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10-thermochemistry-abc - CHM151LL THERMOCHEMISTRY | 1...

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CHM151LL: THERMOCHEMISTRY | 1 Thermochemistry I: Endothermic & Exothermic Reactions OBJECTIVES: Learn elementary concepts of calorimetry and thermochemistry Practice techniques of careful temperature, mass, and volume measurement Determine the sign of H for the reaction of citric acid with sodium hydrogen carbonate DISCUSSION: Until now, we have focused on the composition changes taking place in a chemical system. Starting with this lab, we will broaden our scope to include energy changes , venturing into thermochemistry . Many reactions release energy (e.g. heat, light, gas expansion). When the energy release comes in the form of heat specifically, we call the reaction exothermic . 1 By contrast, endothermic reaction systems take in heat from their thermodynamic surroundings. The laboratory measurement of the heat energy changes that accompany a chemical change is known as calorimetry . You will perform an elementary calorimetric study of the reaction of a citric acid solution with sodium hydrogen carbonate to produce water, carbon dioxide, and sodium citrate: H 3 C 6 H 5 O 7 ( aq ) + 3 NaHCO 3 ( s ) 3 H 2 O ( l ) + 3 CO 2 ( g ) + Na 3 C 6 H 5 O 7 ( aq ) Monitoring the temperature of the reaction system inside a simple calorimeter will enable you to decide whether the reaction is exothermic or endothermic. Remember that your thermometer is part of the thermodynamic surroundings . If heat flows out of the thermometer (if the temperature reading drops) heat must be flowing into the system . (Would that process be exothermic or endothermic?) PROCEDURE: 1. Assemble a calorimeter from a pair of insulating expanded polystyrene (EPS, Styrofoam®) cups. Use a 250-mL beaker for support, if necessary. Insert a liquid-in-glass thermometer or digital thermocouple, as shown in Figure 1. 2. Measure a 30-mL sample of citric acid solution into the EPS calorimeter cup. Before you begin data collection, the temperature probe must come to thermal equilibrium with the solution (this takes at least 45 seconds). 3. Fold a sheet of weighing paper, tare the balance with the paper on the pan, and weigh out 10.0 g of sodium hydrogen carbonate (baking soda). 4. Record temperature values in your data table every 15 seconds. Wait at least 30 s (three temperature readings) to establish a temperature baseline, then add the NaHCO 3 to the citric acid solution in the calorimeter cup. To ensure good mixing, stir the solution gently with the temperature probe. 1 When we don’t care about the form of energy release, we call reactions that release energy exergonic , and reactions that absorb energy endergonic . Chemists almost always focus on heat transfer.
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5. Continue collecting temperature vs. time data until the solution temperature reaches a maximum (or minimum) and begins to fall (or rise) again. If you can’t interpret the trend with certainty, stop after 225 s. 6.
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10-thermochemistry-abc - CHM151LL THERMOCHEMISTRY | 1...

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