Calorimetry: Determining the Heat of Fusion of Ice and the Heat of Vaporization of Liquid Nitrogen - Chemistry I Acc O B J E C T I V E 1. Using a simple calorimeter, Determine the heat of fusion of ice and the heat of vaporization of liquid Nitrogen. Introduction When a chemical or physical change takes place heat is given off or absorbed. That is, the change is either exothermic or endothermic. It is important for chemists to be able to measure this heat. Measurements of this sort are made in a device called a calorimeter. The technique used in making these measurements is called calorimetry. In simplest terms, a calorimeter is an insulated container made up of two chambers (see figure above). The outer chamber contains a known mass of water. In the inner chamber, the experimenter places the materials that are to lose or gain heat while undergoing a physical or chemical change. The basic principal on which the calorimeter works is that when two bodies at different temperatures are in contact with one another, heat will flow from the warmer body to the colder body. Thus, the heat lost by one body will be gained by the other. This exchange of heat of heat will continue until the two bodies are at the same temperature. In a calorimeter, heat is exchanged between the water and the materials undergoing change until the temperatures are the same. The experimenter can thus make a direct measurement of the temperature change of the water. From this information, the heat gained (lost) by the water can be calculated. The experimenter then uses these data to determine the heat lost (or gained) by the materials undergoing change.
Nitrogen is the major (79%) component of air. Even though nitrogen is a gas at room temperature (Boiling Point = 77K) it is possible to obtain it in the liquid state. In part two of this lab we will determine the heat of vaporization of nitrogen by making use of the ideas of conservation of energy, change of state, calorimetry and specific heat. Unlike most calorimeters, the simple Styrofoam-cup calorimeter used in this experiment will have only one chamber. In part 1 of this experiment, you will place the ice directly into a measured amount of water. The heat required to melt ice will be supplied by the water. By measuring the temperature change (DT) of the water, you can calculate the quantity of heat exchanged between the water and the ice. Using these experimental data, you will calculate the heat of fusion of ice. The following relationships will be used in part one of this experiment: heat lost (or gained) original mass change in specific heat a. by the water in = of water in X temperature X capacity the calorimeter the calorimeter of the water of water In symbols, this word formula becomes: q = m X DT X c b. heat given off by the water = heat absorbed by the ice c. heat absorbed by the ice = heat of fusion of ice mass of melted ice The specific heat capacity of a substance is the quantity of heat energy needed to raise the temperature of 1 gram of a substance by 1 C. The specific heat capacity of water is 4.184 J/(g X C). Materials 250-mL beaker water wire gauze 100-mL graduated cylinder ice ring stand lab burner liquid nitrogen thermometer Styrofoam cup Procedure - Part A 1. In a 250-mL beaker, heat about 125 ml of water to a temperature in excess of 50 C. 2. Immediately, measure precisely 75 ml of this heated water in a graduated cylinder and pour it into a Styrofoam cup. Record this volume of water in the data table as V1. 3. Measure accurately and record the temperature of the water in the Styrofoam cup as T1. 4. Immediately add 4-5 ice cubes. Make sure there is always ice in the calorimeter throughout the experiment.
5. Carefully and continuously stir the ice-water mixture with the thermometer. Continuously monitor the temperature of the ice-water mixture until the temperature stops dropping. Record this temperature as T2 6. As soon as the temperature stops dropping, carefully pour the water into a clean dry beaker without transferring any of the remaining ice. 7. Measure and record the volume the water at the end of the experiment as V2. Procedure - Part B 8. In a 250-mL beaker, heat about 100 ml of water to a temperature in excess of 70 C. 9. Determine and record the mass of a Styrofoam calorimeter (call this cup #1) to the nearest 0.1 g. 10. Add the hot water to cup #1 and determine the total mass (mass x). Set the Styrofoam cup in a 600-mL beaker to make the set-up more stable. 11. Take a second Styrofoam cup (cup #2). Place it on the scale and rezero (tare) the scale. Add about 60 g of liquid nitrogen to cup #2, and record the mass to the nearest 0.1 g. Note: The liquid nitrogen will immediately begin to boil to vapor so go immediately to your lab station while your partner is doing step 12. 12. While the liquid nitrogen is being massed determine the temperature of the warm water right before adding the liquid nitrogen to the nearest 0.1 C 13. Record this value as T1 right before you add the liquid nitrogen. The temperature will probably be below 70 C, don t worry (be happy) just use whatever temperature the water is currently. 14. Remove the thermometer and immediately add all of your measured liquid nitrogen to the cup with hot water. Wait for the boiling to stop. Do not put the thermometer back in until the sizzling has stopped. You may have to fan the fog away with your hand. If any ice has formed make sure it has melted before taking you final temperature. 15. Record the lowest temperature you can record of the cooled water (again to the nearest 0.1 C) (T2). 16. Determine the final mass of the Styrofoam cup and cool water (mass y).
