EXPERIMENT 1: Thermochemistry

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1 EXPERIMENT 1: Thermochemistry Heat Flows Associated with Physical Processes Laboratory Techniques You will be expected to measure temperatures precisely and quickly in addition to becomin familiar with constant pressure calorimeters and their setup. Objectives 1. To learn the basic techniques and calculations associated with calorimetry. 2. To become aware of some difficulties involved with measurin heat flows precisely. Part I. Heat Flows Associated with Temperature Chanes of Substances Definitions The absolute temperature (T) of an object is a measure of the kinetic enery associated with the motion of the particles of that object. Heat (q) is the amount of kinetic enery transferred from one object to another as a result of a temperature difference between them. The size of the heat flow to an object is directly proportional to the temperature chane T ( T = Tfinal - Tinitial) of the object. Heat (q) is related to T by the proportionality constant, C, called heat capacity of the object. The heat capacity is the heat required to raise the temperature of a iven amount of a substance by one deree Kelvin (1 deree K = 1 o C). C = q or q = C T (Equation 1.1) T If we are dealin with a homoeneous substance we can talk about its specific heat capacity, c, the amount of heat necessary to raise the temperature of one ram of a substance by one deree. (Note: the word specific in physical sciences often means per ram so the specific heat capacity literally means heat capacity per ram). c = C or C = m c m where m represents the mass in rams of the substance. Equation 1.1 becomes q = m c T (Equation 1.2) If a warm substance comes in contact with a cool substance: The cool substance absorbs heat; Tfinal - Tinitial is a positive number, so q is positive. This an endothermic process for the cool substance. The warm substance ives off heat to the cool substance, and for it, T and q are neative. This is an exothermic process for the warm substance. Notice that the sin of q denotes direction in which heat is flowin

2 Calorimetry Calorimetry is a process by which the heat flow for a particular process is measured by observin the chane in temperature it produces in a known quantity of water or another substance. In any calorimetry experiment we divide the universe into two parts: the system and the surroundins. The system is that part of the universe under study and the surroundins are the rest of the universe. We will limit the size of the surroundins by carryin out the process in an insulated container (calorimeter) that is assumed ideally to not allow heat to enter from or leave to the surroundin laboratory. If a system loses heat, by the Law of Conservation of Enery, its surroundins must ain the same amount of heat. The manitude (or the absolute value) of qsys and qsurr is the same, and since the heat flow has a different direction for each, the sins are opposite. We relate qsys and qsurr in the followin way: qsys = - qsurr (Equation 1.3) In Part I, we will add a known amount of hot water to a known amount of cold water in a calorimeter, measure the temperature chanes involved, and use calculated q s for both hot and cold water to calculate qcalorimeter and estimate Ccalorimeter. We will arbitrarily choose the hot water as bein the system, althouh the cold water or the calorimeter could just as easily have been chosen. The hot water will ive off heat as it cools. The cold water and the calorimeter will therefore be the surroundins. The surroundins will warm as heat is taken in. qsys = - qsurr qhot water = - (qcalorimeter + qcold water) (m c T)hot water = -(C T)calorimeter + -(m c T)cold water (Equation 1.4) Procedure 1. Place a clean dry polystyrene cup plus its stopper in an insulated jacket. Weih the assembled apparatus on the top loadin balance. Pour 50 ml of room temperature water (from the deionized water vat) into the cup and reweih the cup, stopper, jacket and contents. Record all data in the table provided. 5. Without splashin the contents of the cup, carefully add the hot water to the room temperature cup, cover with the insulated stopper and swirl the mixture. Wait one minute, then put the thermometer into the cup. Record the temperature to the nearest 0.1 o C when it has stabilized at its lowest value. 2. Measure and record the temperature of the water to the nearest 0.1 o C (Note: we only use deionized water because it is at room temperature, as is the calorimeter). 3. Place a second clean dry polystyrene cup plus its stopper in an insulated jacket. Weih it on the top loadin balance. Pour 60 ml of hot water (from the hot tap) into the cup and reweih the cup, stopper, jacket and contents. Record all data in the table provided. 4. Record the temperature of the hot water to the nearest 0.1 o C usin the same thermometer you used for the room temperature water

