Investigating Substances:
Investigating Substances: Ideal Gases and Airbags
BACKGROUND
Gases are an important substance in our world, and many situations depend on their interactions and behavior. For example, scuba divers may experience physical pain if they ascend to the surface from deep water too quickly, air travelers my feel their ears “pop” while flying, and temperature changes in hotair balloons help the riders to ascend or descend. The relationship between various properties of gases is often described by the Ideal Gas Law, PV=nRT where P=pressure, V=volume, n=number of moles, T=temperature, and R is the ideal gas constant. Though based on assumptions that gas molecules do not interact with each other and occupy no volume, assumptions you will later learn to be incorrect, the Ideal Gas Law is still incredibly useful in characterizing the properties of gases. In this lab, you investigate the amount of gas produced from various reactions and compare that amount to the values expected from the ideal gas law. For the second part of the lab, you will use the ideal gas law and your estimated deviations to create a model of an airbag. Airbags, as you know, are safety devices required on all automobiles produced today. The reaction in commercial airbags depends on the decomposition of sodium azide: 2NaN3 (s) 2Na (s) + 3N2 (g) There is a good website that discusses the chemistry of airbags at the following website: http://www.chemistry.wustl.edu/~edudev/LabTutorials/Airbags/airbags.html Since our lab is not equipped to work with chemicals as toxic as sodium azide, we will work with the similar gas producing agent sodium carbonate (Na2CO3). When sodium carbonate is allowed to react with acetic acid, carbon dioxide (CO2) and two other products, acetate and water, are formed: Na2CO3 (s) + 2 CH3COOH (aq) 2 CH3COOˉNa + (aq) + H2O (l) + CO2 (g) The stoichiometry of this reaction is not simply 1:1 for all reactants, so some care should be taken when doing your calculations. Because the overall goal of this experiment is to design an ‘airbag’ that inflates rapidly, and fully, without wasting materials, you will need to convert molar amounts of reagents into gram and volume amounts in order to produce the precise volume of CO2 that will fill up your ‘airbag’.
OBJECTIVES
By preparing for and performing this experiment, you will: • • • •
Use a balance to accurately weigh a sample; Develop and perfect a procedure for measuring the amount of gas produced from various reactions; Use the ideal gas law to predict the theoretical moles of gas produced and compare this value to the experimental number of moles obtained; Use your deviations from the part 1 reactions to build a fully inflated "airbag."
MATERIALS
Chemicals (hood/balance bench):
1M HCl
Mg(s) CaCO3(s) Na2CO3(s) 6M Acetic Acid, CH3COOH Equipment (dispensing room):
Lab Pro Kit or GoLink Gas Pressure Sensor Bag Plastic zipclosure Bag Small Vial
You will also need a laptop with LoggerPro software installed.
PROCEDURES Part 1: Investigating the ideal behavior of some gases Each group will work with Mg(s) and one of the other two carbonates. By adding HCl and the solid together, a reaction producing a gas will occur. It is your goal to use the gas pressure sensor to measure the amount of gas produced and compare that to the theoretical amount of moles expected. There is not a specific procedure for you this week, as there are several ways that you can perform this experiment successfully. As you design your procedure, there are some things to keep in mind: • • •
Why should you use excess acid, and how much should be used? Why is it a good idea to mix the two reactants after the container is sealed? What is the best way to achieve this? (Hints: You will need to get the two reactants into an Erlenmeyer flask and seal it with the gas pressure stoppered before they mix. It is also good to have some time (maybe 20 seconds or so) for an initial pressure reading of the sealed flask before you mix the reactants. Other helpful hints are discussed in lecture)
You should also note that the pressure building up in the flask will eventually cause the top to pop off. It is ok to hold the top on the flask to help keep it from popping. You will also need to try to prevent this by limiting the amount of gas produced and controlling the amounts of reactants used. The flask tends to pop off when the pressure reaches 1.2 1.5 atm. The room pressure is generally around 1.0 atm, so you will only have a window of about 0.5 atm to work with. You should have your whole reaction occurring under that pressure change. Compare the amount of gas produced by the reactions to the amount predicted from the ideal gas calculations. Is there a good agreement? If not and your procedural methods are at fault, revise your procedure and recollect data. If you still have errors that you believe not to be “user error,” how can those be explained?
Part 2: Making an "airbag" 1. Determine the volume of your airbag. The easiest method is by filling it with water and sealing it, and then using a graduated cylinder to measure the volume. Just make sure to completely dry your bag before proceeding.
2. Calculate how much sodium carbonate you need, taking into consideration the deviations from ideal behavior. 3. Weigh out your calculated mass of sodium carbonate, making sure to record the exact amount in your notebook. Add this to your bag. 4. Calculate how much 6.0 M acetic acid you need to react with your measured amount of sodium carbonate and measure out this volume. 5. Quickly, but carefully, pour the acetic acid into your "airbag" and seal it. Mix the reactants by shaking the bag. 6. Once the reaction is complete, observe how full the airbag became. Were all of the reactants consumed? 7. Repeat the procedure above once making any adjustments to your calculations. 8. Show your successful airbag to your TA!
REPORT
Make sure to turn your notebook pages in to your instructor before you leave lab. This experiment will require a short report. Details are posted on Bb.
