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Unformatted text preview: Determining the Enthalpy of a Chemical Reaction Determining the Enthalpy of a Chemical Reaction All chemical reactions involve an exchange of heat energy. While it would be useful to follow a reaction by measuring the enthalpy change (∆H), it is not possible to measure the heat energy change of the reactants and products (the system) directly. We can however determine the heat change that occurs in the surroundings indirectly by monitoring temperature changes. If we conduct a reaction between two substances in aqueous solution, then the enthalpy of the reaction can be indirectly calculated with the following equation. q = Cp m ∆T The term q represents the heat energy that is gained or lost. Cp is the specific heat of the solution, m is the mass of the solution, and ∆T is the temperature change of the solution. The energy change that occurs in the liquid containing the reaction (the surroundings) is equal in magnitude and opposite in sign to the energy change that occurs during the reaction itself (the system). Furthermore, according to a principle known as Hess’s Law of Heat Summation, the enthalpy changes of a series of reactions are added to calculate the enthalpy change of a reaction that is the sum of the components of the series. In this experiment, you will measure the temperature change of two reactions, and use Hess’s Law to determine the enthalpy change, ΔH of a third reaction. You will use a Styrofoam cup nested in a beaker as a calorimeter, as shown in Figure 1. For purposes of this experiment, you may assume that the heat loss to the calorimeter and the surrounding air is negligible. OBJECTIVES In this experiment, you will Use Hess’s law to determine the enthalpy change of the reaction between aqueous ammonia and aqueous hydrochloric acid. Compare your calculated enthalpy change with the experimental results and the accepted value. Figure 1 JSM 7/27/2011 Advanced Chemistry with Vernier 13 - 1 Determining the Enthalpy of a Chemical Reaction MATERIALS Vernier computer interface computer Temperature Probe Styrofoam cup 250 mL beaker 50 mL or 100 mL graduated cylinders glass stirring rod 2.00 M hydrochloric acid, HCl, solution 2.00 M sodium hydroxide, NaOH, solution 2.00 M ammonium chloride, NH4Cl, solution 2.00 M ammonium hydroxide, NH4OH, solution ring stand utility clamp fume hood PRE-LAB QUESTIONS 1. You will investigate three reactions in this experiment. In the space provided below, write the balanced molecular and net ionic equations from the descriptions of the reactions. Use the table of standard heats of formation in your text (or another approved resource) to calculate the molar enthalpies of the reactions. Reaction 1: An aqueous solution of sodium hydroxide reacts with an aqueous solution of hydrochloric acid, yielding water. Reaction 2: An aqueous solution of sodium hydroxide reacts with an aqueous solution of ammonium chloride, yielding aqueous ammonia, NH3, and water. Reaction 3: An aqueous solution of hydrochloric acid reacts with aqueous ammonia, NH3, yielding aqueous ammonium chloride. Reaction Balanced reaction equation ΔH (kJ) 1 2 3 2. For each reaction you perform, you will mix together 50.0 mL of 2.00 M acid and 2.00 M base. What are the resulting concentrations of the reactants upon mixing? 3. How many moles of each reactant are present in each trial? 4. What is the total volume of the solutions upon mixing? Assuming a density of 1.03 g/mL for the solution, find the total mass of each reaction mixture. JSM 7/27/2011 Advanced Chemistry with Vernier 13 - 2 Determining the Enthalpy of a Chemical Reaction PROCEDURE 1. Obtain and wear goggles. It is best to conduct this experiment in a fume hood, or in a well-ventilated room. 2. Connect a Temperature Probe to Channel 1 of the Vernier computer interface. Connect the interface to the computer with the proper cable. Use a utility clamp to suspend the Temperature Probe from a ring stand, as shown in Figure 1. 