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Chemistry 55

Chemistry 55. Fall 1999 Chapter 7. Gases and Gas Laws. Some Gases in Our Lives. Air: oxygen O 2 nitrogen N 2 ozone O 3 argon Ar carbon dioxide CO 2 water H 2 O Noble gases : helium He neon Ne krypton Kr xenon Xe Other gases:

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Chemistry 55

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  1. Chemistry 55

  2. Fall 1999Chapter 7 Gases and Gas Laws

  3. Some Gases in Our Lives Air: oxygen O2 nitrogen N2 ozone O3 argon Ar carbon dioxide CO2 water H2O Noble gases: helium He neon Ne krypton Kr xenon Xe Other gases: fluorine F2 chlorine Cl2 ammonia NH3 methane CH4 carbon monoxide CO

  4. Properties of a Gas • Volume V L, mL, cc • Temperature T° C , K • Moles n g/mole • Pressure P mmHg, atm, torr

  5. Units of Pressure One atmosphere (1 atm) • Is the average pressure of the atmosphere at sea level • Is a standard of pressure • P = Force Area 1.00 atm = 760 mm Hg = 760 torr

  6. Measuring Pressure Barometers 760 mmHg atm pressure Hg

  7. Learning Check A. What is 475 mm Hg expressed in atm? 1) 475 atm 2) 0.625 atm 3) 361000 atm B. The pressure of a tire is measured as 29.4 psi. What is this pressure in mm Hg? 1) 2.00 mm Hg 2) 1520 mm Hg 3) 22 300 mm Hg

  8. Solution A. What is 475 mm Hg expressed in atm? 475 mm Hg x 1 atm = 0.625 atm (2) 760 mm Hg B. The pressure of a tire is measured as 29.4 psi. What is this pressure in mm Hg? 29.4 psi x 1.00 atm x 760 mmHg = 1520 mmHg 14.7 psi 1.00 atm (2)

  9. Pressure and Altitude • As altitude increases, atmospheric pressure decreases

  10. Pressure and Boiling Point • As P atm decreases, water boils at lower temperatures and foods cook more slowly

  11. Boyle’s Law Pressure and Volume

  12. Pressure and Volume Experiment Pressure Volume P x V (atm) (L) (atm x L) 1 8.0 2.0 16 2 4.0 4.0 _____ 3 2.0 8.0 _____ 4 1.0 16 _____ Boyle's Law P x V = k (constant) when T,n remain constant P1V1= 8.0 atm x 2.0 L = 16 atm L P2V2= 4.0 atm x 4.0 L = 16 atm L P1V1 = P2V2 = k Use this equation to calculate how a volume changes when pressure changes, or how pressure changes when volume changes. new vol. old vol. x Pfactor new P old P x Vfactor V2 = V1 x P1 P2 = P1 x V1 P2 V2

  13. P and V Changes P1 P2 V1 V2

  14. Boyle's Law • The pressure of a gas is inversely related to the volume when T,n does not change • The PV product remains constant P1V1 = P2V2 P1V1= 8.0 atm x 2.0 L = 16 atm L P2V2= 4.0 atm x 4.0 L = 16 atm L

  15. PV Calculation What is the new volume (L) of a 1.6 L sample of Freon gas initially at 50. mm Hg after its pressure is changed to 200. mm Hg? ( T and n are constant)

  16. HINT • The pressure goes from 50. mmHg to 200. mmHg. Is that an increase or decrease in pressure ? • What will happen to the volume? P V

  17. Finding the New Volume Take the old volume and multiply by a factor of pressures to make the result bigger.

  18. Solution 1.6 L x 200 mmHg = 6.4 L 50 mmHg • Factor greater than 1; answer is larger

  19. Learning Check A sample of nitrogen gas is 6.4 L at a pressure of 0.70 atm. What will the new volume be if the pressure is changed to 1.40 atm? (T and n constant) Explain. 1) 3.2 L 2) 6.4 L 3) 12.8 L

  20. Solution A sample of nitrogen gas is 6.4 L at a pressure of 0.70 atm. What will the new volume be if the pressure is changed to 1.40 atm? (T and n constant) 6.4 L x 0.70 atm = 3.2 L (1) 1.40 atm Volume must decrease to cause an increase in the pressure

  21. Learning Check A sample of helium gas has a volume of 12.0 L at 600. mm Hg. What new pressure is needed to change the volume to 36.0 L? (T and n constant) Explain.

  22. Solution A sample of helium gas has a volume of 12.0 L at 600. mm Hg. What new pressure is needed to change the volume to 36.0 L? (T constant) Explain. 600. mm Hg x 12.0 L = 200. mmHg (1) 36.0 L Pressure decrease when volume increases.

