back in the proper rack, just as you did for the copper cylinder. Clean Up. Pour the water down the sink. Rinse your metal cylinders with distilled water and dry them thoroughly before returning them to the original racks. Follow the directions in Appendix 1 for cleaning your buret. Rinse all of your glassware, as well as the Styrofoam cups with distilled water before drying it. Save your Styrofoam cups and cardboard lids for the next section unless your TA gives you permission to toss them. Part 3: Graphical Determination of ∆ T. For each of your four experimental trials, make a graph with the temperature on the vertical axis and the time on the horizontal axis. The data (both prior and subsequent to mixing) are extrapolated using straight lines to the time of mixing. The temperature difference between the extrapolated lines at the time of mixing is the temperature change ( ∆ T) for the water, as shown in Figure 10–1. CALCULATIONS The sum of the energy changes for the process must be zero (i.e. the energy gained by the water plus the energy lost by the heated metal must equal zero): (2) Using the known masses of the metal and H 2 O, the specific heat capacity of H 2 O, and each temperature change, calculate the specific heat capacity of the metal ( C metal ). In 1819, it was observed by Dulong and Petit that for most metallic elements, the product of the specific heat and atomic mass is close to the value of 25 J mol -1 deg -1 . Theoretically, it was shown by Einstein that this value should be 3 times the gas law constant, 8.3145 J mol -1 deg -1 . Using the relationship: (3) calculate an approximate atomic mass of your unknown and compare this result to the molar mass value of your metal (from the identity established by comparing the experimental heat capacity to the literature values cited below) from the periodic table. Literature Values. The literature heat capacities of several metals at 298 K are: Metal C s (J g -1 deg -1 ) Metal C s (J g -1 deg -1 ) Aluminum 0.897 Nickel 0.444 Copper 0.385 Platinum 0.133 Gold 0.129 Silver 0.235 Iron 0.449 Tin 0.227 Lead 0.130 Zinc 0.388 RESULTS/DISCUSSION Include the following in the summary portion of your report: m H 2 O ⋅ C H 2 O ⋅ Δ T H 2 O = − m metal ⋅ C metal ⋅ Δ T metal C metal J g ° C ( ) ⋅ MM metal g mol ( ) ≈ 25 J mol ° C
Experiment#9 – Heat Capacity & Enthalpy 9 – 5 1. Heat Capacity of Copper. Compare the measured heat capacity for copper with the literature value and comment on the accuracy of your determination. Report the percent error as: (4) 2. Heat Capacity of an Unknown Metal. Report the measured heat capacity for the unknown metal and try to identify it. Report the percent error for your unknown determination based upon the suspected identity of your unknown.
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