Gases, cont. (and finished!)

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

2. 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!

3. 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

4. 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!

5. Ideal vs. Real • 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.

6. Ideal vs. Real • 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!

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

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

9. Ideal Gas Law Problems • 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

10. Ideal Gas Law Problems WORK: 85 g 1 mol = 2.7 mol 32.00 g • Find the volume of 85 g of O2 at 25°C and 104.5 kPa. GIVEN: V=? n=85 g T=25°C = 298 K P=104.5 kPa R=8.315dm3kPa/molK = 2.7 mol PV = nRT (104.5)V=(2.7) (8.315) (298) kPa mol dm3kPa/molKK V = 64 dm3

11. 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.

12. 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:

13. Dalton’s Law of Partial Pressures

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

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

16. 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.

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