HEATS OF REACTION EXPERIMENT
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1 16 Text Reference Section 11.4 HEATS OF REACTION EXPERIMENT PURPOSE To measure the heats of reaction for three related exothermic reactions and to verify Hess s law of heat summation. Time Required 50 minutes Objectives Measure the heat changes for three different exothermic reactions carried out at constant pressure. Demonstrate that heats of reaction are additive. Three related exothermic reactions will be carried out and the heats of reaction determined. Two of the reactions add up algebraically to give the third. Students will verify that the heats of reaction are likewise additive. This is an example of Hess s law. Advance Preparation 0.5M hydrochloric acid Pour 84 ml of concentrated HCl into 1800 ml of distilled water and dilute to 2 L. 1.0M hydrochloric acid Pour 84 ml of concentrated HCl into 900 ml of distilled water and dilute to 1 L. 1.0M sodium hydroxide Dissolve 40.0 g of NaOH in 1 L of distilled water. Add the NaOH slowly, stirring. Review the safety procedures and caution the students about the corrosive nature of the chemicals before they begin to work. BACKGROUND Energy changes occur in all chemical reactions; energy is either absorbed or released. If energy is released in the form of heat, the reaction is called exothermic. If energy is absorbed, the reaction is called endothermic. In this experiment, you will measure the amounts of heat released in these three related exothermic reactions: (1) NaOH(s) y H 10.6 kcal/mol Na (aq) OH (aq) x 1 kcal (2) NaOH(s) H (aq) Cl (aq) y H 23.9 kcal/mol H 2 O Na (aq) Cl (aq) x 2 kcal (3) Na (aq) OH (aq) H (aq) Cl (aq) y H 13.3 kcal/mol H 2 O Na (aq) Cl (aq) x 3 kcal After determining the heats of reaction, you will analyze your data and verify the additive nature of heats of reaction. MATERIALS (PER PAIR) safety goggles sodium hydroxide pellets, 1 plastic-foam cup NaOH ct mL graduated cylinder 1.0M sodium hydroxide, mL beaker NaOH ct 1 50-mL beaker 0.5M hydrochloric acid, 1 thermometer HCl ti centigram balance 1.0M hydrochloric acid, HCl ti 1 spatula distilled water SAFETY FIRST! In this lab, observe all precautions, especially the ones listed below. If you see a safety icon beside a step in the procedure, refer to the list below for its meaning. Caution: Wear your safety goggles. (All steps.) Caution: 0.5M and 1.0M hydrochloric acid are eye and skin irritants. (Steps 4, 7.) Heats of Reaction 123
2 NaOH readily absorbs moisture from the air. Such a substance is said to be hygroscopic. In fact, NaOH is so hygroscopic that it can absorb enough water from moist air to completely dissolve in the absorbed water. If this happens, the NaOH will look just like water on the laboratory bench. Such very hygroscopic substances are said to be deliquescent. Caution: Sodium hydroxide is a very corrosive material that can cause severe skin burns and permanent eye damage. Under no circumstances, handle solid sodium hydroxide with your fingers. Sodium hydroxide pellets absorb water from the air and can eventually be mistaken for a puddle of water. Never leave the sodium hydroxide container uncovered; never leave pellets on the balance or on the laboratory bench. (Steps 2, 5, 7.) Caution: Mercury is extremely toxic, and mercury spills are very difficult to clean up. If you should break a mercury thermometer, immediately report the incident to the teacher. Never use a thermometer as a stirrer. (Steps 3, 8.) Note: Return or dispose of all masterials according to the instructions of your teacher (Steps 3, 6, 9.) PROCEDURE As you perform the experiment, record your data in Data Table 1. Procedure note: After each reaction, dispose of the solution as directed by your teacher and rinse the cup and thermometer with water. Reaction 1 Reaction 2 1. Measure 100 ml of distilled water into a plastic-foam cup. Place the cup inside a 400-mL beaker for support. This assembly, together with a thermometer, will serve as your calorimeter. 2. Measure and record the mass of a 50-mL beaker to the nearest 0.01 g. CAUTION: NaOH is extremely corrosive. Using a spatula, add as close to 2.00 g as possible of sodium hydroxide pellets to the beaker. Measure and record the combined mass of the beaker and sodium hydroxide to the nearest 0.01 g. (Do this operation as quickly as possible to avoid error due to absorption of water by the NaOH.) 3. Measure and record the temperature of the water in the foam cup to the nearest 0.5 C. Add the NaOH pellets to the water in the calorimeter. Stir the mixture gently with the thermometer until all the solid has dissolved. CAUTION: Hold the thermometer with your hand at all times. Record the highest temperature reached during the reaction. 4. CAUTION: Low-concentration hydrochloric acid can irritate your skin. Measure 100 ml of 0.5M HCl into the plastic-form cup and place the cup inside a 400-mL beaker. 5. Using a spatula, measure out 2.00 g of solid NaOH pellets. CAUTION: NaOH is extremely corrosive. 124 Experiment 16
3 Reaction 3 6. Measure and record the temperature of the HCl solution in the foam cup. Add the NaOH pellets to the acid solution and stir gently until the solid is dissolved. Measure and record the highest temperature reached by the solution during the reaction. 7. Place the plastic-foam cup inside a 400-mL beaker. Measure 50 ml of 1.0M HCl into the cup. Rinse the graduated cylinder and fill with 50 ml of 1.0M NaOH. 8. Measure and record the temperature of the HCl solution (in the cup) and the NaOH solution (in the cylinder) to the nearest 0.5 C. Rinse the thermometer between measurements. Step 9. Advise students to make certain that the solutions are stirred sufficiently to ensure complete mixing. Warn them that it is easy to poke a hole in the cup with the thermometer, resulting in lost data and a chemical spill. 9. Pour the NaOH solution into the foam cup. Stirring the mixture gently, measure and record the highest temperature reached. Use the following disposal method for chemical waste. Disposal 3: All solutions in reactions 1, 2, and 3. Heats of Reaction 125
