Gas Laws Introduction

Gas Laws Introduction Although we cannot see gases, we can observe their behavior and study their properties. This lab will apply several concepts from Ideal Gas Laws. You will use your knowledge of chemical reactions and gas phase stoichiometry to predict the theoretical number of moles of gas formed as a product of the reaction of aluminum with hydrochloric acid. You will then use the Ideal Gas Law to predict the theoretical volume of the gas formed. Then you will carry out the actual reaction, measure the actual volume of gas generated, and use Dalton’s law of partial pressures to calculate the actual number of moles of gas produced which will be compared to the theoretical moles predicted. Equipment Set-up and Leveling bulb discussion: A buret will be used to collect our gas product, so we can measure the volume of gas produced accurately. A leveling bulb (a plastic bottle with an opening at its base) is attached to the bottom of the buret with a rubber hose. The opening allows the water in the system to be exposed to the atmospheric pressure in the lab. The leveling bulb is filled with water, and the water moves through the hose to fill the buret. As gas is produced, the water will be pushed out of the buret into the leveling bulb thus preventing excess pressure build up. The reaction chamber is a round-bottomed flask, into which the hydrochloric acid and aluminum metal is placed. There is a second hose that connects the reaction vessel to the top of the buret. When the reaction flask is screwed on, the system is closed. Note that when the reaction flask is removed by unscrewing the screw cap, the water levels in the buret and leveling bulb will be equal (both at atmospheric pressure). Thus, to raise or lower the water level in the buret you simply raise or lower the leveling bulb. Before beginning the reaction with the reaction flask off, adjust the leveling bulb up and down. Verify that as the bulb is raised, the water in the buret rises to match the level of water in the bulb and when the bulb is lowered, the water level in the buret drops to match the level in the bulb, so the water levels remain equal. Figure 1: Experimental Set-up with Leveling Bulb page 1 of 8 Check for leaks. With the reaction flask screwed on and the rubber septum attached, raise and lower the bulb. The water should not flow as freely as before due to the system being closed, and the water levels should not remain the same. With the reaction flask on (system closed), lower the bulb so the water level in the buret is higher than the water level in the bulb. The level of water in the buret should not continue dropping to the same level as the water level in the bulb. If it does, your system has a leak. Check to make sure the reaction flask is screwed on securely and the septum is attached. If you cannot find the source of the leak, consult your instructor. Once leaks have been eliminated, lower the leveling bulb, so its water level is lower than the level of water in the buret. The pressure inside the buret is now less than atmospheric pressure (the gas in the buret is not pressing down as hard as the atmosphere). Now raise the bulb so that the water level in the buret is lower than the water level in the bulb. The pressure inside the buret is now higher than atmospheric pressure (the gas in the buret is pressing down harder than the atmosphere). Remember, when the water levels are equal, the pressures are equal. You will keep the water levels equal while running the reaction so that the pressure inside the reaction chamber and buret is equal to atmospheric pressure. Dalton’s Law of Partial Pressures: As a gas is collected over water it becomes saturated with water vapor. The total pressure in the system at the end of the reaction is, thus, due to both the gas product’s pressure and the water vapor pressure. So the total pressure is the sum of the pressure of the gas (Pgas) and the pressure of the water vapor ( PH 2 O ). This is a result of Dalton’s Law of Partial Pressures. Ptotal = Pgas + PH 2O When the water level in the buret is kept level with the water in the leveling bulb, the total pressure inside the buret is the same as the pressure outside the buret. Thus the total pressure is simply the atmospheric pressure. The pressure of the water vapor depends only on temperature and can be obtained by looking it up in the CRC Handbook of chemistry and physics. In reality, if a reaction is carried out in a closed container to trap any gas product, the pressure inside the container will build up. If too much pressure results, the container could explode. We must avoid building up pressure in the glass buret, and this is accomplished by keeping one end (the leveling bulb end) of the apparatus open to the atmosphere throughout the experiment. For more information: Chemistry: Atom’s First by OpenStax sections 8.2 – “Relating Pressure, Volume, Amount, and Temperature: The Ideal Gas Law” and 8.3 – “Stoiciometry of Gases . . .”. Equations to use for the calculations: PV=nRT P = pressure (atm), V = volume (liters), n = moles, T = temperature (Kelvin) and R = ideal gas constant (0.08206 Latmmol-1K-1) TK = TºC + 273.15 760 torr = 1 atm = 760 mmHg Ptotal = Pgas + PH 2O % Yield = moles H2 actual x 100 moles H2 theoretical page 2 of 8 Materials: gas laws apparatus aluminum can hole punch balance 6M HCl 10-mL graduated cylinder plastic weigh boat ruler DI water bottle calculator Procedure SAFETY PRECAUTIONS: In the presence of a flame, the hydrogen gas given off in this reaction can react explosively with oxygen in the air. No flames are thus allowed in laboratory during this experiment. 1. Obtain 6 round pieces of an aluminum can by using a hole punch. Obtain three weigh boats, and put two of the aluminum pieces in each. (Each trial will use 2 pieces.) Weigh the aluminum pieces on an analytical balance by taring the empty weigh boat first. Record the exact mass of the aluminum pieces for your three trials in your data table. 