Introduction. Objectives. Hazards. Procedure

Transcription

1 Experiment: Exploring Gases Note to Students: Check with your instructor to see which parts of this lab (Parts A, B, or C) you will complete. Introduction Gases are made up of molecules that are in constant motion and exert pressure when they collide with the walls of their container. The velocity (speed) and the number of collisions of these molecules is affected when the temperature of the gas increases or decreases, or if the volume of gas changes as it is compressed or expanded. Objectives In this experiment, you will Investigate what pressure is and how various substances change their properties with changes in pressure. Determine the relationship between the temperature of a gas sample and its pressure. Determine the relationship between the volume of a gas sample and its pressure. Practice graphing data to determine if variables are inversely or directly proportional. Describe how the macroscopic changes that are observed result from a model of how molecules move at the submicroscopic level. Hazards Even though dangerous chemicals are not used, glassware is used so make sure to wear your goggles this entire period. Procedure Parts A, B, and C can be done in any order. However, it helps to get the warm water needed for Part B started on the hot plate earlier in the period. Part A. Exploring Pressure 1. Place a flask ( ml) with about 100 ml of water on a hot plate and cover its ending with a balloon. Turn on the hot plate. While waiting, predict what you expect to happen on the report sheet. Then describe what you actually see and draw a before and after picture of what you think the molecules are doing. 2. Place a similar flask with 100 ml of water on a hot plate. Bring the water to a boil. Remove from the hot plate. Cover the ending with a balloon. Wait for the flask and the water to cool. While waiting, predict what you expect to happen on the report sheet. Then describe what you actually see and draw a before and after picture of what you think the molecules are doing. OPTIONAL: Your instructor may have a few other demonstrations to show you. You can experiment with various substances/objects on your own these objects will be placed at various stations in the lab. Students will rotate to various stations in groups of four. Your job is to predict what will happen at each station, and explain how it works with words and drawings. Have fun! Page 1 of 7

2 Part B. Temperature vs. Pressure Using the apparatus shown below, you will place an Erlenmeyer flask containing an air sample in water baths of varying temperature. Pressure will be monitored with a Pressure Sensor and temperature will be monitored using a Temperature Probe. The volume of the gas sample and the number of molecules it contains will be kept constant (the same). 1. Put about 800 ml of hot tap water into a l-l beaker and place it on a hot plate. Turn the hot plate to a high setting but keep an eye on it you want hot water to be between 50 C and 70 C for this experiment. 2. Prepare a second 1-L beaker with about 700 ml of cold tap water and ice to make 800 ml. 3. Put about 800 ml of room-temperature water into a third 1-L beaker. 4. Obtain a Lab Quest Pro, a temperature probe, and a pressure sensor. 5. Set up the pressure sensor: a. Plug the Pressure Sensor into a channel in the Lab Quest. b. Plug the Temperature Probe into a channel in the Lab Quest. c. Check that a rubber-stopper assembly with a piece of heavywall plastic tubing connected to one of its two valves is attached the Pressure Sensor. Leave its two-way valve on the rubber stopper open (lined up with the valve stem as shown in the figure) until Step 6b. 6. Prepare the gas sample in the flask. a. Insert the rubber-stopper assembly into a 125-mL Erlenmeyer flask. Important: Twist the stopper into the neck of the flask to ensure a tight fit. MAKE SURE THERE ARE NO LEAKS!!! b. Close the 2-way valve above the rubber stopper do this by turning the valve handle so it is perpendicular with the valve stem itself (as shown in the figure below). The air sample to be studied is now confined in the flask. c. Do NOT open this valve for the rest of the experiment! Page 2 of 7

