2The Nature of GasesRegardless of their chemical identity, gases tend to exhibit similar physical behaviorsGas particles can be monatomic (Ne), diatomic (N2), or polyatomic (CH4) – but they all have some common characteristics:Gases have mass.Gases are compressible.Gases fill their containers.Gases diffuse.Gases exert pressure.Pressure is related to temperature
3Kinetic Molecular Theory Theory used to explain the behaviors and experimental characteristics of ideal gases –The theory states that the tiny particles in all forms of matter are in continuous motion.There are 3 basic assumptions of the KMT as it applies to ideal gases.
4KMT Assumption #1 A gas is composed of small particles. The particles have an insignificant volume and are relatively far apart from one another.There is empty space between particles.No attractive or repulsive forces between particles.
5KMT Assumption #2The particles in a gas move in constant random motion.Particles move in straight paths and are completely independent of each otherParticles path is only changed by colliding with another particle or the sides of its container.
6KMT Assumption #3 elastic collisions inelastic collisions All collisions a gas particle undergoes are perfectly elastic.They exert a pressure but don’t lose any energy during the collisions.elastic collisionsinelastic collisions
7Gases have mass.Gases are classified as matter, therefore, they must have mass.
8Gases are squeezableThe gas particles empty space can be compressed by added pressure giving the gas particles less room to bounce around thus decreasing the overall volume.
9Gases are squeezable There are a huge number of applications Storm door closersPneumatic tube delivery devicesTiresAir tanks
10Gases fill their containers Gases expand until they take up as much room as they possibly can.
11Gases fill their containers The random bouncing motion of gases allows for the mixing up and spreading of the particles until they are uniform throughout the entire container.
12Gases diffuse Gases can move through each other rapidly. The movement of one substance through another is called diffusion.Because of all of the empty space between gas molecules, gas molecules can pass between each other until the gases mix uniformly.
16Gases diffuseThis doesn’t happen at the same speeds for all gases though.Some gases diffuse more rapidly then other gases based on their size and their energy.Diffusion explains why gases are able to spread out to fill their containers.It’s why we can all breathe oxygen anywhere in the room.It also helps us avoid potential odoriferous problems.
17Gases exert pressureGas particles exert pressure by colliding with objects in their path.The definition of pressure is the force per unit area – so the total of all of the tiny collisions makes up the pressure exer ted by the gas
18Gases exert pressureIt’s the pressure exerted by the gases that hold the walls of a container outThe pressure of gases is what keeps our tires inflated, makes our basketballs bounce, makes hairspray come out of the can, helps our lungs inflate, allow vacuum cleaners to work, etc.
19Pressure depends on Temp Temperature measures the average kinetic energy of the particles in an object.Therefore, the higher the temperature the more energy the gas particle has.So the collisions are more often and with a higher force.Think about the pressure of a set of tires on a car.
20Pressure depends on Temp Today’s temp: 35°FPressureGauge
21Pressure depends on Temp Today’s temp: 85°FPressureGauge
22Measuring GasesVariables that are very important to studying the behavior of gases:Volume: generally in Liters (1L = 1000 mL)Temperature :given in Celsius but must be converted to Kelvin for gas law problemsKelvin = °C + 273Pressure1 atm=760 mmHg=760 Torr = 14.7 psi = kPaamount generally given in moles
23S T PSince the behavior of a gas is dependent on temperature and pressure, it is convenient to designate a set of standard conditions, called STP in order to study gas behavior.Standard Temperature = 0°C or 273KStandard Pressure = 1atm or 760mmHg or 101.3kPa (depending on the method of measure)
24Atmospheric PressureThe gases in the air are exerting a pressure called atmospheric pressureAtmospheric pressure is a result of the fact that air has mass and is colliding with everything.Atmospheric pressure is measured with a barometer.
