Hess' Law: Calorimetry

Transcription

1 Exercise 9 Page 1 Illinois Central College CHEMISTRY 130 Name: Hess' Law: Calorimetry Objectives The objectives of this experiment are to... - measure the heats of reaction for two chemical reactions. - use Hess' law, in conjunction with the above measurements, to calculate the heat of reaction for the combustion of magnesium metal. Background The heat of reaction ( H) is defined as the heat lost or gained as a reaction proceeds from reactants to products. It is often given as part of a thermochemical equation, such as that shown for the combustion of hydrogen: 1 H 2(g) + O 2(g) H 2 H = kj 2 The negative sign indicates that this reaction is exothermic. Thus, when one mole of hydrogen gas and 1/2 mole of oxygen gas combine to form one mole of liquid water, kj of heat energy are released. Conditions of constant atmospheric pressure (reaction vessel is an open container) and near room temperature (25 o C) are assumed in the above value, although heats of reaction do not vary appreciably with temperature. A calorimeter is a device for measuring the heat of reaction. For those reactions involving aqueous solutions, a simple styrofoam coffee cup works well. The reaction can be carried out in the cup and the resulting change in temperature, ( T), can be measured using a thermistor. Assuming adiabatic conditions (no heat loss), the reaction heat all goes into warming the solution in the cup and the cup itself. This heat quantity can be calculated as follows: Heat = [(specific heat of soln.) x (grams solution) x ( T)] + [(heat capacity of cup) x ( It can be reasonably assumed that the reaction solution is similar to water. Hence use a specific heat value of 4.18 J/(g o C) and a density of 1.00 g/ml for the reaction solution. The estimated cup heat capacity is 10 J/ o C. The final heat of reaction calculation requires a conversion from grams of limiting reactant to moles of limiting reactant. T)].

2 Exercise 9 Page 2 In this experiment you will measure H values for the following reactions: Mg (s) + 2 HCl (aq) MgCl 2(aq) + H 2(g) MgO (s) + 2 HCl (aq) MgCl 2(aq) + H 2 Using the thermochemical equations for these two reactions along with that previously given for the hydrogen/oxygen reaction, you are to then calculate the H value for the combustion of magnesium: 2 Mg (s) + O 2(g) 2 MgO (s) H=??? This calculation is done by applying Hess' Law as described in your text book. Temperature measurement in this experiment is made with a thermistor. The spreadsheet program will be used to graph the temperature vs. time data for the two reactions. From these plots, T values will be determined. Safety Precautions As usual, any skin contacted with chemicals should be washed immediately. Safety goggles must be worn at all times in the lab. Procedure 1. Connect the Go-LinK to the USB connestor of your computer and connect the Stainless Steel Temperature Probe to the Go-LinK. 2. Start the Logger Pro program on your computer. Open the file 18 Hess's Law from the Chemistry with Computers folder. You should now see the screen shown here. This file allows you to collect temperature readings at half-second intervals and display them graphically. 3. Double-click the top of the temperature column to open the the Column Option dialogue box. Choose the Option tab and set the Displayed Precision to 2 decimal places.

3 Exercise 9 Page 3 Thermistor Calibration CHECK WITH YOUR INSTRUCTOR TO SEE IF CALIBRATION IS NECESSARY In most cases, the Stainless Steel Temperature Probe will never need to be calibrated. However, if the need arises the sensor can be custom calibrated. You will need a good thermometer and three containers of water at three different temperatures. 1. Open Logger Pro and from the Menu Bar choose Experiment/Calibrate and then choose the Temperature Probe.Choose Calibrate Now. For each of the three water baths, place the temperature probe in the bath with the thermometer. 2. Allow both readings to stabilize, click, and enter the temperature reading measured by the thermometer. 3. Click Done after the third water bath reading. Note: If you want the calibration to be saved with the file for later use after you calibrate, you need to do two things: Go to the calibration dialogue box and choose Calibration Storage from the pull down menu. Then set the Calibration Retrieval Preference to Experiment file. Save the Experiment file. Running the Experiment Obtain two styrofoam cups (one nested in the other for extra insulation), a cardboard lid, a temperature probe, a magnetic stirrer, and a magnetic stir bar (one inch in length or less). Fit the thermistor through the cardboard lid to a length such that its tip goes deep into the cup but misses the stir bar. Use the rubber stopper to prevent the thermistor from slipping lower. The Mg/HCl Reaction 1. Use distilled water and the graduated cylinders provided to prepare 250 ml of 1.00 M HCl. The HCl provided in the lab is 6.00 M. Pour 100 ml of the 1.00 M HCl solution into the styrofoam cup (nested inside another cup) and add the stir bar. Place the styrofoam cup calorimeter on the magnetic stirrer. Adjust the stirring rate to vigorous but without splashing. Finally, insert the Temperature Probe (with lid) and allow several minutes for temperature equilibration. 2. Weigh approximately grams of magnesium turnings on the top-loading balance. Tare a piece of weighing paper on the balance pan first for supporting the magnesium and record the magnesium weight to at least g. 3. On the Logger Pro Tool Bar, click the Collect button and allow the program to collect temperatures for approximately 30 seconds to establish an accurate initial temperature.

