1. Reactions Involving Gases In addition to this presentation, before coming to lab or attempting the prelab quiz you must also:  Read the introduction to the lab in the coursepack  View the prelab video on Chemiscool.org

2. What’s the point? Introduce some uses of gas laws Chapter 10, Brown, LeMay and BursteinIntroduce some uses of gas laws Chapter 10, Brown, LeMay and Burstein Review stoichiometry Chapter 3, Brown, LeMay and BursteinReview stoichiometry Chapter 3, Brown, LeMay and Burstein Practice collection of gas over waterPractice collection of gas over water

3. Background GasesGases Very few particles per unit volumeVery few particles per unit volume Very weak intermolecular forcesVery weak intermolecular forces At low pressures, gases behave ideally, following the ideal gas lawAt low pressures, gases behave ideally, following the ideal gas law PV = nRT

4. The Ideal Gas Law Relates gas pressure to volume, moles and temperatureRelates gas pressure to volume, moles and temperature PV = nRT P = pressure (in atm) V = volume (in L) n = number of moles of gas T = temperature (in K) R = ideal gas constant 0.08206 L atm mol -1 K -1

5. Dalton’s Law of Partial Pressure When more than one gas is present, the total pressure equals the sum of what all the gas pressures would be by themselves P total = P 1 + P 2 + P 3 +.....When more than one gas is present, the total pressure equals the sum of what all the gas pressures would be by themselves P total = P 1 + P 2 + P 3 +..... This is an important consideration when trying to determine the pressure of an individual gas in a mixtureThis is an important consideration when trying to determine the pressure of an individual gas in a mixture

6. H 2 (g) Generation Over Water Reaction of metals with acids often makes H 2 (g) 2H + (aq) + Mg(s)  H 2 (g) + Mg 2+ (aq) We will carry out the reaction like this: Before Reaction After Reaction A small hole in the stopper allows H 2 SO 4 (aq) to be displaced by the gas

7. Important things to notice at the end: - The vapor over the water contains H 2 (g) AND H 2 O(g) - The total pressure inside the tube must equal the atmospheric pressure outside the tube … plus the pressure from this water … This gas pressure, which is the sum of P H2(g) + P H2O(g) … … equals atmospheric pressure.

8. P atmosphere = P H2(g) + P H2O(g) + P H2O( l ) gases in the tube liquid in the tube P atmosphere can be found from a barometerP atmosphere can be found from a barometer P H2O( l ) can be calculated by measuring the height of the water columnP H2O( l ) can be calculated by measuring the height of the water column The total gas pressure can be found from the difference, P atmosphere - P H2O( l )The total gas pressure can be found from the difference, P atmosphere - P H2O( l ) P H2(g) can be found by subtracting the vapor pressure of H 2 O at the measured temperatureP H2(g) can be found by subtracting the vapor pressure of H 2 O at the measured temperature

9. Calculation Tips It is ESSENTIAL that you use consistent UNITS in gas law calculationsIt is ESSENTIAL that you use consistent UNITS in gas law calculations Make sure all your pressure values are in the same units when using Dalton’s law and related conceptsMake sure all your pressure values are in the same units when using Dalton’s law and related concepts If you are using the ideal gas law, make sure your pressure is in atm, volume in L, and T in KIf you are using the ideal gas law, make sure your pressure is in atm, volume in L, and T in K

10. Sample Calculation What is the pressure of hydrogen gas evolved if atmospheric pressure is 764.5 mm Hg, the height of the water column is 189 mm and the temperature is 25.2 o C? The terms on the right hand side come from, P atmosphere is given P H2O( l ) is from the measured height of the column P H2O(g) is the vapor pressure of H 2 O at the given T  found in a reference text From before: P H2(g) = P atmosphere – P H2O( l ) – P H2O(g)

11. First, convert the water column height (P H2O( l ) ) to mm Hg: (189 mm H 2 O)(1 mm Hg / 13.6 mm H 2 O) = 13.9 mm Hg Next, look up P H2O(g) = 17.26 mm Hg at 25.2 o C (CRC Handbook of Chemistry and Physics) Now we can apply the previous equation: P H2(g) = (764.5 mm Hg) – (13.9 mm Hg) – (17.26 mm Hg) = 733.3 mm Hg

12. Using the Ideal gas Law Find the number of moles of H 2 produced if P H2 = 733.3 mm Hg, the volume of gas is 83.1 mL and the temperature is 25.2 o C. P = 733.3 mm HgP = 733.3 mm Hg V = 83.1 mLV = 83.1 mL T = 25.2 o CT = 25.2 o C R = 0.08206 L atm mol -1 K -1R = 0.08206 L atm mol -1 K -1 We have some unit conversions to do first…

13. P = (733.3 mm Hg)(1 atm / 760 mm Hg) = 0.965 atm V = (83.1 mL)(1 L / 1000 mL) = 0.0831 L T = 25.2 o C + 273.15 = 298.4 K PV = nRTOR n = PV / RT = 3.27x10 -3 mol (0.965 atm)(0.0831 L) (0.08206 L atm mol -1 K -1 )(298.4 K) n =

14. And Some Stoichiometry If 3.27 x 10 -3 mol H 2 were generated, how many grams of Mg reacted? 2H + (aq) + Mg(s)  H 2 (g) + Mg 2+ (aq) mol Mg = (3.27 x 10 -3 mol H 2 )(1 mol H 2 / 1 mol Mg) = 3.27 x 10 -3 mol Mg = 3.27 x 10 -3 mol Mg g Mg = (3.27 x 10 -3 mol Mg)(24.305 g / 1 mol Mg) = 7.95 x 10 -2 g Mg = 7.95 x 10 -2 g Mg

15. Safety Goggles and coat worn at all timesGoggles and coat worn at all times Caution with 3 M H 2 SO 4Caution with 3 M H 2 SO 4 sulfuric acid is a very caustic acidsulfuric acid is a very caustic acid avoid contact with skinavoid contact with skin inform your instructor of spills immediatelyinform your instructor of spills immediately rinse spill areas with copious amounts of waterrinse spill areas with copious amounts of water