3. Start the Logger Pro program on your computer. Open the file “13 Enthalpy” from the Advanced Chemistry with Vernier folder. The Reaction Between Solutions of NaOH and HCl (Part I) 4. Nest a Styrofoam cup in a beaker (see Figure 1). Measure 50.0 mL of HCl solution into the cup. Lower the tip of the Temperature Probe into the HCl solution. CAUTION: Handle the hydrochloric acid with care. It can cause painful burns if it comes in contact with the skin. 5. Measure out 50.0 mL of NaOH solution, but do not add it to the HCl solution yet. CAUTION: Handle the sodium hydroxide solution with care. 6. Conduct the reaction. a. Click to begin the data collection and obtain the initial temperature of the HCl solution. b. After three or four readings have been recorded at the same temperature, add the 50.0 mL of NaOH solution to the Styrofoam cup all at once. Stir the mixture throughout the reaction. c. Data collection will end after three minutes. If the temperature readings are no longer changing, you may terminate the trial early by clicking . d. Click the Statistics button, . The minimum and maximum temperatures are listed in the statistics box on the graph. If the lowest temperature is not a suitable initial temperature, examine the graph and determine the initial temperature. e. Record the initial and maximum temperatures in your data table. 7. Rinse and dry the Temperature Probe, Styrofoam cup, and the stirring rod. Dispose of the solution as directed. The Reaction Between Solutions of NaOH and NH4Cl (Part II) 8. Measure out 50.0 mL of NaOH solution into a nested Styrofoam cup (see Figure 1). Lower the tip of the Temperature Probe into the cup of NaOH solution. 9. Measure out 50.0 mL of NH4Cl solution, but do not add it to the NaOH solution yet. JSM 7/27/2011 Advanced Chemistry with Vernier 13 - 3 Determining the Enthalpy of a Chemical Reaction 10. Conduct the reaction. a. Click to begin the data collection. b. After three or four readings have been recorded at the same temperature, add the 50.0 mL of NH4Cl solution to the Styrofoam cup all at once. Stir the mixture throughout the reaction. c. Data collection will end after three minutes. If the temperature readings are no longer changing, you may terminate the trial early by clicking . d. Examine the graph as before to determine and record the initial and maximum temperatures of the reaction. 11. Rinse and dry the Temperature Probe, Styrofoam cup, and the stirring rod. Dispose of the solution as directed. 12. Perform a second trial of the reaction of NaOH + NH4Cl. Conduct the Reaction Between Solutions of HCl and NH3 (Part III) 13. Measure out 50.0 mL of HCl solution into a nested Styrofoam cup (see Figure 1). Lower the tip of the Temperature Probe into the cup of HCl solution. 14. Measure out 50.0 mL of NH3 solution, but do not add it to the HCl solution yet. 15. Conduct this reaction in a fume hood or in a well-ventilated area. Repeat Step 10 to conduct the reaction and collect temperature data. ANALYSIS 1. Calculate the energy, q, absorbed by the solution (“surroundings”) for each reaction. Use the solution mass calculated in PreLab question 3. Use the specific heat of water, 4.18 J/(g•°C), for all solutions. Relate this value to the energy released for each of the reactions. 2. Calculate the enthalpy change, ∆H, for each reaction in kJ (per mol of reactant). Calculate the average enthalpy change for the two trials of the reaction of NH4OH + HCl. Compare the results to the accepted values you calculated in the Pre-Lab exercise. 3. Use Hess’s Law and your answers from question 2 for the reaction of NaOH + HCl and the reaction of NaOH + NH4Cl to calculate an enthalpy change for reaction of HCl + NH3. How does this value compare to the experimental enthalpy change for the reaction of HCl + NH3 found in question 2? 4. How did the value you found using Hess’s law for the reaction of HCl + NH3 in question 3 compare to the accepted value for the ∆H for this reaction calculated in the in the Pre-Lab exercise? 5. Do your results support Hess’s law? Discuss possible sources of error and suggest ways to improve your results. JSM 7/27/2011 Advanced Chemistry with Vernier 13 - 4 Determining the Enthalpy of a Chemical Reaction DATA TABLE Table 1: Heats of Reaction Reaction NaOH + HCl NaOH + NH4Cl Trial 1 HCl + NH3 Trial 2 Maximum temperature (°C) Initial temperature (°C) Temperature a change (°C) qsurroundings (J) qreaction (J) b c ∆Hrxn(kJ) d [per mole] Average ∆Hrxn(kJ) e [per mole] Using Hess’s Law f Accepted Values ∆Hrxn(kJ) * (per mole) a, b, c, d, e, f * Refer to the appendix for these calculated values in your lab report Values calculated from standard heats of formation (cite the reference in your lab report) JSM 7/27/2011 Advanced Chemistry with Vernier 13 - 5 ...
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