Lab 7. Gain and Loss: Heating and Cooling

Transcription

1 Lab 7. Gain and Loss: Heating and Cooling Why do things get hot or cold? Why do chemical reactions get hot or cold? Part 1. Identify a metal from specific heat. Prelab Spend 5 minutes doing the following activity. Assign a notetaker. Report to class. Add 50 ml of 100 o C water to 50 ml of 25 o C water. a. What happens? What is the final temperature of the water? b. Do a heat calculation to confirm the final temperature that you measured. Introduction When something hot touches something cold, the hot object gets cooler and the cold object gets warmer. In other words, heat is transferred. Three factors determine the amount of heat transferred: mass (the amount of substance present) temperature (how fast or slow the atoms/molecules are moving in a substance) specific heat (the amount of energy required to raise 1 g of a substance 1 o C). You ll look at these factors to help you measure specific heat of a metal and identify different metals. Materials Metal rods: copper, iron, zinc, etc. Computer, Vernier Lab Pro or LabQuest Mini, thermocouple Procedure 1. Measure the specific heat of 2 metals. Modify the Prelab Experiment to measure the specific heat of a metal. Hints: What is gaining heat? What is losing heat? What experimental variables do you want to measure? How will you measure T f? See the Appendix. What measuring devices will you use? What is the uncertainty of each device? When you design your experiment, record your data and results in Table 1. Table 1. Specific of data and results. What else goes in this table? How about density? Metal # Mass of T i of Exp s True s % error 2. Identify each metal. Check your identification with your instructor. Waste Disposal: water in sink. Return the metals to the Stockroom. Report Your instructor will tell you whether your identification is correct or not. If you did not correctly identify the liquid, try again. Grading: A = 1 st try, B = 2 nd try, C = 3 rd try. Question You may have set up your experiment exactly the same way for each metal. Compare the T f you measured for each metal. Based on T f, which metal gained/lost more heat? Which metal is the better conductor? Is conductivity related to specific heat? 49

2 Appendix. Graphical Determination of ΔT. Determination of ΔT is done graphically to correct for the heat loss that occurs while the process is taking place. ΔT = T f - T i, where T is the initial temperature of the water before the small beaker is added and T f is the highest temperature the system would reach if no heat were lost to the surroundings. You will obtain ΔT by measuring the temperature initially, just prior to reaction, and then every 15 seconds for about 5-10 minutes once the reaction has begun. The initial temperature, T i, is the temperature measured just prior to starting the reaction. To determine T f, you must plot the temperature of the reaction along the vertical axis (the y-axis) and the reaction time along the horizontal axis (the x-axis). See Figure 1 for a sample graph. You will notice a steep increase in temperature to a maximum, followed by a slow cooling. After plotting your data points on the graph paper, you will take a straight edge and draw a straight line through the cooling portion of the curve, extrapolating the line back to zero time. The temperature where the line intersects the vertical axis will be taken as T f. It represents the temperature the system would have climbed to if no heat had been lost from the system. Figure 1. Determining the change in temperature, ΔT, from the temperature vs. time graph. 50

3 Part 2. Quest for Fire Prelab Spend 5 minutes doing the following activity. Assign a notetaker. Report to class. 1. A Bunsen burner uses natural gas as a fuel. Natural gas is methane, CH 4. a. A fuel needs an oxidizer. What is the oxidizer in this reaction? Write a balanced chemical equation that represents the combustion of CH 4. b. A combustion reaction is exothermic. Calcuate ΔH of this reaction. c. Draw a reaction energy diagram for the CH 4 combustion reaction. Is the energies of the reactants higher or lower than the energies of the products? 2. Take a look at a Bunsen burner. a. Draw a picture of a Bunsen burner. Where does CH 4 mix with O 2? b. Where and how do you adjust the amount (flow) of gas that enters the burner? There are two adjustments. c. Where and how do you adjust the amount (flow) of oxygen that enters the burner? Objectives (i) Learn how a Bunsen burner works (ii) Relate flame color to temperature and limiting/excess reactants. Introduction Humankind s quest for fire has fascinated us since the dawn of time. The dancing flame gives us a lot of information. Materials Bunsen burner Computer, Vernier Lab Pro or LabQuest Mini, thermocouple Procedure 1. Connect a Bunsen burner to the natural gas valve. Light the burner. Adjust the flame so it is blue and about 3 inches high. 2. Does the amount of oxygen affect how much energy is produced in this reaction? Record your data in Table 2. a. From the blue flame, turn the barrel of the burner clockwise to adjust the amount (flow) of oxygen that enters the burner until the flame turns yellow. b. Next, turn the barrel of the burner counterclockwise to adjust the amount (flow) of oxygen that enters the burner until the flame turns blue. c. Continue turning the barrel of the burner counterclockwise until the flame is yellow again. 51

