CHEM 162, Winter 2010
Lab 4: Heat of Fusion
Monday, February 8
Objective
To verify the heat of fusion of ice using a calorimeter.
Introduction
When a material undergoes a change of state, e.g. from solid to liquid or liquid to gas, the temperature of thematerial remains constant. While changing states, all of the energy (heat) added or removed is used to change states rather than change temperature. The constant temperature at which a solid material changes to a liquid is known as its melting point and the amount of heat necessary to melt the substance is known as the Heat of Fusion (DHf).
The amount of heat transferred during a process can be measured with a device called a calorimeter. There are two types of calorimeters: constant pressure and constant volume (bomb) calorimeters. In lab, we will use a simple calorimeter, which is an insulated vessel with a stirrer, a thermometer, and loose-fitting lid to keep the calorimeter at constant atmospheric pressure. Because the reaction is carried out at constant pressure, transferred heat (q) is equal to the heat of the reaction (DH). How can the temperature change inside a calorimeter be used to calculate the heat of the reaction (DH)?
In this experiment, ice at 0oC is placed into a calorimeter containing water at a temperature Ti. The ice melts and, in time, a final steady state temperature, TF, is obtained. If we make the assumption that the calorimeter is thermally isolated, i.e., no heat is gained or lost to the surroundings, then the heat gained by the ice in the process of melting and then rising to temperature TF is equal to the heat lost by the water in going from Ti to TF. Thus we have the useful equation. The below equation states that the heat gained by the ice in order to change state and change temperature is equal to the heat lost by the water initially placed in the calorimeter.
where / mi / The mass of iceDHf / The heat of fusion for ice
sw / The specific heat of water
mw / The mass of the water at temperature Ti
Mi / The moles of ice
Pre Lab Questions: (Due at the beginning of the lab period)
1. 50.0 mL of water are placed in a calorimeter. When 15.0 g of ice are added to the calorimeter, the temperature drops from 70.0°C to 51.2°C. Calculate the heat of fusion of ice in kJ/g.
2. Calculate the heat of fusion of ice in kJ/mol.
3. How do these results compare to the accepted value?
Procedure:
1. Plug a stainless steel temperature probe into the LabPro®interface and launch Logger Pro 3.2. Set experimental parameters according to your instructor’s directions.
a) Go to the Experiment tab and click Data Collection.
b) Set Length to 600 sec and Sample Rate to 2 Seconds/Sample.
c) I recommend you cut and paste the data into excel for later usage but you may wish to save it in the 2003 format. Don’t forget to email the data to yourself or save it to a USB drive!
2. Start a hot water bath (around 60°C) using a 250 mL beaker and a hot plate. Place a 50 mL graduated cylinder upside down in the water bath to warm it. Several groups can share a hot plate.
3. Obtain 2 nested Styrofoam cups and a magnetic stir bar that will function as your calorimeter for this experiment. Place the calorimeter on a cool hot plate and use the magnetic stir feature.
4. Pour ~30mL of warm water into the warm graduated cylinder and quickly measure the volume (be sure to note volume). Once the volume is measured, pour the water into the calorimeter and begin collecting temperature data with the LabPro temperature probe. Make sure the tip of temperature probe is immersed in the water.
5. Add approximately 10 g of ice to the calorimeter very soon after you begin collecting temperature data. Be sure your ice is as “dry” as possible because the presence liquid water will throw off your calculations. If your ice appears to be “wet” dry it off with a paper towel.
6. Continue collecting temperature data until all the ice has melted and an apparent constant temperature has been reached. Once a constant temperature has been reached you may stop data collection.
7. After the experiment is complete, measure the volume of the water in the calorimeter.
8. Cut and paste your data into excel for later use. You may email this file to yourselves.
9. Repeat the experiment 2 more times if time allows.
* Be sure to include the raw data along with the calculations (below) that you will turn in with your lab.
Calculations
1. Calculate mass of water in the calorimeter initially. Calculate mass of water in the calorimeter after melting has occurred. Calculate the mass of ice that was placed in the calorimeter. Calculate the moles of ice that were placed in the calorimeter.
2. Create a graph of your data by using the Chart Wizard and be sure to title and label the axes and use a legend where necessary. From this graph you should be able to determine the change in temperature, Ti and Tf.
3. Calculate the heat lost by the warm water (and heat absorbed by ice), using 4.179 J/g·°C as the specific heat of liquid water.
4. Calculate the heat of fusion of ice (in kJ/mol) for each trial and determine the average. How does this compare to the values obtained by the rest of the class?
5. Enter all the class data into an Excel spreadsheet and use Excel to calculate the standard deviation and % error for your data as well as the class data (accepted value: DHf for ice = +6.01 kJ/mol). Class data will posted on the webpage for download.
Post-Lab Assignment and Questions
Data: Data Tables should be clearly labeled, and all Excel plots and spreadsheets should be clearly labeled before turning in.
Calculations: Complete all calculations and turn in a set of calculations from at least one trial. Clearly label everything on your Excel spreadsheets including a title, your name, partner’s name and date.
Questions:
1. Using the concept that breaking bonds takes energy and forming bonds releases energy, explain why the temperature remains constant while ice is melting despite the fact that heat is being added during the process.
2. Why is the heat of vaporization generally much higher than the heat of fusion?
3. How would the results for the heat of fusion of water have been affected if the ice were not dried before being placed in the calorimeter? Would this have made the measured heat of fusion larger or smaller? Explain.
4. Use the concept of specific heat to explain why vegetables in a stir fry burn your mouth while meat or tofu in that same stir fry cooked at the same temp might not.
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Edited by Nick Buker 12/23/08