Gas Mixtures--Partial Pressure

1. Gas Mixtures--Partial Pressure • Many gases are actually mixtures of two or more gases: • air: O2, N2 , H2O, etc • How do mixtures of gases behave?

2. Gas Mixtures--Partial Pressure P= 6 psi P= 8 psi P= 9 psi O2 (g) N2 (g) CO2(g)

3. Gas Mixtures--Partial Pressure What happens when you put all three samples of gas together into one container (the same size container as each was in alone)? P

4. Gas Mixtures--Partial Pressure • The gases form a homogeneous mixture. • The pressure in the container increases. • How do you know what the new pressure will be?

5. Gas Mixtures--Partial Pressure • Each gas in a mixture behaves independently of the other gases present. • Each gas exerts its own pressure on the container. • PO = pressure exerted by O2 • PN = pressure exerted by N2 • PCO = pressure exerted by CO2 2 2 2

6. Gas Mixtures--Partial Pressure • Partialpressure: the pressure exerted by a particular gas present in a mixture • Dalton's Law of Partial Pressure:The total pressure of a mixture of gases equals the sum of the pressures that each would exert if it were present alone. Ptotal = P1 + P2 + P3 + ………

7. Gas Mixtures--Partial Pressure • Ptotal = PO + PN + PCO • So for this example: Ptotal = 6 psi + 8 psi + 9 psi = 23 psi 2 2 2

8. Gas Mixtures--Partial Pressure • In other words, at constant T and V, • Ptotal depends only on the total number of moles of gas present • Ptotal is independent of the type (or types) of gases present.

9. Gas Mixtures--Partial Pressure • When describing a mixture of gases, it is useful to know the relative amount of each type of gas. • Mole fraction (X):the ratio of the number of moles of one component compared to the total number of moles in a mixture.

10. Gas Mixtures--Partial Pressure • If a gas mixture contains 5.0 mol O2 (g), 3.0 mol H2O (g), and 12.0 mol N2 (g), • XO = nO = 5.0 mol = 0.25 ntotal 20.0 mol • On the exam, you must be able to calculate the mole fraction of each component of a gas mixture. 2 2

11. Gas Mixtures--Partial Pressure • The partial pressure of a gas in a mixture can be found: PA = XA Ptotal where PA = partial pressure of gas A XA = mole fraction of gas A Ptotal = total pressure of mixture

12. Gas Mixtures--Partial Pressure Example: A mixture of gases contains 0.43 mol N2, 0.28 mol H2, and 0.52 mol NH3. If the total pressure of the mixture is 2.35 atm, what is the partial pressure of H2?

13. Gas Mixtures--Partial Pressure • Gases formed during a reaction are often collected by displacing water from a container. • The gas collected is a mixture that contains water vapor and the gas that was formed. • Ptotal = Pgas + Pwater vapor • The vapor pressure (partial pressure) of water at various temperatures can be found in Appendix B in your text).

14. Gas Mixtures--Partial Pressure Example: The oxygen gas formed during the decomposition of potassium chlorate was collected by displacing the water in a gas measuring tube. What is the partial pressure of O2 in the gas collected if the total pressure of the gas was 745 torr at 25oC?

15. Molecular Effusion & Diffusion • What happens to a helium filled balloon as it sits around for several days?

16. Molecular Effusion & Diffusion • What happens to a helium filled balloon as it sits around for several days?

17. Molecular Effusion & Diffusion • What happens to a helium filled balloon as it sits around for several days?

18. Molecular Effusion & Diffusion • What happens to a helium filled balloon as it sits around for several days?

19. Molecular Effusion & Diffusion • What happens to a helium filled balloon as it sits around for several days?

20. Molecular Effusion & Diffusion • Many materials such as rubber or plastics have very small openings or pores through which gases can pass. • The escape of a gas molecule through a tiny hole is called effusion.

21. Molecular Effusion & Diffusion • The rate of effusion (r) of a gas is inversely proportional to the square root of its molar mass, M. • Graham’s Law: r1 = 1 M1 1/2

22. Molecular Effusion & Diffusion A molecule with smaller molar mass escapes faster than a molecule with higher molar mass.

23. Molecular Effusion & Diffusion • Another phenomenon, diffusion, also depends on the molar mass of the gas. • Diffusion:the spreading of one substance throughout another • perfume molecules spreading throughout the air in a room

24. Real Gases • Real gases do not completely follow the ideal gas law. • In kinetic molecular theory, the following assumptions are made: • gas molecules occupy no space • gas molecules have no attraction for each other

25. Real Gases • Neither assumption is correct. • Real gas molecules have a finite volume. • Real gas molecules do attract each other.

26. Real Gases • The greatest deviation from ideal gas behavior occurs at: • high pressure • higher density of gas molecules • Molecules are closer together so: • finite volume of gas molecules more important • attraction between molecules more important

27. Real Gases • Low temperature • Attractive forces between molecules becomes more important. • Average kinetic energy decreases. • Gas molecules have less energy to overcome attractive forces.