Name Date Calorimetry: Determining the Heat of Fusion of Ice and the Heat of Vaporization of Liquid Nitrogen - Chemistry I Acc Prelab Questions 1. As the warm water cools down explain what happens in terms of energy on a molecular level. 2. In this lab when the water loses energy where does the energy go? 3. What does a low heat of vaporization value for a substance tell you about its behavior and the forces that hold it together? 4. Why do we use a Styrofoam cup and not a metal cup as a calorimeter? Data and Observations Part A V1 = ml T1 = C V2 = ml T2 = C Part B cup 1 mass g mass of the liquid N 2 g cup 1 plus water mass at the start (mass x) = g T1 = C cup 1 plus water mass at the end (mass y) = g T2 = C
Information needed for Calculation question #B6 Today s Outside Temperature F ( F = (9/5) C + 32) C Today s Atmospheric Pressure inches Hg Calculations - part A 1. Using the known density of water, find the mass (m1) of the original volume water (V1). 2. Find the volume of the water produced from the melted ice (V3 = V2 - V1) 3. Find the mass(m3) of this water(v3) produced from the ice. 4. Find the change in the temperature of the ice (DT = T1 T2). 5. Find the heat lost by original mass of water (q = m1 X DT X c) 6. Find the heat of fusion of ice (q/m3) 7. Calculate % error. (the accepted value is 336 J/g) Calculations - part B 1. Calculate the change in temperature of the water in part B.
2. Calculate the energy (heat) lost by the water in joules. 3. Calculate the heat of vaporization of nitrogen in joules/gram N 2. 4. Using the results of this experiment, determine the number of joules required to vaporize one mole of liquid nitrogen. This is called the molar heat of vaporization. 5. The accepted value for the heat of vaporization of nitrogen is 5.58 kj/mol. Calculate your percent error. 6. Calculate the volume of the nitrogen gas that you produced under today s conditions of pressure and temperature. Discussion and Synthesis 1. List possible sources of error in this experiment.
2. How might the use of a calorimeter as shown in Figure 1 reduce some of these errors? 3. One source of error is flow of heat between the water in the cup and surroundings. Explain how this error is reduced by starting with water at a temperature above room temperature and ending with water at a temperature below room temperature? 4. In what way does calorimetry make use of the law of conservation of energy? 5. Is the process of melting exothermic or endothermic? Give evidence to support your answer. 6. Write a chemical equation to represent each of the following for nitrogen and include the energy term in joules/mol in the equation. State whether the process is endothermic or exothermic. a. Vaporization b. Condensation 7. What is the identity of the fog that was produced when the liquid nitrogen was added to the water? Explain.
8. Pigs don t sweat. Explain why on hot days they roll in the mud using ideas from this lab. 9. Which posses more kinetic energy, liquid N 2 at its boiling point or N 2 gas at its boiling point? Explain. 10. A solid substance with a mass of 200.0 g is at its melting point temperature in a calorimeter. While the substance changes from a solid to a liquid at the same temperature, the 400.0-grams of water in the calorimeter goes from an initial temperature of 80.0 C to a final temperature of 30.0 C. a. How much heat did the water in the calorimeter lose while the substance melted? b. What is the heat of fusion of the substance that melted? 11. The result listed below were obtained in an experiment on an unknown liquid (liquid Y). Calculate the heat of vaporization of the unknown liquid in units of J/g. Show all work. Mass of liquid-y: 42.6 g Mass of water: 96.1 g T1 of water : 55.0 C T2 of Water: 44.5 C
12. The heat of fusion of nitrogen is 25.7 J/g. Why is the heat of fusion lower than that for the heat vaporization? 13. Explain how your refrigerator works in terms of energy transfer to keep food cold. Where does the heat from the food go and how?