4 Exercise 9 Page 4 4. Quickly add the magnesium turnings and reposition the Temperature Probe and lid assembly. As the reaction occurs, you should observe the temperature climb. Continue collecting data until a final, constant temperature plateau is well established. The experiment can be stopped at that point by clicking the stop button on the Tool Bar. 5. On the Menu Bar select File/Save As and assign your data a file name. Do not overwrite any existing file names in the current folder. The MgO/HCl Reaction 1. The second experiment is conducted in a manner identical to that described above except that MgO is used (instead of Mg metal) and the weighed amount is different. Use a top-loading balance to weigh approximately gram of MgO powder (record to the nearest g). 2. Set up the calorimeter with 100 ml of fresh 1.00 M HCl, insert the Temperature Probe and allow time for equilibration. Begin the program by clicking the Collect button (the current graph will automatically be erased) and again collect data for at least 30 seconds to establish the initial temperature of the solution. 3. Quickly add the MgO powder and reposition the Temperature Probe and lid assembly. When you transfer the powder to the calorimeter, take care to transfer it as completely as possible, leaving little or no residue on the weighing paper and no powder on the inside walls of the calorimeter. As before the temperature will eventually stabilize. Continue taking data until there is a well established plateau. The experiment can be stopped at this point by clicking the stop button on the Tool Bar. 4. On the Menu Bar select File/Save As and assign your data another file name. Do not overwrite any existing file names in the current folder. Data Analysis 1. After you have saved your data for Experiment 2, select File/Open from the Menu Bar and open your data file from experiment From a printout of this plot (or from a close examination of the data) determine the T as accurately as possible for the reaction and record this value on your Report Sheet. 3. Repeat steps (1) and (2) for the data obtained in experiment Using the T's obtained from your graphs along with the masses and specific heats of each solution to determine the H of each reaction in kj/mol. (note that an entire mole was not used in each experiment, consequently you must convert the heats obtained to kj/mol) 5. Attach printouts of both graphs to your Report Sheet (no data tables necessary).

5 Exercise 9 Page 5 Illinois Central College CHEMISTRY 130 Name: REPORT SHEET Hess' Law: Calorimetry Magnesium/HCl Reaction Magnesium Oxide/HCl Reaction Grams Mg Grams MgO Final temperature Final Temperature Initial Temperature Initial Temperature T T H Calculations 1. Calculate H for the Mg/HCl reaction in kj/mole of Mg. Show your work! H= kj/mol 2. Calculate H for the MgO/HCl reaction in kj/mole of MgO. Again, show all your work. H= kj/mol

6 Exercise 9 Page 6 Hess' Law Calculation 2 Mg (s) + O 2(g) 2 MgO (s) H =??? 1. Write the three thermochemical equations needed to calculate H for the reaction above. Include H values for each reaction. 2. In the space provided, neatly rewrite the three equations (multiply, reverse directions, etc.) such that when added, the result is the thermochemical equation above for the combustion of magnesium. Calculate the for this reaction. Show your work. H H= 3. Calculate the % error between your value for the combustion of magnesium and the value for the for MgO(s) found in your text book. (Remember..your value is for two moles Mg) H f o

7 Exercise 9 Page 7 Illinois Central College CHEMISTRY 130 Name: SHOW YOUR WORK PRELAB: Exp.9 Hess' Law: Calorimetry 1. How much heat is required to raise the temperature of 30.0 grams of silver from 18.2 o C to 35.6 o C? (the specific heat of silver is J/g o C). J 2. A reaction of 1.00 gram of NaOH with 100 ml of 1.0 M HCl in a styrofoam calorimeter causes a 10.5 o C increase in the temperature of the reaction solution. Assuming the specific heat of the solution is J/g o C, density is 1.00 g/ml, and the heat capacity of the styrofoam cup calorimeter is 10.0 J/ o C, calculate the heat of this reaction per mole of NaOH. H= kj/mol 3. Use Hess' law to calculate H o for the reaction C 2 H 2(g) + 2 H 2(g) C 2 H 6(g) from the combustion data: C 2 H 2(g) + 5/2 O 2(g) 2 CO 2(g) + H 2 H o = kj H 2(g) + 1/2 O 2(g) H 2 H o = -286 kj C 2 H 6(g) + 7/2 O 2(g) 2 CO 2(g) + 3 H 2 H o = kj H = kj

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