Heat of Fusion & Heat of Vaporization Lab

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1 Name: Period: Purpose Heat of Fusion & Heat of Vaporization Lab In this two-part activity, you will determine the change in heat of fusion, and heat of vaporization for water. Introduction In ancient times, people used to put rocks from the fire into pools of water to warm the water up. The hot rock would pass energy to the cool water until they both have the same final temperature. A fundamental law of nature is that when a hot object is in contact with a cold object, heat energy will pass from the hot object to the cool object until the two objects are the same temperature. The same amount of energy that is passed from the hot object enters the cold object (law of conservation of energy), but the two objects don t necessarily have the same change in temperature. When you put an ice cube in a drink, the drink immediately begins to cool down, but the ice cube will remain at 0 o C until it melts completely. How is that possible? The answer is that there is more energy in liquid water at 0 o C than in solid water at 0 o C. This extra energy in the liquid water is latent or hidden energy. For the solid to liquid (or liquid to solid) phase change, this hidden energy is potential energy and is called the latent heat of fusion. The amount of energy required to melt (or freeze) a substance is different for different substances. The heat of fusion is the amount of energy required to melt (or freeze) one gram of a substance, while the molar heat of fusion is the amount of energy required to melt one mole of a substance. When a substance changes from liquid to gas, there is a different amount of hidden or latent energy. The latent heat of vaporization (liquid to gas) is different than the latent heat of fusion for the same substance. In this lab, you will try to determine the latent heat of fusion and vaporization for water. When a substance gains or loses heat energy without a phase change, the formula is: q = mc T Where q is the heat energy in Joules, m is mass of the substance, C is the specific heat of the substance in units of J/g o C, and T is the difference between starting and ending temperature in o C. When a phase change happens the formula is: q=m Hvap or q=m Hfus depending on the phase change. The Hvap is the heat of vaporization for the change between liquids and gasses while the Hfus is the heat of fusion for the change between solids and liquids. To calculate the total energy that steam which condenses and then cools down gives away, you will need to add the heat calculated from q=mc Hvap (the energy given away during condensation to q=mc T (the energy given off while the hot water cools to its final temperature). To calculate the total energy that an ice cube which melts and then warms up would have absorbed, you will need to add the heat calculate from q=mc Hfus (the energy absorbed while the ices is melting) to q=mc T (the energy it absorbed while the cold liquid water warmed up to its final temperature.

2 Procedure Part A: Heat of Vaporization -Use extreme caution due to hot steam. Equipment Setup: 1. Set up a steam generator as shown with about mL of water. 2. Find the mass of a dry Styrofoam beaker. Record this as m1. 3. Fill the Styrofoam beaker about ½ full with water and obtain the mass of the water and beaker. Record this as m2. 4. Lower the temperature probe into the water in the Styrofoam Beaker (to about 1 cm from the bottom). Record Data 1. Set your SPARK to the digital display. 2. When ready, press to begin recording the temperature. 3. Watch the digits display for the temperature to reach equilibrium. Record this temperature as the initial temperature (Ti) of the water. 4. Turn on the hot plate/burner to begin boiling the water in the steam generator apparatus. Position the glass tubing so that steam will be added to the Styrofoam beaker when it boils. Be careful to avoid the escaping steam! 5. Monitor the temperature and continue stirring until the temperature climbs above 75 C. Turn off the hot plate and stir the water in the Styrofoam beaker. Remove the tubing when it is cool enough to safely remove it from the Styrofoam beaker (use tongs if necessary). Watch the temperature to see that it has reached its highest value. Record this temperature as the final temperature (Tf) of the water. 6. Press to end data collection. 7. Allow the stream generator and all the equipment to cool before disassembling. 8. When sufficiently cool to handle, find the mass of the Styrofoam beaker containing the water plus the condensed steam, Record this value as m4 in the appropriate data table. Data for Part A Heat of Vaporization Initial Temperature (Ti) Final Temperature (Tf) Change in Temperature ( T) Mass of Styrofoam Beaker (m1) Mass of Styrofoam Beaker and Water (m2) Mass of Initial Water (m3) Mass of Styrofoam Beaker, Water, and Steam (m4) Mass of Steam (m5) Heat gained by the water (q1) Heat lost by steam to the water (q2) Heat lost by condensed steam (q3) Heat of vaporization for water in this experiment Molar heat of vaporization for water Note: Be sure to include proper units on all data in the table. Note: Be sure tubing is immersed in the water for best results

