Specific Heat Lab December 13, 2006

Transcription

1 LAB REPORT ESTIMATION OF ATOMIC MASS FROM SPECIFIC HEAT DATA DUE NO LATER THAN 6:00 PM, SUNDAY 12/17/06 Data - Part I - Determination of Heat Capacity of Calorimeter Please note: all temperatures were measured in degrees Celcius. Volume of cold water (.x ml) Temp. readings of cold water Initial Temp. of cold water at time of mixing Volume of hot water (.x ml) Temp. readings of hot water Initial Temp. of hot water at time of mixing 50 ml ml Temp. readings after mixing Maximum Temp. after mixing

2 Data - Part II - Determination of Specific Heat of Metal Please note: all temperatures were measured in degrees Celcius. Metal Used Mass of Metal (.xxxx g) Volume of water (.x ml) Temp. Readings of Hot Water Bath (i.e., Metal) (.xºc) Lead g ml Initial Temp. of Metal at time of mixing Temp. Readings of (room temp.) water (.xºc) Initial Temp. of (room temp.) water at time of mixing Temp. readings after mixing 21.0 Maximum Temp. after mixing 21.0

3 Calculations: Part I: Determine the heat capacity of the calorimeter (Ccal) in cal/ºc. Show all work and tabulate results in the table above. Refer to Example 1 for reference. If you carried out more than one trial, record your results for each trial and then determine and record the average value above in cal/ºc. Use the average value (Ccal, average) for any calculations in Part II. q hot = (m hot )(c water )( T hot ) and q cold = (m cold )(c water )( T cold ) q hot = 50g x 1.00cal/g x o C q cold = 50g x 1.00cal/g x 14.9 o C q hot = -895 calories q cold = +745 calories q cal = (c cal )( T cold ) q cal = +c cal x 14.9 o C calories q total = 0 = q hot + q cold + q cal 0 = -895 calories calories + c cal x 14.9 o C C cal = (895 calories 745 calories) = 150 cal = cal/ o C 14.9 o C 14.9 o C Part II: 1. Determine the specific heat of your metal (c metal ) in cal/gºc. Show all work and tabulate results in the table above. Refer to Example 2 for reference. If you carried out more than one trial, record your results for each trial and then determine and record the average value above in cal/gºc. Use the average value (c metal, average ) for the next question. q hot metal = (m lead )(c lead )( T lead ) q hot metal = 80.0g x c lead x (21.0 o C 98.0 o C) q hot metal = 80.0 x -77 x c lead q hot metal = -6160g/ o C x c lead q water = (m water )(c water )( T water ) q water = 100.0mL x 1.00cal/g o C x (21.0 o C o C) q water = 100.0mL x 1.00cal/g o C x (2.5 o C) q water = calories q cal = (c cal )( T water ) q cal = (10.07 cal/ o C) (2.5 o C) q cal = calories

4 q total = 0 = q lead + q water + q cal 0 = -6160g o C x c lead cal cal cal = -6160g o C x c lead -6160g o C -6160g o C = c lead c lead = ()(4.184 J/cal) = 0.17J/ g o C The periodic table states that the specific heat of lead is 0.129J/g o K. Since this is given in a different dimension than my results, I wonder how it affects my ability to correctly calculate the atomic weight. I do not know how to change these dimensions although I know the conversion factor of Celcius to Kelvin is o C Using the Law of Dulong &Petit, estimate the atomic weight (AW) of your metal in grams per mole. Refer to Example 2 for reference. Show all work and tabulate the result in the table above. C molar = c lead x AW 6.0 cal/mole o C AW lead = 6.0 cal/mole o C = 6.0 cal/mole o C = g/mole c lead 3. Referring to the periodic table, calculate the percent error between your value from the previous question and the listed value. The atomic weight of lead on the periodic table is The difference between the two numbers is 57.2 = 27.61% error based on the formula: Error = ((actual experimental)/actual) x % error is significant. Tabulation of Results (From Calculations above) C cal (in cal/ºc) C cal, average (in cal/ºc) cmetal (in cal/ gºc) c metal, average (in cal/gºc) AW of metal (g/mole) cal/ o C cal/ o C g/mole

5 Questions: 1. How would a calculation for the atomic mass be affected if the hot metal sample cooled off while being transferred to the calorimeter? The difference between the actual atomic weight and the experimental/calculated atomic weight would be significant if the hot metal sample cooled off while being transferred to the calorimeter. This is what we believe happened in our experiment. 2. What are possible sources of error in this experiment? In our experiment, there were three sources of actual error. We turned off the heat source before taking the last measurement of the water bath. In addition, there was a delay in grabbing and transferring the hot metal to the calorimeter cup. I also do not recall that any of us actively swirling the contents between readings.