4 Table 2. Bunsen burner gas and oxygen data and results. Burner setting Flame color Flame height Flame temperature Barrel yellow Clockwise Limiting reactant CH 4 to O 2 ratio (low, just right, high) 3. Does the amount of gas (fuel) affect how much energy is produced in this reaction? Record your data in Table 2. a. Adjust the burner so the flame is blue again. Turn the knob at the bottom of the burner clockwise to adjust the amount (flow) of gas that enters the burner until the flame turns yellow. b. Next, turn the knob at the bottom of the burner counterclockwise to adjust the amount (flow) of gas that enters the burner until the flame turns blue. c. Continue turning the knob at the bottom of the burner counterclockwise until the flame is yellow again. Questions 1. a. Show your Table 2. b. How is flame color related to flame temperature? c. How is flame temperature related to the amount of excess reactant? d. How can you use flame temperature to determine the correct ratio of reactants? 2. As you did this experiment, the heat by the methane combustion reaction = the heat by the air. 52

5 a. When the Bunsen burner flame is yellow, is the heat by the air greater than, equal to, or less than the heat by the air when the burner flame is blue? b. When the Bunsen burner flame is yellow, is the heat by the methane combustion reaction greater than, equal to, or less than the heat by the methane combustion reaction when the burner flame is blue? c. ΔH = q. If your answer to 2b is greater than or less than, does it mean that ΔH for the methane combustion changes? Give reasons. 3. You light a candle. The flame is. a. The fuel is. b. The energy of the fuel is (higher/lower/same) as the energy of the products. c. The limiting reactant is. d. How do you think you can make a candle flame blue, like a Bunsen burner? 4. a. You have a gas (CH 4 ) stove at home. What color should the flame be? b. You light a butane (C 4 H 10 ) lighter. The flame is yellow. (i) What should be the O 2 to butane ratio by mass? (ii) What does the flame color tell you? More Practice: Some cars, e.g., Honda Civic GX, run on natural gas. a. Calculate the O 2 to fuel ratio by mass. b. Since the source of O 2 is air and air is 20% O 2 and 80% N 2 by moles, calculate the air to fuel ratio by mass. c. Most cars run on octane (C 8 H 18 ). The air:fuel ratio is approximately 16:1. Do a calculation to confirm this ratio. 53

6 Part 3. Flex Fuel Students: bring a Clean and dry 0.5 liter plastic bottle (water bottle or soda bottle) Prelab Spend 5 minutes doing the following activity. Assign a notetaker. Report to class. 1. Ethanol, C 2 H 5 OH, is a fuel. It is also used for other purposes. a. A fuel needs an oxidizer. What is the oxidizer in this reaction? Write a balanced chemical equation that represents the combustion of C 2 H 5 OH. b. Is this reaction exothermic or endothermic? Calcuate ΔH to support your answer. 2. Isopropanol, C 3 H 7 OH, is a fuel. It is also called rubbing alcohol and used as a disinfectant. a. Write a balanced chemical equation that represents the combustion of C 3 H 7 OH. b. Is this reaction exothermic or endothermic? Calcuate ΔH to support your answer. c. For this reaction, are the reactants more stable or less stable than the products? Objectives (i) Compare combustion of a liquid fuel to gaseous fuel. (ii) Measure % yield of combustion reaction. Introduction With our quest for fire came the desire to do something with the fire. What can we do with fire? Materials Students: bring a Clean and dry 0.5 liter plastic bottle (water bottle or soda bottle) Ethanol or rubbing alcohol Procedure 1. How does liquid alcohol burn? (You did an ignition test with ethanol in an eariler Chem 1A lab.) Which alcohol are you using? Pour about 1 ml of liquid alcohol on the lab bench. Using a match or wooden splint, light the alcohol on fire. What color is the flame? Is this reaction exothermic or endothermic? How can you tell? Is this reaction fast or slow? How can you tell? Is the speed of this reaction fast enough to run a car engine? 2. How does gaseous alcohol burn? a. Measure the mass of your clean and dry 0.5 liter plastic bottle. b. Add fuel. Add about 5 drops of alcohol to the bottle. Measure the mass of the bottle and alcohol. Record your data in Table 3. c. Cap the bottle and shake it. Turn it upside down. Hold the bottle in your hands and let your warm hands vaporize the alcohol. Make sure all of the alcohol has evaporated! How can you tell all of the alcohol has evaporated? d. Ignition. Remove the cap. Light a splint. Carefully, place the burning splint over the top of the open bottle. Keep your hair and face and body parts away from the mouth of the bottle. What happened? 54

