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Chapter 14 – Gases

Chapter 14 – Gases. Kinetic Molecular Theory (KMT). Defn – describes the behavior of gases in terms of particle motion Makes assumptions of size, motion, & energy of gas particles. KMT and Gases. Ideal Gases – imaginary gas that conforms to assumptions of KMT. Assumptions of KMT.

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Chapter 14 – Gases

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  1. Chapter 14 – Gases

  2. Kinetic Molecular Theory (KMT) • Defn – describes the behavior of gases in terms of particle motion • Makes assumptions of size, motion, & energy of gas particles

  3. KMT and Gases • Ideal Gases – imaginary gas that conforms to assumptions of KMT

  4. Assumptions of KMT • A) gas particles do not attract or repel each other no intermolecular forces occurring. gases are free to move in their containers without interference from other particles

  5. Assumptions of KMT • B) gas particles have NO volume almost all of the volume of a gas is empty space. The particles are insignificant in size compared to all the space

  6. Assumptions of KMT • C) gas particles are in constant motion they move in straight lines until they collide with each other or the walls of container

  7. Assumptions of KMT • D) no kinetic energy is lost when gas particles collide with each other or walls aka elastic collision. There is no loss in speed of the particles

  8. Assumptions of KMT • E) ALL gases have same average kinetic energy at a given temperature as temperature increases, more energy. As temp decreases, less energy

  9. Assumptions of KMT • ***KEEP IN MIND IDEAL GASES DO NOT EXIST!!!!!!!*** • APPLIES AT ALL TEMPERATURES AND ALL PRESSURE

  10. Real Gases • Defn – do not behave according to assumptions of KMT • Characteristics • i) real gas particles have volume • ii) real gas particles exert attractive forces on each other

  11. Real Gases • When do real gases act like ideal gases? at very low pressure and very high temp - at low P, molecules far from each other - at high temp, molecules move too fast to have intermolecular interactions

  12. Characteristics of Gases • 1) expansion • 2) fluidity • 3) low density • 4) compressibility • 5) diffusion

  13. 4 measurable variables of gases • Pressure • Temperature • Volume • Amount

  14. Pressure • Defn – force/area; pressure caused by collisions of molecules • Units: atmosphere (atm), millimeter of mercury (mm Hg),torr, kiloPascal (kPa) • Conversions: 1 atm = 760 mm Hg = 760 torr = 101.3 kPa

  15. Sample problem #1 • Convert 2.3 atm to torr 2.3 atm 760 torr = 1748 torr 1 atm

  16. Sample problem #2 • Convert 450 mm Hg to kPa 450 mmHg 101.3 kPa = 60 kPa 760 mm Hg

  17. Measuring air pressure • What device is used to measure air pressure????? BAROMETER

  18. Standard Temp and Pressure (STP) • P = 1 atm • T = 0°C

  19. Temperature • Unit • Kelvin (K) Kelvin = °C + 273

  20. Temperature • Convert 45°C to Kelvin 45°C + 273 = 318 K

  21. Temperature • Absolute zero – 0 K • Molecules stop moving • Question: what is absolute zero temperature in Celsius? -273°C

  22. Volume • Units: • mL • L

  23. Amount • Units • grams • moles

  24. Gas Laws • Each gas law relates the 4 variables: • Pressure • Temperature • Volume • Amount For the next 4 gas laws, amount is not a factor • We will only pay attention to P, T, & V

  25. Boyles’ Law: P/V relationship • Defn – at constant T, volume of a fixed amount of gas varies inversely with pressure • inversely – as one variable increases, other variable decreases P V

  26. Boyle’s Law • Formula P1V1 = P2V2 P1 & V1 are initial conditions P2 & V2 are final conditions

  27. Boyle’s Law • A balloon filled with helium gas has a volume of 500 mL at 1.00 atm. When it rises to a higher altitude, the pressure is reduced to 0.50 atm. If the temperature is constant, what is the volume of the balloon? 1000 mL

  28. Charles’ Law: V/T relationship • Defn – at constant P, volume of fixed amt of gas varies directly with Kelvin temp • Directly – as one variable increase, the other increases V T

  29. Charles’ Law • Formula • V1 & T1 are initial conditions • V2 & T2 are final conditions

  30. Charles’ Law • A helium balloon has a volume of 2.75 L at 20°C. On a cold day, the balloon is placed outside where it shrinks to 2.46 L. What is the temperature outside? 262 K

  31. Gay-Lussac’s Law: P/T relationship • Defn – at constant V, pressure of fixed amt of gas varies directly with Kelvin temp • Directly – as one variable increase, the other increases P T

  32. Gay-Lussac’s Law • Formula • P1 & T1 are initial conditions • P2 & T2 are final conditions

  33. Gay-Lussac’s Law • Before a trip from Sugar Land to South Padre, the pressure in an automobile tire reads 2.1 atm at 27°C. Upon arriving in Padre, the gauge reads 2.3 atm. What is the temperature in South Padre? 329 K

  34. Combined Gas Law: P/T/V • Defn – relationship between P,V, & T of fixed amount of gas • Formula

  35. Combined Gas Law • The volume of a gas is 27.5 mL at 22.0°C and 740 mm Hg. What will be its volume at 15°C and 755 mm Hg? 26 mL

  36. Avogadro’s Principle • Defn – equal volumes of gases at same temp and pressure contain equal number of molecules - the size of the molecule does not influence the volume a gas occupies - for example, 1000 relatively large Kr gas molecules occupies the same volume as 1000 smaller He gas molecules

  37. Molar Volume of a Gas • Molar Volume – volume that one mole of a gas occupies at STP (0°C, 1 atm) 1 mole = 22.4 L

  38. Molar Volume of a Gas • Ex prob #1: calculate the volume 0.881 mol H2 will occupy at STP. 0.881 mol H2 0.881 mol H2 x 22.4 L H2 1 mol H2 = 19.7 L H2

  39. Molar Volume of a Gas • What is the mass of 0.0752 L of oxygen gas at STP? 0.0752 L O2 0.0752 L O2 x 1 mol O2 22.4 L O2 0.0752 L O2 x 1 mol O2 x 32 g O2 22.4 L O2 1 mol O2 = 0.107 g O2

  40. Ideal Gas Law • Defn – relationship between P,V,T and # of moles, n • Formula PV = nRT ideal gas constant

  41. 3 different R constants R = 0.0821 L·atm mol·K = 62.4 L· mm Hg mol·K = 8.314 L·kPa mol·K pay attention to what unit of pressure is given in problem then use the appropriate R Ideal Gas Constant

  42. Ideal Gas Law • Ex prob 1: what is the volume of 0.250 moles of oxygen gas at 20°C and 740 mm Hg? which R do we use? 6.18 L O2

  43. Ideal Gas Law • Ex prob 2: calculate the pressure, in atm, of 4.75 L NO2 containing 0.86 mol at 27°C. 4.46 atm

  44. Ideal Gas Law • Ex prob 3: calculate the grams of N2 present in a 0.600 L sample kept at 1.0 atm and 22°C. first determine # of moles 0.025 mol N2 convert to grams 0.70 g N2

  45. 4 gas laws vs. ideal gas law • The differences between ideal gas law and the other 4 are: • Ideal gas law utilizes an amount (moles), other 4 don’t • The 4 gas laws have a change in condition; ideal gas law does not

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