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Molecular Composition of Gases

Molecular Composition of Gases. Chapter 11. Gay-Lussac’s law of combining volumes of gases. At constant temperature and pressure, the volumes of gaseous reactants and products can be expressed as ratios of small whole numbers. Example.

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Molecular Composition of Gases

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  1. Molecular Composition of Gases Chapter 11

  2. Gay-Lussac’s law of combining volumes of gases • At constant temperature and pressure, the volumes of gaseous reactants and products can be expressed as ratios of small whole numbers

  3. Example • When 2 L of hydrogen react with 1 L of oxygen 2 L of water vapor are produced. • Write the balanced chemical equation:

  4. You try • When 1 L of hydrogen gas reacts with 1 L of chlorine gas, 2 L of hydrogen chloride gas are produced. • Write the balanced chemical equation:

  5. Avogadro's Law • Equal volumes of gases at the same pressure and temperature contain the same number of molecules • Atoms can’t split  diatomic molecules • Gas volume is proportional to the number of molecules

  6. Molar Volume • 1 mole of any gas contains 6.022 x 1023 molecules. • According to Avogadro’s law, 1 mole of any gas must have the same volume. • Standard molar volume: molar volume of 1 mole of any gas at STP • 22.4 L

  7. Example • You are planning an experiment that requires 0.0580 mol of nitrogen monoxide gas. What volume in liters is occupied by this gas at STP? • 1.30 L NO

  8. You try • A chemical reaction produces 2.56 L of oxygen gas at STP. How many moles of oxygen are in this sample? • 0.114 mol O2

  9. Example • Suppose you need 4.22 g of chlorine gas. What volume at STP would you need to use? • 1.33 L Cl2

  10. You try • What is the mass of 1.33 x 104 mL of oxygen gas at STP? • 19.0 g O2

  11. Discuss • Explain Gay-Lussac’s law of combining volumes • State Avogadro’s law and explain its significance.

  12. Review • Boyles Law: • Charles Law: • Avogadro’s Law:

  13. Math • A quantity that is proportional to each of several quantities is also proportional to their product. Therefore:

  14. More math • Convert a proportionality • to an equality by multiplying by a constant

  15. Therefore • We can covert • to

  16. More neatly

  17. This means…. • The volume of a gas varies directly with the number of moles and the temperature in Kelvin. • The volume varies indirectly with pressure.

  18. What if… • n and T are constant? • nRT is a constant, k • Boyle’s Law • n and P are constant? • nR/P is a constant, k • Charles’s Law

  19. What if… • P and T are constant? • RT/P is a constant, k • Avogadro’s law

  20. The ideal gas constant • R • Value depends on units • SI units:

  21. Other units

  22. Solving ideal gas problems • Make sure the R you use matches the units you have. • Make sure all your units cancel out correctly.

  23. Example • A 2.07 L cylinder contains 2.88 mol of helium gas at 22 °C. What is the pressure in atmospheres of the gas in the cylinder? • 33.7 atm

  24. You try • A tank of hydrogen gas has a volume of 22.9 L and holds 14.0 mol of the gas at 12 °C. What is the reading on the pressure gauge in atmospheres? • 14.3 atm

  25. Example • A reaction yields 0.00856 mol of oxygen gas. What volume in mL will the gas occupy if it is collected at 43 °C and 0.926 atm pressure? • 240. mL

  26. You try • A researcher collects 9.09 x 10-3 mol of an unknown gas by water displacement at a temperature of 16 °C and 0.873 atm pressure (after the partial pressure of the water vapor has been subtracted). What volume of gas in mL does the researcher have? • 247 mL

  27. Finding mass • Number of moles (n) equals mass (m) divided by molar mass (M).

  28. Example • What mass of ethene gas, C2H4, is contained in a 15.0 L tank that has a pressure of 4.40 atm at a temperature of 305 K? • 74.0 g

  29. You try • NH3 gas is pumped into the reservoir of a refrigeration unit at a pressure of 4.45 atm. The capacity of the reservoir is 19.4 L. The temperature is 24 °C. What is the mass of the gas in kg? • 6.03 x 10-2 kg

  30. Example • A chemist determines the mass of a sample of gas to be 3.17 g. Its volume is 942 mL at a temperature of 14 °C and a pressure of 1.09 atm. What is the molar mass of the gas? • 72.7 g/mol

