Unit 10: Gases

1. Unit 10: Gases Chemistry I Mr. Patel SWHS

2. Topic Outline • MUST have a scientific calculator (not graphing)!! • Properties of Gases (13.1, 14.1) • Kinetic Molecular Theory (13.1) • Temperature and Pressure Conversions (14.1) • Empirical Gas Laws (14.2) • Ideal Gases and Real Gases (14.3) • Dalton’s Law (14.4) • Graham’s Law (14.4) • Gas Law Stoichiometry

3. Nature and Properties of Gases • Gases are easily compressible • Flow easily (fluids) • Assume the shape and volume of container • Gases are described by four variables: • Amount of gas (n) • Volume (V) • Pressure (P) • Temperature (T) • Kinetic theory – all particles are in constant motion …one exception

4. Kinetic Molecular Theory • Particles have insignificant volume • Large distance between particles • No attractive or repulsive forces • Motion is rapid and random • Travel in straight line path until collisions • Collisions are completely elastic • KE is transferred without loss

5. Heat vs. Temperature • Heating  Rise in Temperature • Heat and Temperature are NOT the same thing! • Heat (is energy) • Total energy of molecular motion • Depends on number, speed, and type of particle • Temperature (measure of energy) • Measure of the average kinetic energy of a particle • Depends only on the speed of particle • A bathtub full of water has more heat/thermal energy than a cup of water at the same temperature • Energy transfers from hotter objects to cooler objects

6. Temperature • When heating a gas, most energy goes into motion • Average KE is related to temperature (K) • All particles same average KE at room temp • All motion stops at absolute zero • 0 K or -273⁰C

7. Temperature Conversions • Celsius to Kelvin: K = 273 + ⁰C • Kelvin to Celsius: ⁰C = K - 273

8. Ex: Convert 924.8 K to Celsius.

9. Ex: The hottest planet in the solar system measured a temperature of 480 K on a certain day. What is this temperature in Celsius?

10. Ex: The Rhinovirus reproduces especially well at nostril temperatures of 32.8⁰C. What is this temperature in Kelvin?

11. Gas Pressure • Force exerted by gas per unit surface area • Result of billions of simultaneous collisions • Ex: Helium balloon • Vacuum – No pressure • no gas particles • Barometer – device used to measure atmospheric pressure • Gas particles move from highpressure to low pressure

14. Pressure Conversions • Standard Temperature and Pressure (STP) • 0⁰C = 273 K • 1 atm • 1 atm = 760 mm Hg = 760 torr = 101.3 kPa (SI) • These are conversion factors

15. Ex: Convert 25.3 atm to mm Hg.

16. Ex: The air pressure for a certain tire is 109 kPa. What is this pressure in atmospheres?

17. Ex: The weather news gives the atmospheric pressure as 1.07 atm. What is this atmospheric pressure in mm Hg?

18. Ex: Convert 1460 mm Hg to torr.

19. Practice • An experiment at Sandia National Labs in New Mexico is performed at 758.7 torr. What is this pressure in atm? • A bag of potato chips is sealed in a factory near sea level. The atmospheric pressure at the factory is 761.3 mm Hg. The pressure inside the bag is the same. What is the pressure inside the bag of potato chips in kPa?

20. Practice Solutions • Answer: 0.998 atm • Answer: 1.01 kPa

21. The Empirical Gas Laws • There are four empirical gas laws • These laws relate the gas variables • Empirical – based on observation

22. Boyle’s Law • Relates Pressure and Volume(Constant Temperature) • Law: P1 V1 = P2 V2 • Inverse relationship • If V then P

25. Ex: If the volume of the lungs is 2.400 L during exhalation and the pressure is 101.70 KPa, and the pressure during inhalation is 101.01 KPa, what is the volume of the lungs during inhalation? 

26. Charles’s Law • Relates Temp and Volume(Constant Pressure) • Law: • Direct relationship • If T then V

28. Ex: If the volume of a gas at 25⁰C is 25.5 L, what is the temperature when the volume is 75.0 L?

29. Gay-Lussac’s Law • Relates Pressure and Temp(Constant Volume) • Law: • Direct relationship • If T then P

31. Ex: If the pressure of a gas at 75⁰C is 12.5atm, what is the temperature in Celsius when the pressure is 3.75 atm?

32. Combined Gas Law • Combination of the previous empirical laws

33. Avogadro’s Law • Relates Moles and Volume • Law: • Direct relationship • If n then V

34. Ex: If 3.75 moles of N2gas are used to inflate a balloon to 0.250L, what is the volume when the moles of gas are tripled?

35. Tips on Solving Gas Law Problems • Write down all the variables that are known. • Determine which variable you are looking for. • Convert all variables to the correct units. • Write down the correct law/equation. • Rearrange equation. • Plug in values and solve. • See if your answer makes sense.

36. Ex: The gas in a spray can has a pressure of 103 kPa at 25 ⁰C. The can is heated to 928 ⁰C. What is the resulting pressure?

37. Ex: A balloon has a volume of 30.0L at 313K and a pressure of 1147.9 mmHg. What is the volume at STP?

38. The Empirical Laws • The previous laws are useful when only two variables are changing…everything else is constant. • They are laws for specific scenarios. • It is possible to use the laws to create a law that is general in nature – not specific.

39. Ideal Gas Law • Combines the previous observations. • One of the most fundamental and important laws in chemistry. • Describes a SINGLE gas. PRESSURE TEMPERATURE MOLES VOLUME GAS CONSTANT

40. Ideal Gases • This is for ideal gases…not real gases. • Many real gases behave as ideal gases. • An ideal gas completely follows all the tenants of the Kinetic Molecular Theory.

41. Gas Constant – R • This is a universal constant. • It is important to memorize the value of this number with the CORRECT units. • The value changes based on the units. R = 0.0821 atm∙L mol∙K

42. Implications of R on PV=nRT • Previous laws – we could use any units as long as we were consistent. • The units used in the ideal gas law are determined by our choice of R. • If R = 0.0821 atm∙L/mol∙K is used: • P must be in atm • V must be in L • T must be in K

43. Ex: A 5.78L container with a pressure of 1285 torr holds 7.50 moles of an ideal gas. What is the temperature (in Celsius) within the container?

44. Ex: A container with a volume of 3.25L holds 32.0 g H2O gas at 25.0⁰C. What is the pressure within the container?

45. Real Gases • An ideal gas does not exist • Gas particles do have volume and attraction/repulsion which is typically negligible • Real Gases differ most from an ideal gas at: • Low Temperature • High pressure • van der Waals Equation:

46. Dalton’s Law • Focuses on a mixture of different gases • Pressure depends on particle quantity – not type of particle • Gases are independent of one another • Partial Pressure (PX) – pressure of a single gas within a mixture of gases • Dalton’s Law of Partial pressure • Total pressure is the sum of the partial pressures PTotal = P1 + P2 + P3 + …

47. Dalton’s Law

48. Dalton’s Law

49. Ex: A gas mixture containing carbon dioxide, oxygen, and nitrogen has a total pressure of 32.9 kPa. If the pressure due to O2 is 6.6 kPaand N2 is 23.0 kPa, what is the partial pressure of CO2?

50. Graham’s Law • Diffusion – particles move from areas of high concentration to areas of low concentration • Spraying air freshener in the corner of a room • Effusion – process of gas particles escaping a container through a single hole • Gas escaping from a balloon causing it to deflate • Diffusion and Effusion depend on the type of particle