  23. Worksheet 7-1 • Do the problems from Worksheet 7-1 • You can work these problems alone or with others around you. • You may use your notes and textbook. • When you have finished, compare answers with someone else.

  24. Charles’ Law T = 273 K T = 546 K Observe the V and T of the balloons. How does volume change with a temperature increase ?

  25. Charles’ Law: V and T At constant pressure, the volume of a gas is directly related to its absolute (K) temperature V1 = V2 T1 T2 1. If final T is higher than initial T, final V is (greater, or less) than the initial V. 2. If final V is less than initial V, final T is (higher, or lower) than the initial T.

  26. Charles’ Law: V and T At constant pressure, the volume of a gas is directly related to its absolute (K) temperature V1 = V2 T1 T2 1. If final T is higher than initial T, final V is (greater) than the initial V. 2. If final V is less than initial V, final T is (lower) than the initial T.

  27. V and T Calculation A balloon has a volume of 785 mL when the temperature is 21°C. As the balloon rises, the gas cools to 0°C. What is the new volume of the balloon? Think about what happens to T;always use K !!!

  28. Solution 785 mL x 273 K =729 mL 294 K Factor less than 1; answer is smaller

  29. Learning Check A sample of oxygen gas has a volume of 420 mL at a temperature of 18°C. What temperature (in °C) is needed to change the volume to 640 mL? 1) 443°C 2) 170°C 3) - 82°C

  30. Solution A sample of oxygen gas has a volume of 420 mL at a temperature of 18°C. What temperature (in °C) is needed to change the volume to 640 mL? T2 = 291 K x 640 mL = 443 K 420 mL = 443 K - 273 K = 170°C (2) 170°C 3) - 82°C

  31. P and T P (mm Hg) T (°C) 936 100 761 25 691 0 When temperature decreases, the pressure of a gas (decreases or increases). When temperature increases, the pressure of a gas (decreases or increases).

  32. Gay-Lussac’s Law • Pressure and Absolute temperature are directly proportional T P

  33. P and T Calculation A gas has a pressure at 2.0 atm at 18°C. What will be the new pressure if the temperature rises to 62°C? (V,n constant) T = 18°C T = 62°C

  34. Solution 2.0 atm x 335 K =2.3 atm 291 K Factor more than 1; answer is larger

  35. Learning Check Answer with 1) Increases 2) Decreases 3) Does not change A. Pressure _________, when V decreases B. When T decreases, V __________ C. Pressure ____________ when V changes from 12.0 L to 24.0 L (constant n and T) D. Volume _______when T changes from 15.0 °C to 45.0°C (constant P and n)

  36. Solution Answer with 1) Increases 2) Decreases 3) Does not change A. Pressure 1) Increases, when V decreases B. When T decreases, V 2) Decreases C. Pressure 2) Decreaseswhen V changes from 12.0 L to 24.0 L (constant n and T) D. Volume 1) Increaseswhen T changes from 15.0 °C to 45.0°C (constant P and n)

  37. Worksheet 7-2 • Do the problems from Worksheet 7-2. • You can work these problems alone or with others around you. • You may use your notes and textbook. • When you have finished, compare answers with someone else.

  38. Combined Gas Law • CGL gives the result of changing 2 properties P1V1 P2V2 = T1 T2

  39. Problem • Oxygen gas has a pressure of 0.15 atm when the volume is 15. L and the temperature is 27º C. What will the new volume be if T becomes 127 º C and the pressure becomes 900. mmHg?

  40. Answer Change T to Kelvin: 27°C +273 = 300 K,127°C +273 = 400 K Change mmHg to atm: 900. mmHg x 1 atm = 1.18 atm 760 mmHg

  41. Algebraic solution 15. L x 0.15 atm x 400 K = 1.2 L 300 K 1.18 atm

  42. Alternate solution 15 L x 0.15 atm x 400 K = 1.2 L 1.18 atm 300 K P V T V

  43. Worksheet 7-3 • Do the problems from Worksheet 7-3. • You can work these problems alone or with others around you. • You may use your notes and textbook. • When you have finished, compare answers with someone else.

  44. Avogadro’s Law • Volume is directly related to the number of moles of gas

  45. Avogadro’s Law 0.60 moles of O2 gas has a volume of 50.L. What is the volume when 1.0 moles of O2 is added?

  46. Does a balloon get bigger or smaller when air is added? • Add air

  47. Does a balloon get bigger or smaller when air is added? Add air

  48. Solution 50 L x 1.6 moles = 133 L 0.6 moles Factor more than 1; answer is larger

  49. STP • Standard temperature 0 °C or 273 K • Standard pressure 760 mmHg or 1 atm

  50. Molar Volume • At STP, 1 mole of gas has a volume of 22.4 L. 1 mole = 22.4 L (at STP)

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