  31. Density

  32. You try • The density of dry air at sea level (1 atm) is 1.225 g/L at 15 °C. What is the average molar mass of the air? • 29.0 g/mol

  33. Stoichiometry • Involves mass relationships between reactants and products in a chemical reaction • For gases, the coefficients in the balanced chemical equation show volume ratios as well as mole ratios • All volumes must be measured at the same temperature and pressure

  34. Volume-Volume calculations • From volume of one gas to volume of another gas • Use volume ratios just like mole ratios in chapter 9

  35. Example • Xenon gas reacts with fluorine gas to produce the compound xenon hexafluoride, XeF6. Write the balanced equation for this reaction. • Xe(g) + 3F2(g)  XeF6(g) • If a researcher needs 3.14 L of XeF6 for an experiment, what volumes of xenon and fluorine should be reacted? • 3.14 L of Xe and 9.42 L of F2

  36. Example • Nitric acid can be produced by the reaction of gaseous nitrogen dioxide with water.3NO2(g) + H2O(l)  2HNO3(l) + NO(g) • If 708 L of NO2 gas react with water, what volume of NO gas will be produced? • 236 L

  37. You try • What volume of hydrogen gas is needed to react completely with 4.55 L of oxygen gas to produce water vapor? • 9.10 L

  38. You try • At STP, what volume of oxygen gas is needed to react completely with 2.79 x 10-2 mol of carbon monoxide gas, CO, to form gaseous carbon dioxide? • 0.313 L

  39. You try • Fluorine gas reacts violently with water to produce hydrogen fluoride and ozone according to the following equation:3F2(g) + 2H2O(l)  6HF(g) + O3(g) • What volumes of O3 and HF gas would be produced by the complete reaction of 3.60 x 104 mL of fluorine gas? • 1.20 x 104 mL O3 and 7.20 x 104 mL HF

  40. You try • Ammonia is oxidized to make nitrogen monoxide and water4NH3(g) + 5O2(g)  4NO(g) + 6H2O(l) • At STP, what volume of oxygen will be used in a reaction of 125 mol of NH3? What volume of NO will be produced? • 3.50 x 103 L O2 and 2.80 x 103 L NO

  41. Volume-mass and mass-volume • Converting from volume to mass or from mass to volume • Must convert to moles in the middle • Ideal gas law may be useful for finding standard conditions

  42. Example • Aluminum granules are a component of some drain cleaners because they react with sodium hydroxide to release both heat and gas bubbles, which help clear the drain clog. The reaction is:2NaOH(aq) + 2Al(s) + 6H2O (l)  2NaAl(OH)4(aq) + 3 H2(g) • What mass of aluminum would be needed to produce 4.00 L of hydrogen gas at STP? • 3.21 g

  43. Example • Air bags in cars are inflated by the sudden decomposition of sodium azide, NaN3 by the following reaction:2NaN3(s)  3N2(g) + 2Na(s) • What volume of N2 gas, measured at 1.30 atm and 87 °C, would be produced by the reaction of 70.0 g of NaN3? • 36.6 L

  44. You try • What volume of chlorine gas at 38°C and 1.63 atm is needed to react completely with 10.4 g of sodium to form NaCl? • 3.54 L Cl2

  45. Example • A sample of ethanol burns in O2 to form CO2 and H2O according to the following reaction.C2H5OH + 3O2 2CO2 + 3H2O • If the combustion uses 55.8 mL of oxygen measured at 2.26 atm and 40.°C, what volume of CO2 is produced when measured at STP? • 73.3 mL CO2

  46. You try • Dinitrogen pentoxide decomposes into nitrogen dioxide and oxygen. If 5.00 L of N2O5 reacts at STP, what volume of NO2 is produced when measured at 64.5 °C and 1.76 atm? • 7.02 L NO2

  47. Review • Diffusion: the gradual mixing of gases due to their random motion • Effusion: gases in a container randomly pass through a tiny opening in the container

  48. Rate of effusion • Depends on relative velocities of gas molecules. • Velocity varies inversely with mass • Lighter particles move faster

  49. Kinetic energy • Depends only on temperature • Equals • For two gases, A and B, at the same temperature • Each M stands for molar mass

  50. Algebra time

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