25Atmospheric Pressure Atmospheric pressure varies with altitude The lower the altitude, the longer and heavier is the column of air above an area of the earth.Example: Recipes likethe back of a cake boxthat describes how tocook a cake at a higheraltitude
27Or to the volume if we changed the pressure? Boyle’s Mathematical Law:If we have a given amount of a gas at a starting pressure and volume, what would happen to the pressure if we changed the volume?Or to the volume if we changed the pressure?since PV equals a constantP1V1 = P2V2
28Boyle’s Mathematical Law: Ex: A gas has a volume of 3.0 L at 2 atm. What will its volume be at 4 atm?List the variables or clues given:P1 = 2 atmV1 = 3.0 LP2 = 4 atmV2 = ?Plug in the variables & calculate:(2 atm)(3.0 L) =(4 atm)P1V1 = V2 P2(V2)1.5 L
30Or to the temperature if we changed the volume? Charles’s Mathematical Law:If we have a given amount of a gas at a starting volume and temperature, what would happen to the volume if we changed the temperature?Or to the temperature if we changed the volume?since V/T = k=V1 V2T T2
31Charles’s Mathematical Law: Ex: A gas has a volume of 3.0 L at 400K. What is its volume at 500KList the variables or clues given:T1 = 400KV1 = 3.0 LT2 = 500KV2 = ?Plug in the variables & calculate:3.0LX L=500K400K3.8 L
33Or to the temp. if we changed the pressure? Gay-Lussac’s Mathematical Law:If we have a given amount of a gas at a starting temperature and pressure, what would happen to the pressure if we changed the temperature?Or to the temp. if we changed the pressure?since P/T = kP P2T T2=
34Gay-Lussac’s Mathematical Law: Ex: A gas has a pressure of 3.0atm at 400K. What is its pressure at 500K?List the variables or clues given:T1 = 400KP1 = 3.0 atmT2 = 500KP2 = ?Plug in the variables & calculate:3.0atmX atm=3.8 atm500K400K
37Avogadro’s LawThere is a lesser known law called Avogadro’s Law which relates Volume & moles (n).It turns out that they are directly related to each other.As number of moles increases then Volume increases.V/n = k
38Ideal Gas Law PV = R nT PV = nRT If we combine all of the laws together including Avogadro’s Law we get:Where R is the universal gas constantPVnT= RNormallywritten asPV = nRT
39Ideal Gas Constant L•atm R =.0821 mol•K L•mmHg R=62.4 mol•K L•kPa Because of the different pressure units there are 3 possibilities for our ideal gas constantR =.0821L•atmmol•KIf pressure is given in atmIf pressure is given in mmHg or torrR=62.4L•mmHgmol•KR=8.314L•kPamol•KIf pressure is given in kPa
40PracticeUse the Ideal Gas Law to complete the following table for ammonia gas (NH3).PressureVolumeTempMolesGrams2.50 atm0C32.0768 mmHg6.0 L100C
41Variations of the Ideal Gas Law We need to know that the unit mole is equal to mass divided by molar mass.PV = nRTn = m/MM
42Variations of the Ideal Gas Law We can then use the MM equation to derive a version that solves for the density of a gas.Remember that D = m/VMW = dRT or MW = mRTP VPKitty cats say meow and they put dirt on their pee!!
43Dalton’s Law of Partial Pressure States that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the component gases.PT=P1+P2+P3+…What that means is that each gas involved in a mixture exerts an independent pressure on its container’s walls
44Simple Dalton’s Law Calculation Three of the primary components of air are CO2, N2, and O2. In a sample containing a mixture of these gases at exactly 760 mmHg, the partial pressures of CO2 and N2 are given as PCO2= 0.285mmHg and PN2 = mmHg. What is the partial pressure of O2?
46Dalton’s Law of Partial Pressure Partial pressures are also important when a gas is collected through water.Any time a gas is collected through water the gas is “contaminated” with water vapor.You can determine the pressure of the dry gas by subtracting out the water vapor
47Ptot = Patmospheric pressure = Pgas + PH2O The water’s vapor pressure can be determined from a list and subtract-ed from the atmospheric pressure
49Simple Dalton’s Law Calculation Determine the partial pressure of oxygen collected by water displace-ment if the water temperature is 20.0°C and the total pressure of the gases in the collection bottle is mmHg.PH2O at 20.0°C= 17.5 mmHg
51Mole FractionMoles of gasx x PT = PxTotal molesA mixture of 4.00 moles of O2 and 3.00 moles of H2 exert a total pressure of 760 torr. What is the partial pressure of each gas?4.00 moles of O2 x 760 torr = 434 torr7.00 total moles3.00 moles of H2 x 760 torr = 326 torr
52Graham’s LawThomas Graham studied the effusion and diffusion of gases.Diffusion is the mixing of gases through each other.Effusion is the process whereby the molecules of a gas escape from its container through a tiny hole
54Graham’s LawGraham’s Law states that the rates of effusion and diffusion of gases at the same temperature and pressure is dependent on the size of the molecule.The bigger the molecule the slower it moves the slower it mixes and escapes.
55The velocities of two different gases are inversely proportional to the square roots of their molar masses.Rate of effusion of A=Rate of effusion of BMMBMMA
56Graham’s Law Example Calc. If equal amounts of helium and argon are placed in a porous container and allowed to escape, which gas will escape faster and how much faster?Rate of effusion of A=Rate of effusion of BMBMA
57Graham’s Law Example Calc. Rate of effusion of He40 g=Rate of effusion of Ar4 gHelium is 3.16 times faster than Argon.