Thermochemistry I: Endothermic & Exothermic Reactions

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1 THERMOCHEMISTRY I 77 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: In the lab class so far, 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 as well. With this step, we venture into thermochemistry. Many chemical reactions release energy (e.g. as heat, light, gas expansion). When the energy release comes in the form of heat specifically, we can 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 (baking soda) 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 temperature drops) it must be flowing into the system. (Would that process be exothermic or endothermic?) PROCEDURE: 1. Assemble a simple calorimeter from an insulating expanded polystyrene (EPS, Styrofoam) cup and a supporting 250-mL beaker. Insert a liquid-in-glass thermometer or digital thermocouple, as shown in Figure 1. Figure 1. Calorimeter Assembly 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 as the form of energy transfer.

2 78 CHM151LL: GENERAL CHEMISTRY I LAB MANUAL 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. 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. Dispose of the reaction products in the proper waste container. Graphical Data Display Your instructor may require that you prepare a graph of your experimental results. If so, you will receive additional instructions on data analysis with the lab computer or on saving your data for export to a print-enabled computer outside the lab. If you need to save the file, give it a name that will make it possible to identify easily. A filename like THERMO1 may be descriptive, but PAT_THERMO_419 might work better if your name is Pat and you are performing the experiment on 4/19. Data Display Options Computer software offers options for changing the appearance of the data display. In making choices about the graphical display of data, consider the following: Assume that the audience is intelligent. Show the data. Graphic design is intelligence made visible. In the best graphics, nearly all of the ink and nearly all of the white space communicate. Add clarifying detail (figure number, descriptive title, axis labels, explanatory notes). Make marks or labels as small as clarity permits. Avoid legends, interpolated lines, and non-standard abbreviations. Avoid sans-serif fonts, ALL-UPPER-CASE text, & non-horizontal text.

3 THERMOCHEMISTRY I 79 Name Date Score Prelaboratory Assignment FOR FULL CREDIT, SHOW DETAILED CALCULATION SETUPS. REMEMBER TO FOLLOW THE SIGNIFICANT FIGURES CONVENTION, AND TO SHOW MEASUREMENT UNITS FOR EACH QUANTITY. 1. Give brief definitions for these terms: a) enthalpy change b) standard molar enthalpy of formation ( H f ) c) specific heat d) heat capacity 2. Underline the correct choice: a) If a reaction absorbs heat, the calorimeter and its contents will (increase, decrease) in temperature. b) If the calorimeter and its contents increased in temperature, the reaction must have (absorbed, released) heat. 3. A student mixed 50.0 ml of HCl (aq) and 75.0 ml of NaOH (aq) in a calorimeter. Each of the reactant solutions had a density of 1.04 g/ml and an initial temperature of 22.6 C. After they reacted, the mixed solution reached a maximum temperature of 26.3 C. Calculate the heat released or absorbed by the reaction process. Assume the product solution has a specific heat of 4.0 J/g C. 4. Is the reaction described in item 3 exothermic or endothermic? Explain your choice.

4 80 CHM151LL: GENERAL CHEMISTRY I LAB MANUAL 5. Hess s law of heat summation states that if you can conceive of a process as the sum of a set of steps, then the enthalpy change of the overall process is equal to the sum of the enthalpy changes of the individual steps. Modify reactions (2), (3), and (4) as needed so that the modified reactions add up to give reaction (1), the target reaction. Mg (s) + 1 / 2 O 2 (g) MgO (s) (1) H 2 (g) + 1 / 2 O 2 (g) H 2 O (l) (2) Mg (s) + 2 HCl (aq) MgCl 2 (aq) + H 2 (g) (3) MgO (s) + 2 HCl (aq) MgCl 2 (aq) + H 2 O (l) (4)

5 THERMOCHEMISTRY I 81 Name Partner Lab Report I Data Sheet for Citric Acid + Baking Soda Calorimetry RECORD TEMPERATURES TO THE THERMOMETER S FULL AVAILABLE PRECISION. time (s) T ( C) time (s) T ( C) time (s) T ( C) Minimum or maximum temperature, T f ( C) Initial temperature, T i ( C) Temperature change, T ( C) = T f T i 1. Is the reaction you studied exothermic or endothermic? Explain your reasoning. 2. Would H rxn have a positive or a negative sign? Explain your choice.

6 82 CHM151LL: GENERAL CHEMISTRY I LAB MANUAL