Gases, cont. (and finished!) Ideal vs. Real Ideal Gas Law Dalton’s Law Diffusion

Chemistry Joke A Chemistry Prison Pun The silicon put his neon the window ledge. He climbed out and then krypton along the wall to meet his buddy. I hope the guard cesium before they argon!

Ideal vs. Real Ideal Gases Follow gas laws at all conditions Conform to the Kinetic Molecular Theory Insignificant volume No attraction / repulsion to each other

Ideal vs. Real Ideal Gases don’t exist!!! Molecules do take up space There are attractive forces between them Otherwise gases would never liquefy But…at many temperatures and pressures real gases act like ideal gases. This makes for easier math!

Ideal vs. Real We can ignore their volume. Real Gases behave ideally… At HIGH TEMPERATURES and LOW PRESSURES… In these conditions, a gas will stay a gas. At low pressures, the molecules are far apart. We can ignore their volume. At high temperatures, molecules are not together very long. We can ignore their attractions.

Ideal vs. Real Think of Dry Ice At LOW TEMPERATURES and HIGH PRESSURES… Gases experience both particle volume and attraction Can be liquefied or solidified Think of Dry Ice We’ll continue to study ideal gases!

PV = nRT n represents number of moles R is the universal gas constant Ideal Gas Law PV = nRT n represents number of moles R is the universal gas constant

Units for P and V have to match “R” Ideal Gas Law Units for P and V have to match “R”

Ideal Gas Law Problems P = ? atm n = 0.412 mol T = 16°C = 289 K Calculate the pressure in atmospheres of 0.412 mol of He at 16°C & occupying 3.25 L. GIVEN: P = ? atm n = 0.412 mol T = 16°C = 289 K V = 3.25 L R = 0.0821Latm/molK WORK: PV = nRT P(3.25)=(0.412)(0.0821)(289) L mol Latm/molK K P = 3.01 atm

Ideal Gas Law Problems V = ? n = T = 25°C = 298 K P = 1.03 atm Find the volume of 85 g of O2 at 25°C and 104.5 kPa. GIVEN: V = ? n = T = 25°C = 298 K P = 1.03 atm R = 0.0821 WORK: 85 g 1 mol = 2.7 mol 32.00 g 2.7 mol PV = nRT (1.03)V=(2.7) (0.0821)(298) atm mol K V = 64 L

Dalton’s Law of Partial Pressures For a mixture of gases in a container… P1 represents the “partial pressure” or the contribution by that gas. Dalton’s Law is particularly useful in calculating the pressure of gases collected over water.

Dalton’s Law of Partial Pressures If the gases in the first three containers are all put into the fourth, we can find the pressure in the 4th container by adding up the pressures in the first 3:

Dalton’s Law of Partial Pressures

Diffusion Mixing of molecules Molecules moving from areas of high concentration to areas of low concentration. Example: perfume molecules spreading across the room.

Diffusion With diffusion, the mass of the particle is important: Gases of lower molar mass diffuse faster than gases of higher molar mass.

Diffusion Example: compare diffusion rates of helium and nitrogen. The molar mass of He = 4.0 g The molar mass of N2 = 28.02 g Therefore… Helium diffuses faster than nitrogen.

Chemistry Joke Q: What did the electron say to the proton to make it unhappy? A: Something negative!