Physical Characteristics of Gases

1. Physical Characteristics of Gases

2. Kinetic Molecular Theory • The Kinetic-Molecular Theory is based on the idea that particles of matter are always in motion. It can explain the properties of solids, liquids, and gases in terms of the energy of particles and the forces that act between them.

3. An Ideal Gas • An ideal gas is an imaginary gas that perfectly fits all the assumptions of the kinetic-molecular theory. The kinetic-molecular theory is based on 5 assumptions.

4. Assumption #1 1. Gases consist of large numbers of tiny particles that are far apart relative to their size. • Gases have low densities • Gases are easily compressed • Volume occupied by gases is mostly empty space

5. Assumption #2 2. Collisions between gas particles and between particles and container walls are elastic collisions. An elastic collision is one in which there is no net loss of kinetic energy.

6. Assumption #3 • Gas particles are in continuous, rapid, random straight line motion until they collide. They, therefore, posses kinetic energy, which is energy of motion.

7. Assumption #4 4. There are no forces of attraction or repulsion between gas particles. • When gas molecules collide they do not stick together but instead immediately bounce apart.

8. Assumption #5 • 5. The average kinetic energy of gas particles depends on the temperature of the gas. • KE of gas particles increases as temperature increases and decreases as temperature decreases • KE = 1_ mv2 2

9. Physical Properties • Expansion • Fluidity • Low Density • Compressibility • Diffusion and Effusion

10. Property #1: Expansion • Gases do NOT have definite shape or a definite volume. • Gases completely fill any container in which they are enclosed and take its shape.

11. Property #2: Fluidity • Gas particles glide easily past one another. • Because gases, like liquids, have the ability to flow they are referred to as fluids.

12. Property #3: Low Density • Because gas particles are so much farther apart than solids or liquids, their density is 1/1000 that of solids or liquids.

13. In diffusion, gases spread out and mix with one another, even without being stirred. In effusion, gas particles escape through a tiny opening. Molecules of low mass will effuse faster than molecules of higher mass. Property #5: Diffusion and Effusion

14. The volume of a gas can be greatly decreased during compression. Increasing the pressure of a gas will increase it’s volume. Property #4: Compressibility

15. Ideal VS Real Gases • Ideal gases always obey the kinetic theory. (Closest to ideal would be the noble gases.) • Real gases vary from the kinetic theory at various temperatures and pressures.

16. Real Gases Real gases act like ideal gases when their particles are far enough apart (large volume) and they have enough KE (higher temperatures). • Real gases do NOT behave like ideal gases at high pressures and low temperatures because at low temperatures, real gases exert more attractive forces between their particles.

17. Volume, Pressure, Temperature, Number of Moles (Descriptions of Gases) Volume – refers to the space matter (gas) occupies. Measured in liters (L). 1L = 1000mL

18. Pressure – the number of times particles collide with each other and the walls of the container (force exerted on a given area). Measured in atmospheres (atm). 1atm = 760 mm Hg 1atm = 760 torr 1atm = 101.3 kPa – kilopascals 1 torr = 1 mmHg

19. Temperature – as temperate increases gas particles move faster, as temperature decreases gas particles move slower. Measured in Kelvin (K). K = 273 + C

20. Number of Moles – tells you how much of a certain gas you have 1 mole = number of grams of the compound or element (molar mass) STP – “standard temperature and pressure”which is 0C and 1.00 atm.

21. Gas Laws - How do all of pressure, temperature, volume, and amount of a gas relate to each other? Rules for solving gas law problems: 1st write down what is given and what is unknown, 2nd identify the gas law you want to use, and 3rd rearrange the formula to solve for the unknown and then solve the problem. (If temperature is involved, it MUST be converted to Kelvin! K = 273 + C)

22. The Gas Laws • Boyle’s Law: Pressure-Volume Relationship • Charles’ Law: Volume-Temperature Relationship • Gay-Lussac’s Law: Pressure-Temperature Relationship • Combined Gas Law: Pressure-Temperature-Volume • Dalton’s Law of Partial Pressure • Avogadro’s Law • Ideal Gas Law

23. Boyle’s Law As pressure increases, volume decreases and as pressure decreases, volume increases

24. Boyle’s Law - Pressure and Volume (when temperature remains constant) V1 = initial or old volume V1P1 = V2P2 V2 = final or new volume P1 = initial or old pressure P2 = final or new pressure Inverse Relationship

25. Boyle’s Law

26. Robert Boyle(1627-1691) • Boyle was born into an aristocratic Irish family • Became interested in medicine and the new science of Galileo and studied chemistry.  • A founder and an influential fellow of the Royal Society of London • Wrote extensively on science, philosophy, and theology.

