Lab #2: Coffee Cup Calorimetry

Transcription

1 lab Lab #2: Coffee Cup Calorimetry Name INTRODUCTION In this experiment, you will determine the specific heat for an unknown metal. The metal sample will be heated to a hih temperature (100 o C) then placed into a coffee cup calorimeter containin a known amount of water. If you can find out how much heat was ained 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 ive 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 handlin 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 lass stirrin rod. Do not fill the calorimeter with water yet. Assemble the experimental setup as shown in the fiure below. 400 ml beaker 2) One of the partners do this step: Fill a 600 ml beaker two-thirds full of water, add three boilin chips (located on the back counter of the lab). Place the beaker on a hot plate. Turn the heatin control knob on the hot plate to full heat. Continue heatin the water until it boils. When the water beins to boil, decrease the heat settin to about 50%. This boilin water will be shared by both lab partners.

2 3) Each person will weih an empty, extra lare test tube and stopper. Record the mass of the empty, stoppered tube in your data table (use all of the diits displayed on the balance). 4) Transfer your unknown metal to the extra lare test tube that you just weihed and replace the stopper. (the boilin water bath would remove the labels from the tubes your metals came in). Weih your sample of unknown metal in the extra lare, 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 oriinal 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 boilin water in the beaker. Do not put water in the tube with the metal! Do not put the stopper tihtly 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 hih enouh so that the top of the metal is below the water surface. Turn the heatin control knob on the hot plate to full heat. Continue heatin the metal in the water for at least 15 minutes after the water beins to boil to ensure that the metal attains the temperature of the boilin water (100.0 o C). Add water to the beaker as necessary to maintain the water level. 6) While the water is boilin, weih the empty calorimeter (both styrofoam cups and cover only; no water, no stirrin rod, no thermometer, no 400ml beaker). Record the mass of the empty calorimeter in your data table, record this mass usin three numbers after the decimal point. 7) Place about 30 ml of tap water in the calorimeter and weih it aain. Record the mass of the calorimeter and water in your data table. (styrofoam cups, water, and cover only; no stirrin rod, no thermometer, no 400ml beaker) Do not be concerned that the last diit on the balance is not stable, you are seein the water evaporate! Because of evaporation occurrin, record this mass usin 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 anle so that the thermometer bulb is completely under the water. Record, to 0.1 C (one place to the riht of the decimal), the temperature of the water (T initial water) in the calorimeter. 9) Insert the stirrer and thermometer into the calorimeter throuh the 2 holes in the cover. 10) Take the test tube out of the beaker of boilin 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 lass stirrer. Record, to 0.l o C, the maximum temperature reached by the water in the calorimeter (this is the T final 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 reusin it; this can be done usin several paper towels. Be sure to dry the metal completely or you will introduce error to your measurements. 2

3 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 Mass of empty test tube and stopper Mass of stoppered test tube plus metal Mass of empty calorimeter Mass of calorimeter and water Initial temperature of water in calorimeter (T initial water) C C Initial temperature of metal (assume C) (T initial metal) C C Equilibrium temperature of metal and water in the calorimeter (T final water = T final metal) C C CALCULATIONS Be sure to use the correct number of sinificant fiures and to use units with every number you write!!!! 1) Calculate T metal 2) Calculate T water 3

4 3) Calculate the mass of the water in the calorimeter (m water ). (Think about how you et 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 (m metal ). (Think about how you et this from the data table values.you have the mass of the metal in the tube and the mass of the empty tube ) 5) Calculate the heat enery ained by the water (Q water ). Did you use the correct number of si. fis.? 6) Knowin that the heat ained 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. Did you use the correct number of si. fis.? 7) If you did two trials, take the averae of the specific heats calculated in the two trials (above). 4

5 Averae specific heat of metal (if you did 2 trials only).. (Did you use the correct number of sinificant fiures 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 Specific Heat (J/ o C) Aluminum Bismuth Tin Nickel My metal is (circle one): Aluminum Bismuth Tin Nickel 5