3 Analyze (Record your results in your data table as you complete your analysis.) 1. Subtract Tf Ti to determine T, the change in water temperature. 2. Subtract m2 - m1 to obtain the mass of Initial Water (m3). 3. Subtract m4 m2 to determine the mass of the steam (m5). 4. The total amount of heat energy gained by the water (q1) is equal to the amount of heat energy lost by the steam (q2), plus an additional amount of energy required to cool the steam to bring the temperature to equilibrium (q3). In other words, q3 represents the energy needed to lower the temperature of the steam to the final temperature. 5. To determine how much energy was required to warm the water, you will need to use the general equation for calculating heat that was discussed in the introduction. Follow the steps below to calculate the heat of vaporization (in Joules/g) for the water you warmed. Record your values in the table: A. Calculate the total amount of heat energy gained by the water. Record this as q1. q = mc T q 1 = m 3(4.18J/g C) T B. Calculate the amount of heat lost by the steam (q2) as it cools from 100 o C to the final temperature. q = mc H vap q 2 = m 5 (4.18J/g C) (100 o C T f) C. Calculate the amount of heat lost by steam when changing its phase to water. q 3 = q 1 q 2 6. The energy you just calculated was the amount of energy that the steam imparted to the water. Use this value to calculate the amount of energy per gram for the water gained in the Styrofoam beaker (q3/m5). We will call this the experimental value for the heat of vaporization. 7. Using your experimental value for the heat of vaporization of water, calculate the Molar heat of vaporization for water. Do this by multiplying your experimental value for heat of vaporization by the molar mass for water (18g/mol). Part B: Heat of Fusion Equipment Setup: 1. Obtain the mass of Styrofoam beaker and record this as m1. 2. Fill the foam cup about ½ full with tap water and obtain the mass of the water and Styrofoam beaker. Record this as m2. 3. Set up the Styrofoam beaker and temperature probe as shown. Lower the temperature probe into the water to about 1.0 cm from the bottom. Record Data 1. Set your SPARK to the digital display. 2. When ready, press to begin recording the temperature. 3. Watch the Digits display for the temperature to reach a constant temperature. Record this temperature as the initial temperature (Ti) of the water. 4. Shake excess water from several (5 or 6) small ice cubes, or dry them with a paper towel, then add them to the water. NOTE: You want about 10 grams of ice. Use the balance for a more exact measurement.

4 5. Stir the ice water mixture with your stirring rod and monitor the temperature. NOTE: Do not use the temperature probe to stir the water; friction could add extra heat to the water. 6. When the ice is completely melted, stir the mixture one more time and record this temperature as the final temperature (Tf). 7. Press again to end data recording. 8. Obtain the mass of the Styrofoam beaker, water, and melted ice. Record this value as m3. Data for Part B Heat of Fusion Initial Temperature (Ti) Final Temperature (Tf) Change in Temperature ( T) Mass of Styrofoam Beaker (m1) Mass of Styrofoam Beaker and Water (m2) Mass of Styrofoam Beaker, Water and Melted Ice (m3) Mass of Water (m4) Mass of Ice (m5) Heat energy lost by surrounding water (q1) Heat absorbed by ice as it melted (q2) Heat absorbed by ice water as temperature equilibrium established (q3) Heat of fusion for water in this experiment Molar heat of fusion for water Note: Be sure to include units on all data in the table. Analyze (Record your results in your data table as you complete your analysis.) 1. Draw a graph like the one in the first part of the lab to describe energy change. Label each segment using the provided key. Let: q1 = water giving up energy q2 = ice gains energy from water q3 = liquid water gains energy from water 2. Subtract m2 m1 to find the mass of the water. Record as m4. 3. Subtract m3 m2 to find the mass of the ice. Record as m5. 4. Subtract Tf Ti to determine T, the change in water temperature. 5. The total amount of heat energy lost by the surrounding water (q1) is equal to the amount of heat energy absorbed by the ice as it melted (q2), plus an additional amount of energy required to bring the temperature to equilibrium (q3). In other words, q3 represents the energy needed to raise the temperature of the melted ice water to the final temperature.

5 6. To determine how much energy was required to melt the ice, you will need to use the general equation for calculating heat that was discussed in the introduction. Follow the steps below to calculate the heat of fusion (in Joules/g) for the ice you melted. Record your values in the table: A. Calculate the total amount of heat energy lost by the water (used to melt the ice as well as to warm the melted ice). Record this as q1. q = mc T q 1 = m 4 (4.18J/g C) T B. Calculate the amount of energy it takes for the zero degree water (formerly the ice cubes) to rise to the equilibrium temperature, this is q3. q = mct q 3 = m 5 (4.18J/g C) T i C. Calculate the amount of heat absorbed by the ice as it melted by finding the difference between these values. q2 is equal to the heat of fusion for the amount of ice you used in this experiment. q 2 = q 1 q 3 7. Calculate the heat required to melt the ice in the experiment by dividing the heat absorbed by your ice (q2) by the mass of the ice (m5). 8. Using your experimental value for the heat of fusion of ice, calculate the Molar heat of fusion of water. Do this by multiplying your experimental value for heat of fusion by the molar mass for water (18g/mol). Conclusion Questions 1. Would one mole of steam condensing release enough heat to melt one mole of ice? Explain. 2. How is it possible to add heat to something without changing its temperature?