27. 1. A gas occupies 3.00L at 1.00atm of pressure. What volume does it occupy at 5.00atm?

28. Charles’ Law -Volume and Temperature (when pressure is constant) V1 = V2 T1 = initial or old temperature T1 T2 T2 = final or new temperature Direct Relationship As temperature increases, volume increases and as temperature decreases, volume decreases.

29. Jacques Charles (1746-1823) French Physicist Part of a scientific balloon flight on Dec. 1, 1783 – was one of three passengers in the second balloon ascension that carried humans This is how his interest in gases started It was a hydrogen filled balloon – good thing they were careful!

30. 1. A gas has a volume of 500.mL at 298K. What volume does it have at 373K?

31. Gay-Lussac’s Law - Pressure and Temperature (when volume is constant) • P1 = P2 • T1 T2 • Direct Relationship • As temperature increases, pressure increases and as temperature decreases, pressure decreases.

32. Joseph Louis Gay-Lussac (1778 – 1850) • French chemist and physicist • Known for his studies on the physical properties of gases. • In 1804 he made balloon ascensions to study magnetic forces and to observe the composition and temperature of the air at different altitudes.

33. 1. The gas in an aerosol can is at 3atm of pressure at 298K. What would the gas pressure in the can be at 325K?

34. Combined Gas Law - Pressure, Temperature, moles, and Volume V1P1 = V2P2 n1T1 n2T2

35. 1. A helium filled balloon has a volume of 50.0mL at 298K and 1.08atm. What volume will it have at 0.855atm and 203K?

36. Avogadro’s Law • Avogadro’s Law states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.

37. Daltons Law of Partial Pressures Daltons Law of Partial Pressure states that the total pressure of a mixture of gases is equal to the sum of the partial pressures of the component gases.

38. PT = P1 + P2 + P3 + ……. PT = total pressure P# = the partial pressures of the individual gases

39. If the first three containers are all put into the fourth, we can find the pressure in that container by adding up the pressure in the first 3: 2 atm + 1 atm = 6 atm + 3 atm 4 3 2 1

40. Daltons Law applied to Gases Collected by Water Displacement Ptotal = Pgas + PH2O

41. Daltons Law applied to Gases Collected by Water Displacement Ptotal = Pgas + PH2O

42. Daltons Law applied to Gases Collected by Water Displacement Ptotal = Pgas + PH2O

43. Daltons Law applied to Gases Collected by Water Displacement – Figure 10-15 page 324 Patm or PT= Pgas + PH2O Patm or PT = barometric pressure or total pressure Pgas = pressure of the gas collected PH2O = vapor pressure of water at specific temperature (Found on page 899 of you textbook.)

44. Ideal Gas Law (PV = nRT) – to use this law, all units must be as follows: • P = pressure in atm • V = volume in liters • n = number of moles • T = temperature in Kelvin • R = (0.0821L) (1atm) (1mol) (1K) R is the ideal gas constant (page 342 in book describes where this constant came from.)

45. Molar Volume of Gases Recall that 1 mole of a compound contains 6.022 X 1023 molecules of that compound – it doesn’t matter what the compound is. One mole of any gas, at STP, will occupy the same volume as one mole of any other gas at the same temperature and pressure, despite any mass differences. The volume occupied by one mole of a gas at STP is known as the standard molar volume of a gas. It has been found to be 22.4liters. We can use this as a new conversion factor 1mol of gas/22.4L of same gas. (Avogadro’s Law states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.

46. 1 mol = 22.4L (molar volume of any gas at STP)

47. 1h. What volume, in L, is occupied by 32.0 grams of oxygen gas at STP?

48. Stoichiometry of Gases Just like mole ratios can be written from an equation so can a volume ratio-same concept! • 2CO(g) + O2 (g) 2CO2 (g)

49. 1i. Using the above equation, what volume of oxygen gas is needed to react completely with 0.626L of carbon monoxide to form carbon dioxide? • 2i. How many grams of solid calcium carbonate must be decomposed to produce 5.00L of carbon dioxide gas at STP? • 3i. How many liters of hydrogen gas at 35.0C and 0.980atm are needed to produce 8.75L of gaseous water according to the following equation? • WO3(s) + 3H2(g) W(s) + 3H2O(g)

50. Graham’s Law IV. Effusion and Diffusion • Effusion is the process whereby the molecules of a gas confined in a container randomly pass through a tiny opening in the container. (onions on page 352)