7 What color is the flame? Is this reaction exothermic or endothermic? How can you tell? Is this reaction fast or slow compared to the liquid alcohol? Is the speed of this reaction fast enough to run a car engine (or rocket)? Table 3. Alcohol combustion data and results. Anything else go in this table? Alcohol = Run 1 Run 2 Mass of bottle Mass of bottle + alcohol Mass of Flame color Is reaction exothermic or endothermic? Is reaction fast or slow? Mass of bottle + liquid after burning Mass of liquid Identify of liquid Moles of alcohol that burned (theory) Moles of water produced (theory) Theoretical yield of water % yield of water Volume of bottle Moles of air Moles of O 2 Alcohol to O 2 ratio (low, just right, high) Limiting reactant e. Cap the bottle again. Let the bottle and contents inside the bottle cool to room temperature. Do you see a liquid inside the bottle? What is this liquid? Where did it come from? Remove the cap. Measure the mass of this liquid. Record your data in Table 3. f. Based on the mass of alcohol that you used, calculate the theoretical yield of water. Then, calculate the % yield of water. 3. Can you make it burn better? a. Determine the limiting reactant. Air is 20% O 2. Calculate the moles of air in the bottle from the ideal gas law, PV = nrt. Given: P = 1 atm, V = liters, R = l atm/mole K, T = K. Solve for n = moles of air. Once you know the moles of air, multiple by to calculate the moles of O 2. Compare the moles of alcohol to the moles of O 2. Is this ratio too low, just right, or too high? Give reasons. What is the limiting reactant? 55

8 b. Based on your answer to 3a, make any adjustments in the amount of alcohol used and do Step 2 again. Waste Disposal: Ethanol, isopropanol in sink. Plastic bottles in trash. Questions 1. a. Show your Table 3. b. (i) Which burned better, liquid alcohol or gaseous alcohol? (ii) Why do you think one burned better than the other? (iii) Is liquid alcohol more stable, less stable, or the same stability as gaseous alcohol? Give reasons based on ΔH f. c. Compare your Run 1 to Run 2. Was there a difference in how the alcohol burned? 2. a. (i) Based on your data in Table 3, what reactant is the limiting reactant? (ii) What reactant should be the limiting reactant? Give reasons. b. Based on the limiting reactant for Run 1, (i) calculate the moles of alcohol that burned. (ii) Then, calculate q for this reaction. c. Based on the limiting reactant for Run 2, (i) calculate the moles of alcohol that burned. (ii) Then, calculate q for this reaction. d. Compare b(ii) to c(ii). Which reaction would you use in a rocket engine? Why? 3. Was your % yield of water 100%. If not, identify a specific step in the experimental procedure that could have lowered the yield. 4. Name 2 useful applications of burning alcohol in a bottle. 56

