proportional to temperature in Kelvin0 degrees K means what?
8 Kinetic Energy ½ mv2 Energy of motion What does this mean in the equation above?As mass increases, velocity must decreaseAs mass decreases, velocity increasesA smaller particle moves faster!
9 Variables that describe gases Average kinetic energy is proportional to temperatureThe average number of molecules is moving at an average speed, with an average KESome move higher, and some lower
10 Volume Volume = length x height x width Can be measured in cm3, mL, L, m3Gas volumes are often measured by water displacementThe amount of water the gas displaces is equal to its volume
11 Pressure Pressure is force per unit area Pressure = force/unit areaFor gases, it is a measure of the force when molecules collide with the container.The atmosphere exerts pressure on the earth
12 STP = Std. Temp. and Press. “Standard” temperature and pressure is: 0°C and atmThis is the same as:K and kPa (kilopascals)K and 760 mm HgK and 760 torr
13 Volatile Liquids Liquids that are flammable!! They have high vapor pressures.They boil at low temperaturesThey have weak attractive forcesThey often have strong smells!
14 Vapor pressure The pressure of a vapor over its liquid Equilibrium VP: when molecules leave the liquid at the same rate as they enter.Boiling: when vapor pressure equals atmospheric pressure, liquids boil.
15 Gas Laws Dalton’s Law of Partial Pressure The total pressure of a gas is the sum of the pressures of all its parts.Ptot = Pa + Pb + Pc + … + PzExample:.
16 Gas LawsCharles’ LawVolume is proportional to temperature
17 Gas LawsBoyle’s LawPressure and volume are inversely proportional
18 The combined gas law Combines Boyle’s and Charles’ Laws: What variable must we assume is held constant?Example: A helium balloon with a volume of 410 mL is cooled from 27 to –27°C. The pressure is reduced from 110 kPa to 25 kPa. What is the volume of the gas at the lower temperature and pressure?
19 The ideal gas lawThe ideal gas law relates all 4 variables, P,V,T, and n.PV = nRTWhat is R?It is a constant: L•kPa/mol•KOr, L•atm/mol•KWhich one do I use? Use the one that will cancel out units appropriately.
20 Example:How many liters of hydrogen gas can be produced at 300K and 104kPa if 20g of sodium metal react with water?2Na + 2H2O 2NaOH + H2
21 Gases and condensation These two assumptions from KMT aren’t exactly true:Volume of gases is zeroNo attractive forces between particlesAt high pressure, gases don’t condense like you think because they DO have volume!And, they do have attractive forces, especially if they are close together.
22 Phase diagramsA graph showing the relationships between solid, liquid, gas, and temperature and pressureNormal boiling point: the temperature at which a substance boils at atm.
23 Things you’ll see on a phase diagram Sublimation: when a solid goes to a gasTriple Point: temperature and pressure where solid, liquid, and gas all coexistCritical Point: Is it a gas or liquid – who knows?Interactive Phase Diagram
24 Gases: a reviewA child receives a balloon filled with 2.30 L of helium from a vendor at a fair. The outside temperature is 311K. What will the volume be when she goes home to her air conditioned house at 295K?
25 2. At a deep sea station 200 m below the surface of the pacific ocean, workers live in a highly pressurized environment. How many liters of gas at STP on the surface must be compressed to fill the underwater environment with 2x107 L of gas at 20 atm? Assume constant temperature.
26 Real GasesIn the real world, the behavior of gases only conforms to the ideal-gas equation at relatively high temperature and low pressure.
27 Real GasesEven the same gas will show wildly different behavior under high pressure at different temperatures.
28 Deviations from Ideal Behavior The assumptions made in the kinetic-molecular model (negligible volume of gas molecules themselves, no attractive forces between gas molecules, etc.) break down at high pressure and/or low temperature.
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