BACKGROUND
Gases are an important substance in our world, and many situations depend on their interactions and behavior. For example, scuba divers may experience physical pain if they ascend to the surface from deep water too quickly, air travelers my feel their ears “pop” while flying, and temperature changes in hotair balloons help the riders to ascend or descend. The relationship between various properties of gases is often described by the Ideal Gas Law, PV=nRT where P=pressure, V=volume, n=number of moles, T=temperature, and R is the ideal gas constant. Though based on assumptions that gas molecules do not interact with each other and occupy no volume, assumptions you will later learn to be incorrect, the Ideal Gas Law is still incredibly useful in characterizing the properties of gases. In this lab, you investigate the amount of gas produced from various reactions and compare that amount to the values expected from the ideal gas law. For the second part of the lab, you will use the ideal gas law and your estimated deviations to create a model of an airbag. Airbags, as you know, are safety devices required on all automobiles produced today. The reaction in commercial airbags depends on the decomposition of sodium azide: 2NaN3 (s) 2Na (s) + 3N2 (g) There is a good website that discusses the chemistry of airbags at the following website: http://www.chemistry.wustl.edu/~edudev/LabTutorials/Airbags/airbags.html Since our lab is not equipped to work with chemicals as toxic as sodium azide, we will work with the similar gas producing agent sodium carbonate (Na2CO3). When sodium carbonate is allowed to react with acetic acid, carbon dioxide (CO2) and two other products, acetate and water, are formed: Na2CO3 (s) + 2 CH3COOH (aq) 2 CH3COOˉNa + (aq) + H2O (l) + CO2 (g) The stoichiometry of this reaction is not simply 1:1 for all reactants, so some care should be taken when doing your calculations. Because the overall goal of this experiment is to design an ‘airbag’ that inflates rapidly, and fully, without wasting materials, you will need to convert molar amounts of reagents into gram and volume amounts in order to produce the precise volume of CO2 that will fill up your ‘airbag’.
OBJECTIVES
By preparing for and performing this experiment, you will: • • • •
Use a balance to accurately weigh a sample; Develop and perfect a procedure for measuring the amount of gas produced from various reactions; Use the ideal gas law to predict the theoretical moles of gas produced and compare this value to the experimental number of moles obtained; Use your deviations from the part 1 reactions to build a fully inflated "airbag."
MATERIALS
Chemicals (hood/balance bench):
1M HCl
Mg(s) CaCO3(s) Na2CO3(s) 6M Acetic Acid, CH3COOH Equipment (dispensing room):
Lab Pro Kit or GoLink Gas Pressure Sensor Bag Plastic zipclosure Bag Small Vial
You will also need a laptop with LoggerPro software installed.
PROCEDURES Part 1: Investigating the ideal behavior of some gases Each group will work with Mg(s) and one of the other two carbonates. By adding HCl and the solid together, a reaction producing a gas will occur. It is your goal to use the gas pressure sensor to measure the amount of gas produced and compare that to the theoretical amount of moles expected. There is not a specific procedure for you this week, as there are several ways that you can perform this experiment successfully. As you design your procedure, there are some things to keep in mind: • • •
Why should you use excess acid, and how much should be used? Why is it a good idea to mix the two reactants after the container is sealed? What is the best way to achieve this? (Hints: You will need to get the two reactants into an Erlenmeyer flask and seal it with the gas pressure stoppered before they mix. It is also good to have some time (maybe 20 seconds or so) for an initial pressure reading of the sealed flask before you mix the reactants. Other helpful hints are discussed in lecture)
You should also note that the pressure building up in the flask will eventually cause the top to pop off. It is ok to hold the top on the flask to help keep it from popping. You will also need to try to prevent this by limiting the amount of gas produced and controlling the amounts of reactants used. The flask tends to pop off when the pressure reaches 1.2 1.5 atm. The room pressure is generally around 1.0 atm, so you will only have a window of about 0.5 atm to work with. You should have your whole reaction occurring under that pressure change. Compare the amount of gas produced by the reactions to the amount predicted from the ideal gas calculations. Is there a good agreement? If not and your procedural methods are at fault, revise your procedure and recollect data. If you still have errors that you believe not to be “user error,” how can those be explained?
Part 2: Making an "airbag" 1. Determine the volume of your airbag. The easiest method is by filling it with water and sealing it, and then using a graduated cylinder to measure the volume. Just make sure to completely dry your bag before proceeding.
2. Calculate how much sodium carbonate you need, taking into consideration the deviations from ideal behavior. 3. Weigh out your calculated mass of sodium carbonate, making sure to record the exact amount in your notebook. Add this to your bag. 4. Calculate how much 6.0 M acetic acid you need to react with your measured amount of sodium carbonate and measure out this volume. 5. Quickly, but carefully, pour the acetic acid into your "airbag" and seal it. Mix the reactants by shaking the bag. 6. Once the reaction is complete, observe how full the airbag became. Were all of the reactants consumed? 7. Repeat the procedure above once making any adjustments to your calculations. 8. Show your successful airbag to your TA!
REPORT
Make sure to turn your notebook pages in to your instructor before you leave lab. This experiment will require a short report. Details are posted on Bb.