3 FIGURE 1 (Calorimeter) Introduction Part II. Heat Flows Associated with Phase Chanes of Substances When a solid is melted at its meltin point, enery is absorbed without a chane in temperature. The enery is used to cause the phase chane rather than to raise the temperature of the substance. The heat absorbed (at constant pressure) in this way is call the enthalpy of fusion. The molar enthalpy of fusion is the amount of heat required to melt one mole of a substance at its meltin point. The amount of heat required to melt a iven amount of solid at its meltin point must be calculated from its molar DHfusion (sometimes written as quantity). H fus where the bar represents the fact that DHfus is a molar The formulas in Part I cannot be used because there is no T for a phase chane. (Equation 1.5) qphase chane = n DHfus where q has units of kj, n is the number of moles of substance melted, and DHfus, a molar quantity, has units of kj mol -1. In Part II, a sample of ice at 0 o C will be added to hot water in a calorimeter. If the hot water and calorimeter are the surroundins, then the ice is the system. The ice melts at 0 o C and the resultin meltwater warms from 0 o C to Tfinal. qsys = - qsurr qice = - (qhot water + qcal) We won t presume to think our Ccal from Part I is precise enouh to use in further calculations, so we will assume qcal = 0. qice phase chane + qmelt water warmin = -qhot water coolin (Equation 1.6) - 3 -

4 Procedure Record all masses to two decimal places (for Weihin by Difference technique, see Appendix D). 1. Place a clean dry polystyrene cup plus its stopper in an insulated jacket. Weih it on the top loadin balance. Pour 100 ml of hot water (from the hot tap) into the cup and reweih the cup, stopper, jacket and contents. Record all data in the table provided. 2. Collect an ice cube (at 0.0) from the ice machine usin a piece of paper towel. Leave it wrapped in the towel to absorb any melt water while you measure the temperature of the water in your cup to the nearest Without splashin the cup s contents, carefully add the dried ice cube and stir the ice-water mixture until the ice has melted. Record the temperature to the nearest 0.1 when it has stabilized at its lowest value. 4. Weih the cup, alon with its stopper, jacket and contents (hot water + melt water) and record the mass in the table provided

5 Laboratory Report Date: Lab. Slot: Part 1. Heat Flows Associated with the Temperature Chanes of a Substance Table 1 Mass of calorimeter + water Mass of calorimeter Mass of water Final temperature of water Initial temperature of water T of water ( Tf - Ti ) Room temperature water Hot water Calculations (show all details - this includes identifyin all numbers, showin the correct sinificant fiures showin all units and the correct values at every step of the calculation) 1. Calculate qhot water (specific heat of water = 4.18 J -1 oc -1 ). 2. Calculate qcold water

6 3. Use Equation 1.4 to calculate qcalorimeter. 4. If qcal = (C T)calorimeter, calculate the apparent Ccalorimeter. 5. What percentae of the heat iven off by the hot water was ained by the calorimeter? - 6 -

7 Part II. Heat Flows Associated with the Phase Chanes of a Substance In this experiment the hot water loses heat to the meltin ice. The ice is first MELTED (a phase chane process involvin enthalpy of fusion) and that water is then WARMED (a process involvin specific heat of liquid water). The heat taken from the hot water can be calculated directly usin m C T. mass of cup + stopper + jacket + hot water mass of empty cup + stopper + jacket mass of hot water final temperature of hot water, Tfinal initial temperature of hot water, Thwi chane in temperature of hot water, Thw Hot Water Data Ice Data mass of cup + stopper + jacket + hot water + melt water mass of cup + stopper + jacket + hot water (above) mass of ice final temperature of melted ice, Tfinal initial temperature of ice, Tice chane in temperature of melted ice, Tmelted ice Calculations (Show all details below) 1. Calculate qhot water. 2. Calculate qmelt water

8 3. Use Equations 1.6 and 1.5 to calculate q for the phase chane of ice and then Hfus in kj mol Calculate Hfus for ice in units of J Calculate the % error for your experimental value of the molar heat of fusion by comparin it with the literature value for Hfusion H2O(s) at 273 K (inside cover of the lab manual). 6. Use your calculated Hfusion H2O(s) to determine the heat required to melt k of ice at 0 o C? - 8 -

9 Prelaboratory Exercise Date: Lab. Slot: Experiment 1: Enthalpy of Fusion of Water A student obtained the followin sample data: mass of cup + stopper + hot water mass of empty cup + stopper mass of hot water final temperature of hot water, Tfinal initial temperature of hot water, Thwi chane in temperature of hot water, Thw mass of cup + stopper + hot water + ice mass of cup + stopper + hot water mass of ice final temperature of melted ice, Tfinal initial temperature of ice, Tice chane in temperature of melted ice, Tmelted ice Calculate qhot water. 2. Calculate qmelt water. 3. Use Equations 1.6 and 1.5 to calculate q for the phase chane of ice and then Ηfus in kj mol Calculate Hfus for ice in units of J