Lab-2-specific-heat 02 14 09 PDF

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Chemistry 108 lab

Name_________________________

Lab #2: Coffee Cup Calorimetry INTRODUCTION In this experiment, you will determine the specific heat for an unknown metal. The metal sample will be heated to a high temperature (100oC) then placed into a coffee cup calorimeter containing a known amount of water. If you can find out how much heat was gained by the water in the calorimeter then you will know how much heat was lost by the metal. You can then calculate and compare the specific heat of your unknown metal to known values of metal specific heats and identify your metal. EXPERIMENTAL 1) Set up a calorimeter. The instructor will give each student a sample of an unknown metal. Each student must do their own unknown. Record your unknown number in the DATA section below. (The thermometer is very expensive, so be careful when handling it.) The calorimeter consists of two polystyrene coffee cups fitted with a styrofoam cover (placed in a 400 ml beaker for balance). There are two holes in the cover for a thermometer and a glass stirring rod. Do not fill the calorimeter with water yet. Assemble the experimental setup as shown in the figure below.

400 ml beaker

2) One of the partners do this step: Fill a 600 ml beaker two-thirds full of water, add three boiling chips (located on the back counter of the lab). Place the beaker on a hot plate. Turn the heating control knob on the hot plate to full heat. Continue heating the water until it boils. When the water begins to boil, decrease the heat setting to about 50%. This boiling water will be shared by both lab partners.

Chemistry 108

Heat Capacity Lab

3) Each person will weigh an empty, extra large test tube and stopper. Record the mass of the empty, stoppered tube in your data table (use all of the digits displayed on the balance). 4) Transfer your unknown metal to the extra large test tube that you just weighed and replace the stopper. (the boiling water bath would remove the labels from the tubes your metals came in). Weigh your sample of unknown metal in the extra large, stoppered test tube. Record the mass of the stoppered tube and metal in your data table. Record all of the units displayed on the balance read-out. • You will return your metal back to the original tube at the end of the experiment, so keep track of your own tube and do not mix it up with your partner’s. 5) Place the loosely stoppered tube with the metal into the boiling water in the beaker. Do not put water in the tube with the metal! Do not put the stopper tightly in to the tube or the tube may explode as the air in the tube is heated. The water level in the beaker should be high enough so that the top of the metal is below the water surface. Turn the heating control knob on the hot plate to full heat. Continue heating the metal in the water for at least 15 minutes after the water begins to boil to ensure that the metal attains the temperature of the boiling water (100.0oC). Add water to the beaker as necessary to maintain the water level. 6) While the water is boiling, weigh the empty calorimeter (both styrofoam cups and cover only; no water, no stirring rod, no thermometer, no 400ml beaker). Record the mass of the empty calorimeter in your data table, record this mass using three numbers after the decimal point. 7) Place about 30 ml of tap water in the calorimeter and weigh it again. Record the mass of the calorimeter and water in your data table. (styrofoam cups, water, and cover only; no stirring rod, no thermometer, no 400ml beaker) Do not be concerned that the last digit on the balance is not stable, you are seeing the water evaporate! Because of evaporation occurring, record this mass using three numbers after the decimal point. 8) Measure the initial temperature of the water contained in calorimeter. Note that you will need to hold the calorimeter at an angle so that the thermometer bulb is completely under the water. Record, to 0.1°C (one place to the right of the decimal), the temperature of the water (Tinitial water) in the calorimeter. 9) Insert the stirrer and thermometer into the calorimeter through the 2 holes in the cover. 10) Take the test tube out of the beaker of boiling water, remove the stopper, and pour the metal into the water in the calorimeter. Replace the calorimeter cover and stir the water/metal mixture as best you can with the glass stirrer. Record, to 0.loC, the maximum temperature reached by the water in the calorimeter (this is the Tfinal of the water and the metal). 11) OPTIONAL If you have 40 more minutes before the end of lab time, if you wish, you can repeat the experiment a second time (trial 2). Be sure to dry your metal before reusing it; this can be done using several paper towels. Be sure to dry the metal completely or you will

introduce error to your measurements. 2

Chemistry 108

Heat Capacity Lab

WHEN FINISHED: The metal used in this experiment is to be dried with paper towels and returned to the front counter in the test tube in which you obtained it.

DATA Your unknown number ________________________ Trial 1

Trial 2

Mass of empty test tube and stopper g

g

g

g

g

g

g

g

°C

°C

100.0 °C

100.0 °C

°C

°C

Mass of stoppered test tube plus metal Mass of empty calorimeter Mass of calorimeter and water Initial temperature of water in calorimeter (Tinitial water) Initial temperature of metal (assume 100.0°C) (Tinitial metal) Equilibrium temperature of metal and water in the calorimeter (Tfinal water = Tfinal metal)

CALCULATIONS Be sure to use the correct number of significant figures and to use units with every number you write!!!! 1) Calculate ∆Tmetal Trial 1

Trial 2

Trial 1

Trial 2

2) Calculate ∆Twater

3

Chemistry 108

Heat Capacity Lab

3) Calculate the mass of the water in the calorimeter (mwater). (Think about how you get this from the data table values….you have the mass of the calorimeter with the water in it and the mass of the empty calorimeter……) Trial 1 Trial 2 (OPTIONAL)

4) Calculate the mass of the metal (mmetal). (Think about how you get this from the data table values….you have the mass of the metal in the tube and the mass of the empty tube……) Trial 1 Trial 2

5) Calculate the heat energy gained by the water (Qwater). Trial 1

Trial 2

Did you use the correct number of sig. figs.? 6) Knowing that the heat gained by the water is equal and opposite the heat lost by the metal, use equation (6) or (7) in the prelab to calculate the specific heat of your metal. Trial 1 Trial 2

Did you use the correct number of sig. figs.? 7) If you did two trials, take the average of the specific heats calculated in the two trials (above). 4

Chemistry 108

Heat Capacity Lab

Average specific heat of metal (if you did 2 trials only)……..________________________ (Did you use the correct number of significant figures and the correct units?)

CONCLUSION Re-write your unknown number here: ____________________ Your unknown metal is one of the metals in the table below. Use this table of specific heats to determine the identity of your metal. Choose the metal with the specific heat that is closest to your experimental value. Metal Aluminum Bismuth Tin Nickel

Specific Heat (J/goC) 0.900 0.126 0.226 0.443

My metal is (circle one): Aluminum

Bismuth

Tin

Nickel

5...