4 OBSERVATIONS DATA TABLE 1: EXPERIMENTAL DATA Parameter Reaction 1 Reaction 2 Parameter Reaction 3 mass (beaker) 0.41 g 0.44 g initial temperature (HCl solution) 21.5 C mass (beaker NaOH) 2.41 g 2.44 g initial temperature (NaOH solution) 22.5 C mass (NaOH) 2.00 g 2.00 g average initial temperature 22.0 C initial temperature 22.0 C 22.5 C final temperature 28.0 C final temperature 27.0 C 34.5 C ANALYSES AND CONCLUSIONS 1. Determine the change in temperature, T, for each reaction. Record your results in Data Table Calculate the mass of the reaction mixture in each reaction. (To do this, first determine the total volume of the solution. Then calculate the mass of the solution, based on the assumption that the density of the solution is the same as that of pure water, 1.0 g/ml.) Record your results in Data Table 2. rxn g (H 2 O) 2.00 g (NaOH) 102 g sol. rxn g (HCl) 2.00 g (NaOH) 102 g sol. rxn 3 50 g (HCl) 50 g (NaOH) 100 g sol. 3. Calculate the total heat released in each reaction, assuming that the specific heat of the solution is the same as that of pure water, 1.0 cal/(g C). Remember: heat of reaction specific heat T mass Record your results in Data Table 2. rxn 1 rxn 2 rxn 3 cal heat C 102 g g C cal cal heat C 102 g g C cal cal heat C 100 g g C cal Heats of Reaction 127
5 4. Calculate the number of moles of NaOH used in reactions 1 and 2, and record the results in Data Table 2. number of moles (NaOH) 2.00 g 40.0 g/mol mol 5. In reaction 3, the number of moles of NaOH can be calculated from the concentration of the solution (1.0M, or 1.0 mole of NaOH per liter of solution) and the volume used. The calculation is shown below. 1 mol NaOH 50.0 ml NaOH mol NaOH ml NaOH Enter this result in Data Table 2. mass (NaOH) gram formula weight (NaOH) 6. Calculate the energy released per mole of NaOH for each reaction and enter your results in Data Table 2. rxn cal cal/mol NaOH mol 10 kcal/mol NaOH rxn cal cal/mol NaOH mol 24 kcal/mol NaOH rxn cal cal/mol NaOH mol 12 kcal/mol NaOH DATA TABLE 2: RESULTS OF CALCULATIONS Rxn Number Mass of Rxn Mixture T Total Heat Released mol NaOH Consumed Heat Released per mol NaOH g 5.0 C cal mol cal or 10 kcal g 12.0 C cal mol g 6.0 C cal mol cal or 12 kcal 7. Show that the equations for reactions 1 and 3, which are given in the Background section, add up to give the equation for reaction 2. Include the energy released per mole of NaOH in each equation. NaOH(s) y Na (aq) OH (aq) 10 kcal Na (aq) OH (aq) H (aq) Cl (aq) y H 2 O Na (aq) Cl (aq) 12 kcal cal or 24 kcal NaOH(s) H (aq) Cl (aq) y H 2 O Na (aq) Cl (aq) 22 kcal 128 Experiment 16
6 8. Examine all the class data. Does the sum of the energy released per mole of NaOH for reactions 1 and 3 equal the energy released per mole of NaOH in reaction 2? What factors might account for any difference? The experimental results are close. Possible reasons for the discrepancy include the following: 1. loss of heat from the calorimeter 2. use of incorrect masses due to the assumptions made concerning the densities of the solutions 3. inability to obtain an accurate measurement of the mass of NaOH(s) 4. inaccuracy of volume measurements made with a graduated cylinder 5. the heat capacity of the solutions is not equal to 1.0 cal/(g C) 9. Calculate the percent difference between the heat given off in reaction 2 and the sum of the heats given off in reactions 1 and 3. Assume that the heat given off in reaction 2 is correct. heat 2 (heat 1 heat 3 ) 100% heat2 24 kcal percent difference (10 kcal 12 kcal) 100% 24 kcal 2 kcal 100% 24 kcal 2 100% % 8% 10. Would changing the amount of NaOH used in reaction 1 affect the value obtained for the energy given off per mole of NaOH? Explain. An increase in the mass of NaOH dissolved in water would increase the amount of heat energy evolved. It would not change the amount of heat energy evolved per mole of NaOH. percent difference (in evolved heat) 11. State in your own words what is meant by the additive nature of heats of reaction. If the equations for two chemical reactions can be algebraically added together to form the equation for a third chemical reaction, then the heat of reaction for the third reaction is the sum for the heats of reaction for the first two equations. Heats of Reaction 129
7 12. Taking into account your answer to problem 4, explain why you were asked to use exactly 2.00 g of NaOH in reactions 1 and 2, and an equivalent number of moles of NaOH in reaction 3. For the heats of reactions to be additive, the same number of moles of reactant must be used in each reaction. GOING FURTHER Develop a Hypothesis Suppose nitric acid (HNO 3 ) were substituted for HCl in reactions 2 and 3. Propose a hypothesis about how the heat change for Reaction 2 would be affected. Nitric acid and hydrochloric acid are both strong acids. Each is completely dissociated in solution. Because the anion of the acid is a spectator ion in this reaction, no change is expected. Design an Experiment Propose an experiment to test your hypothesis. If resources are available and you have your teacher s permission, perform the experiment. Experimental design should be identical to the one used in this experiment. 130 Experiment 16
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