2. Carry out the required theoretical calculations on your report sheet and obtain your instructors initials before moving on to step 3. 3. Adjust the leveling bulb and check for leaks as described in the Experimental Set-up section. 4. Obtain about 3.0 mL of 6M hydrochloric acid from the reagent station. See technique “Transferring liquids”. Return to your bench taking care not to spill any of the acid. Hydrochloric acid can cause burns if in comes in contact with skin. If any acid is spilled on your skin, wash immediately with soap and water, tell your instructor, and rinse for 15 minutes. Safety goggles must be worn at all times because contact with HCl(aq) for even a short time can cause permanent eye damage. 5. Remove the reaction flask, and pour the hydrochloric acid inside. 6. Lift the leveling bulb so that the buret water level is around 15 mL. When everyone is ready in your group, drop the two aluminum pieces for trial one into the flask. Screw the reaction flask back on while holding the leveling bulb steady so that the water levels in the bulb and buret are equal. Quickly record the exact volume in the buret as the initial volume for trial 1 making sure the bulb and buret water levels are still equal. You may use a ruler to help you eye ball the level between the buret and leveling flask. The reaction takes a couple of seconds to get started…so quickly record the volume. 7. While the reaction proceeds, keep the leveling bulb and buret water levels equal. 8. When the reaction has stopped, hold the bulb steady with the water levels equal, and record the exact final buret volume. See technique “Reading a meniscus in various pieces of glassware”. Again, you may use a ruler to help you eye ball the level between the buret and leveling flask 9. Unscrew the reaction flask, empty the contents into your waste beaker, and rinse with DI water twice. 10. Repeat steps 3-9 for the remaining two trials with the other aluminum can pieces. Please refer to the Laboratory Techniques Document on the CHM151LL Course Website for more detailed techniques and images of lab equipment. Clean-Up: Pour all waste in the desigated waste container in the hood. Rinse everything well with soapy tap water followed by a quick DI water rinse. Clean your benchtop. Put all equipment back exactly where you found it. page 3 of 8 Name: _____________________________ Partners: ________________________ Gas Laws Lab Report Turn in Pages 4-7 as your graded lab report Data: Temperature of the lab room: _______________ Barometer Reading: ________________ Partial Pressure H2O:_____________ Data Table for Gas Laws Experiment: Trial 1 Trail 2 Trail 3 Mass of Aluminum Buret VInitial Buret VFinal Observations: Calculations Temperature Conversion: Pressure Conversions: Barometer Pressure Conversion: Partial Pressure of Water Vapor Conversion: Write and balance the equation for the reaction between solid aluminum and hydrochloric acid. page 4 of 8 Calculate the theoretical moles of hydrogen gas that should be produced in the reaction based on the mass of aluminum for each trial. Calculate the theoretical volume of hydrogen. ***Have your instructor confirm your calculations and initial here BEFORE starting the experiment: ___________________________ page 5 of 8 Actual volume H2 gas calculation (Find VTotal from the buret readings): Actual moles H2 gas calculation: Note: You will use Dalton’s Law of Partial Pressures and the partial pressure of water at the temperature in the lab room today to find the actual pressure of hydrogen gas for your calculations. Percent yield calculation for moles of hydrogen gas produced: page 6 of 8 Results: Results Table For Hydrogen Gas Produced: Trial 1 Theoretical moles H2 gas produced Trial 2 Trial 3 Theoretical volume H2 gas produced Actual volume of H2 gas produced (VTotal) Actual moles of H2 gas produced Percent Yield Conclusion: (5 pts) Summarize the results for determining the moles of gas produced for this experiment and the percent yield. Use data to support. page 7 of 8 No Post-Lab Quiz Next Week – Only the Lab Final Exam. Post-Lab Questions – These questions will not be graded as part of your lab report grade. You will be responsible for the information in these questions and able to answer these or similar questions on the FINAL EXAM. Questions will also be similar to your lab report data, observations, calculations, and results. 1. Using the Activity Series circle the metals that would react with hydrochloric acid to produce hydrogen gas. Cu(s) Ni(s) Au(s) Mg(s) Pb(s) Ag(s) Mn(s) Fe(s) 2. Why was the exact volume of the hydrochloric acid in the flask not recorded? 3. Draw a picture representing 5 molecules of hydrochloric acid in the beaker of water. Hint: Is hydrochloric acid a strong, weak, or non-electrolyte? 4. Would the following hypothetical errors cause a higher or lower percent yield for moles of gas? Explain your answer in each case. a. A student forgot to subtract the vapor pressure of water from the atmospheric pressure while doing the calculations. High Low b. A student held the leveling bulb way below the buret’s water level while reading the final buret volume. High Low 5. What would happen if the temperature in the lab room was 45.6ºC on the day the experiment was run? Calculate the theoretical volume of hydrogen gas produced from 0.0150g of aluminum with excess hydrochloric acid at 45.6ºC and 0.9555 atm to see how the volume is affected by the temperature. Is it what you thought? 6. There is a difference between your theoretical and actual volume of hydrogen gas. Assuming your technique was good and there was no human error, explain TWO possible reasons for the difference. page 8 of 8