9 Part 4. Hot Packs and Cold Packs Prelab Spend 5 minutes doing the following activity. Assign a notetaker. Report to class. 1. a. Add some water to a beaker or test tube. Add NaCl to the water. What happens? How would you make a hot/cold pack out of these materials? b. Would you use an exothermic reaction or endothermic reaction for a hot pack? c. In a hot pack, what is gaining heat? What is losing heat? 2. a. Read the Plan below. b. When dissolved in water, NaCl dissociates into its ions. (i) Calculate or look up ΔH of dissolution for each salt in the Materials section. (ii) Based on ΔH, determine whether the salt can be used in a hot pack or cold pack. (iii) Calculate the mass of salt that raises or lowers the temperature of water based on the design considerations in the Plan. (iv) Look up the cost of the mass of each salt you calculated in Step 2b(iii) in a chemical supplier catalog, e.g., Sigma-Aldrich. (v) If the hot/cold pack breaks and spills in the user, there could be a safety problem. Look up the MSDS or NFPA Rating of each compound. Determine the safety of each compound. Record your information in Table 4. Table 4. Hot and Cold Pack data. Ionic Compound ΔH reaction, kj/mole Hot pack or cold pack? NaCl Mass to use in hot/cold pack Cost/pack, $ Solubility in 100 g H 2 O Safety (NFPA Ratings) Plan Your group will design a hot pack and cold pack using these design specifications and considerations: (i) Your hot pack must be at a temperature of 40 o C for 10 minutes. Your cold pack must be at a temperature of 0 o C for 10 minutes. Record temperature and time data using a Vernier temperature probe. (ii) Since your pack is portable, you want it to be relatively small. Use a maximum of 25 ml of water. Based on the heat of dissolution of the ionic salt in 25 ml of water, calculate the mass of ionic salt that you need to raise or lower the temperature to the design specification. (iii) Try two different salts for each pack. Check the price of each salt. See a chemical catalog for prices. Limit: 2 runs per salt. Make sure you plan before you start you start this experiment. (iv) If the pack breaks and spills on the user, what will be the effect of this salt on the user? Look up safety information on the salts. Objectives (i) Choose an ionic compound to use in a hot pack and cold pack. (ii) Make a hot pack and cold pack. (iii) Identify the factors that make the best hot/cold pack. Introduction While you were driving to school with the steering wheel in one hand and a cup of coffee in the other, you hit a pothole and spill some hot coffee on your hand. (What water temperature will burn your tongue (or skin)?) You wish you had some ice. Since ice melts unless refrigerated, another source of something cold is a chemical reaction. You will apply your knowledge of thermochemistry to design a cold pack. While you re at it, you figure you might as well design a hot pack. In hot packs and cold packs, a solid ionic salt is separated from water by some sort of barrier in a small plastic bag. To use the bag, the barrier is broken and mixes the salt and water together. This dissolution reaction either heat (hot pack) or heat (cold pack). 57

10 Materials ionic salts: NaCl, LiCl, NH 4 Cl, NH 4 NO 3, KNO 3, NaNO 3, CaCl 2, MgSO 4, CaSO 4,... Styrofoam cup or beaker or flask and cardboard lid Computer, Vernier LabPro or LabQuest Mini, temperature probe Procedure 1. Design a hot pack and cold pack. Refer to the design considerations in the Plan. a. Fill in Table 4 for each salt in the Materials section. b. Based on, choose two salts to test for a hot pack and two salts to test for a cold pack. Identify the criteria you used to make this choice. 2. a. Do an experiment to test the performance (see Design Specifications) of the salts you choose in Step 1b. Add additional columns or rows to your Table 4 to show the data you collected and results you calculated. b. Based on, which salt would you use for the hot pack? Identify the criteria you used to make this choice. c. Based on, which salt would you use for cold hot pack? Identify the criteria you used to make this choice. Waste Disposal: hot/cold pack ionic solutions in sink. (None of the metals are heavy metals.) Questions 1. Prepare a 5 minute presentation of your data and results to your lab class. a. Show your Table 4. b. Compare the two salts you tested for the hot pack. Which salt did you choose and why? Include numbers! c. Compare the two salts you tested for the cold pack. Which salt did you choose and why? Include numbers! 2. You calculated ΔH of dissolution for various ionic compounds using Hess law. a. For a hot pack, are the reactants more stable or less stable than the products? b. For a cold pack, are the reactants more stable or less stable than the products? c. How do the stabilities of reactants compared to products determine whether a reaction is exothermic or endothermic? 58