3 7. Collect pressure vs. temperature data for your gas sample. DO NOT ZERO THE PRESSURE READINGS! a. Place the flask into the ice-water bath. Make sure the entire flask is covered (see figure above). Stir. b. Place the temperature probe into the ice-water bath. When the temperature stabilizes, record it in your data table. 8. Repeat Step-7 using the room-temperature bath. 9. a) Check the temperature of the hot water bath. If it is above 70 o C the stopper will pop out! Add cold water or ice to bring the temperature below 70 o C. b )Repeat Step-7 using the hot-water bath: Use a utility clamp to hold the flask and temperature probe in the boiling-water bath. To keep from burning your hand, the tubing, or the wire, hold the tubing, the probe wires, and the clamp together in one hand using a glove or a cloth. After the temperature and pressure have stabilized, repeat Step-7. CAUTION: Do not burn yourself or the probe wires with the hot plate. Part C. Volume vs. Pressure The gas we use will be air, and it will be confined in a syringe connected to a Pressure Sensor. When the volume of the syringe is changed by moving the piston, a change occurs in the pressure exerted by the confined gas. This pressure change will be monitored using a Pressure Sensor. It is assumed that temperature will be constant throughout the experiment 1. Prepare the Pressure Sensor and an air sample for data collection. a. Plug the Pressure Sensor into a channel of the Lab Quest. You don t need a temperature probe for this one. b. With the 20-mL syringe disconnected from the Pressure Sensor, move the piston of the syringe until the front edge of the inside black ring is positioned at the 10.0 ml mark, as shown in the figure. c. Attach the 20-mL syringe directly to the white valve stem of the Gas Pressure Sensor with a gentle half-turn, as shown in the figure below. 2. Collect the pressure vs. volume data. a. Move the piston to position the front edge of the inside black ring at the 5.0-mL line on the syringe. Hold the piston firmly in this position until the pressure value stabilizes but avoid placing your hand around the barrel of the syringe. b. When the pressure reading has stabilized, record it in your data table. c. Continue the procedure for volumes of 10.0, 15.0, and 20.0 ml. Page 3 of 7

4 Report Exploring Gases Name Section Lab Partner 1. Define pressure and look up at least three units of pressure you may encounter. 2. Define temperature and look up at least three units of temperature. 3. Circle the correct answer for each statement below. (a) As gas molecules get colder (decrease in temperature), do they move faster or slower? Will the pressure exerted by these molecules on the container increase or decrease? (b) As gas molecules are compressed (decrease in volume), do they collide more or less? Will the pressure exerted by these molecules on the container increase or decrease? 4. If two variables increase or decrease together, and with a constant ratio, the variables are said to be directly proportional to each other. If two variables oppose one other (one increases, the other decreases), and their product is a constant, the variables are inversely proportional to each other. Based on this description, label each graph from plotting two different sets of variables as directly proportional or inversely proportional. variable A variable C variable B variable D GRAPH 1 GRAPH 2 Page 4 of 7

5 Report Exploring Gases Name Section Lab Partner Data for Part A 1. Place a flask ( ml) with about 100 ml of water on a hot plate and cover its ending with a balloon. Turn on the hot plate. While waiting, predict what you expect to happen. Describe what you actually see and draw a before and after picture of what you think the molecules are doing. To show movement of molecules, use long arrows for faster molecules and shorter arrows for slower molecules. BEFORE AFTER 2. Place a similar flask with 100 ml of water on a hot plate. Bring the water to a boil. Remove from the hot plate. Cover the ending with a balloon. Wait for the flask and the water to cool. While waiting, predict what you expect to happen. Describe what you actually see and draw a before and after picture of what you think the molecules are doing. BEFORE AFTER Data and Questions for Part B Page 5 of 7

6 Water Bath Temperature ( C) Pressure (kpa) Ice Room temperature Hot water Plot your data below. Make sure you provide axes labels and a best-fit line. Is the relationship linear or non-linear? Directly proportional or inversely proportional? 1. Based on the data and graph that you obtained for this experiment, express in words the relationship between gas pressure and temperature. 2. Describe what the gas molecules are doing when the temperature increases. How does that affect the pressure of the gas in the flask? 3. If the gas was held in a balloon rather than in a flask, and the temperature increased significantly, how would that affect the pressure of the gas? Explain. Data and Questions for Part C Page 6 of 7

7 Volume Pressure (kpa) (ml) (to three sig figs) PxV (multiply P and V) P/V (divide P by V) Plot your data below. Do not draw a best fit line until you answer the next question. It might be difficult to tell if the relationship is directly or inversely proportional. 4. Check your data table above. If P x V is nearly constant for all data pairs, then the relationship is inversely proportional. If P / V is nearly constant, the relationship is directly proportional. After you determine which it is, draw the best-fit line appropriate for this plot (a straight line for a direct plot; a curve for an inverse plot). 5. Based on the data and graph that you obtained for this experiment, express in words the relationship between gas volume and pressure. 6. Describe what the gas molecules are doing when the volume increases. How does that affect the pressure of the gas in the syringe? Page 7 of 7