A Simplified Undergraduate Calorimetry Experiment
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Difficulties experienced by students with conventional experiments for determining AH for the reaction H30+ + O H 2H10 (e.g. neutralization of NaOH with conc. HCI) have led us to develop a new version, which increases both understanding and precisian. The method now involves a low-energy-equivalent ( h -70 JK-1) glass calorimeter made from a cut down beaker, insulated (see figure) with expanded polystyrene and furnished with a Ge,;t, magnetic-stirrer paddle, and a nickel wire heating coil secured to brass rods. The resistance of the coil (-3 ohm) is determined with a conductance bridge, and the calorimeter calibrated by connecting a measured D.C. supply (-8 V) across the coil for a recorded time sufficient t o give a water-temperature rise similar to that obtained on neutralization, ATN. One hundred milliliters of standard NaOH ( M -0.25) is neutralized by admitting eonc. HCI (-2.8 g), previously intraduced from a calibrated dispenser into a glass tube (stopT "'* pered with a PTFE-sprayed cork) in the rubber-bung lid of the calorimeter. This procedure is now repeated but with 100 ml of H1O in place of NaOH in the calorimeter. Differences in repeated deliveries of HCI (0.4%) and the enor introduced by assuming equal weights of liquid in both neutralization and dilution stages (1%) are less than the error (>2%) in the measurement of the temperature rise on dilution, ATo. Moreover, the specific heat capacities of 0.25 M NaCl and HC1 are the same (4.09 JK-I g-') to within 0.3% so that the temperature rise due toneutralization alone issimply (ATN - ATD) Kelvin. Then,
I,,,....
Routine values of AH within + 2 kJ mol-' of the literature value (-56.8 kJ mol-I) for this reaction and concentration can be obtained. Maximum utilization of equipment is permitted by different sequences for the three stages of the erperiment. K. D. Bartle P.M. Osborn University of Leeds Leeds LS2 9JT, United Kingdom
Volume50, Number 9 , September 1973 I637
