Heat of Fusion Lab v5Chemistry p.1
Molar Heat of Fusion of Ice
The amount of energy required to convert a solid to a liquid, at constant temperature and pressure depends on the amount of substance present. However, the amount of heat needed to melt a specific amount, known as the heat of fusion, is always the same. It is often useful to refer to the amount of heat needed per gram or per mole of substance. The latter value is called the molar heat of fusion. The molar heat of fusion is therefore the energy needed to change one mole of a solid into liquid at the melting point and is specific to that substance. The involvement of energy can be shown in equation form as follows:
H2O (s) + molar heat of fusion H2O (l)
You will perform an experiment to determine the molar heat of fusion of ice. An excess amount of ice will be placed in a calorimeter (styrofoam cup) with a known amount of hot water. You will allow enough ice to melt to bring the temperature of the ice water to the melting point of ice, approximately 0°C. At this time the extra ice will be removed. The volume of the melted ice and water in the cup will be used with temperature data for the calorimeter to calculate the molar heat of fusion of ice. The density of water (1 g/mL) will be used to convert volume to mass where needed.
Objectives:
- To determine the molar heat of fusion of ice.
Pre-lab: Complete on a separate sheet.
1. Prepare a data table for this lab. (Hint only 4 values are recorded)
2. Sketch what is in the main styrofoam cup after each of these steps: 1,4,5,7
Do not worry about the thermometer.
3. How will you know how much ice melted?
4. What equation can be used to determine the heat lost by water?
5. What is the specific heat of water? give two values; one using cal, and one using J:
SAFETY: Wear goggles, gloves, and aprons when working with hot water.
Materials:
large styrofoam cup600 mL beakerhot water (from thermos or hot plate)
100 mL graduated cylinder150 mL beakerthermometer
ice, 5 cubestest tube holder scoop spatula
Procedure: Your data will be an important part of your lab test! Be prepared! The steps after step 2 need to be performed without significant delays between steps. Know what you are doing (see figure 1)! Make sure you can operate your test tube holder easily before starting.
1. Place a large dry styrofoam cup into a 600 mL beaker for support. Have all your equipment ready including your ice cubes in a separate styrofoam cup.
2. In the 150 mL beaker, obtain about 100 mL of hot water at 70°C or a little higher. You will either pour the water from a hot source (CAREFUL!), or heat the water yourself on a hot plate. Do NOT record this temperature or volume!
3. From the 150 mL beaker, pourabout 90 mL of hot water into the graduated cylinder. Record the exact volume. If the water is above 60ºC, let it air cool to 60°C in the graduated cylinder. Do NOT record this temperature!
4. Pour the water into the styrofoam cup. Rest the thermometer in the cup. The water should be between 55°C and 60ºC. If it cooled to below 50°C go back to step 2.
5. Record the exact temperature of the water and immediately add 5 full-size (6 if a bit smaller) ice cubes to the cup. Make sure you add only ice, not water. Also be careful not to splash!
6. Stir constantly and monitor the temperature in the middle of the water near the floating ice (not on the bottom). Watch the temperature go down and observe that the ice is melting. You are waiting until the temperature of the ice and water pretty muchstops going down...meaning it takes about 15 seconds to drop by 1ºC usually somewhere between 0 and 5°C, depending on your thermometer. [** You must keep the ice from all or mostly all melting during this step! If your ice pieces are getting too small (small grape size), add one or two more ice cubes! IT IS OK to add more ice at this time!] RECORD the low (final) temperature and go quickly to step 7. It is better to stop a bit early than to wait too long before step 7.
7. Remove the remaining ice from the water as quickly as possible without losing any of the water. It is usually best done with a test tube holder. A spatula may also help for very small pieces.
8. Carefully determine the final volume of the cold water in the styrofoam cup by pouring it back into a graduated cylinder and measuring (CAREFUL: don’t overflow the graduated cylinder!) If you encounter any difficulties, figure out how to do this. Record this volume.
9. Clean-up / disposal... Empty dry and put away all equipment.
10. Start to calculate the heat of fusion.
Processing: Present your data,perform calculations and answer questions neatly in your lab notebook. (see sample calculation for hints). Even though the steps are laid out for you, you MUST practice explaining clearly what you are doing in each step! Pretend your only instruction was to “calculate the heat of fusion.” Multi-step calculations such as this must be clear and easy to follow! Remember to use sig figs!
1. Calculate the change in temperature of the original hot water in the calorimeter.
2. Calculate the heat lost by this water in calories and joules.
3. Calculate the heat used to melt the ice.
4. Calculate the volume of ice melted.
5. Calculate the mass of ice melted.
6. Calculate the moles of ice melted.
7. Calculate the heat of fusion in the following units: J/g J/mol
Questions:
8. Which values from #7 above represent the molar heat of fusion?
9. What was your percent error? (accepted value is 5999 J/mol)
10. If you repeated the experiment with 10 mL less hot water, how would the molar heat of fusion be expected to differ (higher, lower, or the same?) Explain.
11. What assumptions did you make about the temperature of the ice before and after it melted?
12. If heat was lost from your calorimeter (styrofoam cup) while the ice was melting, would it have made the value for the molar heat of fusion that you calculated higher or lower than the accepted value? Explain clearly.
13. What other sources of error were present in this experiment? Specifically, how could they affect your result?
Sample calculation:
Volume of hot water: 90.4 mLFinal minimum temp (water and ice): 0.7°C
Temperature of hot water: 53.6°CVolume (hot water + melted ice): 151.9 mL
1. Calculate the change in temperature of the original hot water in the calorimeter:
The water cooled from 53.6°C to 0.7°C. T = 53.6°C - 0.7°C = 52.9°C
2. Calculate the heat lost by this water (Qw) in calories and joules:
Since the density of water is 1g/mL, the mass of 90.4 mL of hot water is 90.4 g
Qw=mw•c•T = (90.4 g)(4.18J/goC)(52.9°C) = 2.00x104J
3. Calculate the heat used to melt the ice (Qi). Assume that all of the heat lost by the water went into melting the ice. Therefore, Qi=Qw= 2.00x104J
4. Calculate the volume of water that is from ice melted. The difference between the final volume and initial volume is due to melted ice. Vi = 151.9 mL - 90.4 mL = 61.5 mL.
5. Calculate the mass of ice melted. The melted ice is now water. Use the volume you just calculated. With a density of 1 g/mL, 61.5 mL = 61.5 g
6. Calculate the moles of ice melted. Use the molar mass to convert. Ice is simply H2O
7. Calculate the heat of fusion. For example, in cal/g. Values with other units may be calculated similarly, or may be obtained by converting the units in this answer.:
Heat of fusion = = 5.85x103 J/mol
Practice: Not required for this lab, but will help in studying for your lab test.
120.5 mL of water are measured in a graduated cylinder and are placed in a styrofoam cup. The temperature of the water is found to be 40.2°C and some ice is immediately added. The ice water is stirred until the temperature reaches a minimum temperature of 1.7°C. The excess ice is quickly removed. The remaining water is poured into graduated cylinders and measures 182.2 mL. Based on this data, calculate the heat of fusion of ice in cal/g and in J/mol.
Answers: 75.2 cal/g and 5.66x103 J/mol