Determination of Enthalpy

Published: 2020-07-16 02:50:06
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Determining the Enthalpy of a Chemical Reaction All chemical reactions involve an exchange of heat energy; therefore, it is tempting to plan to follow a reaction by measuring the enthalpy change (? H). However, it is often not possible to directly measure the heat energy change of the reactants and products (the system). We can measure the heat change that occurs in the surroundings 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. = Cp ( m ( ? T The term q represents the heat energy that is gained or lost. Cp is the specific heat of water, m is the mass of water, and ? T is the temperature change of the reaction mixture. The specific heat and mass of water are used because water will either gain or lose heat energy in a reaction that occurs in aqueous solution. Furthermore, according to a principle known as Hess’s law, the enthalpy changes of a series of reactions can be combined 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, ? H 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 nthalpy change with the experimental results. [pic] Figure 1 MATERIALS |LabQuest |2. 0 M hydrochloric acid, HCl, solution | |LabQuest App |2. 0 M sodium hydroxide, NaOH, solution | |Temperature Probe |2. 0 M ammonium chloride, NH4Cl, solution | |glass stirring rod |2. M ammonium hydroxide, NH4OH, solution | |Styrofoam cup |ring stand | |250 mL or 400 mL beaker |utility clamp | |50 mL or 100 mL graduated cylinders |fume hood | PRE-LAB EXERCISE You will conduct the following three reactions in this experiment.
In the space provided below, write the balanced net ionic reaction equations from the descriptions. Use the table of thermodynamic data in your text (or another approved resource) to calculate the molar enthalpy 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 |? H (kJ/mol) | |1 | | | |2 | | | |3 | | | 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 the Temperature Probe to LabQuest and choose New from the File menu. If you have an older sensor that does not auto-ID, manually set up the sensor. 3. Use a utility clamp to suspend the Temperature Probe from a ring stand as shown in Figure 1. Part I Conduct the Reaction Between Solutions of NaOH and HCl 4. Nest a Styrofoam cup in a beaker, as shown in Figure 1. Measure out 50. 0 mL of 2. 0 M HCl solution into the foam cup. Lower the Temperature Probe into the 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. 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. Start data collection and obtain the initial temperature of the HCl solution. a. 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. Use a glass stirring rod to stir the reaction mixture continuously until the temperature reaches a peak and begins to fall. b. Data collection will end after three minutes. If the temperature readings are no longer changing, you may stop the trial early. . To examine the data pairs on the displayed graph, select any data point. As you move the examine line, the temperature values of each data point are displayed to the right of the graph. d. 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. Part II Conduct the Reaction Between Solutions of NaOH and NH4Cl 8. Measure out 50. 0 mL of 2. 0 M 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 2. M NH4Cl solution, but do not add it to the NaOH solution yet. 10. Conduct the reaction. a. Start data collection. e. 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. Use a glass stirring rod to stir the reaction mixture continuously until the temperature reaches a peak and begins to fall. f. Data collection will end after three minutes. If the temperature readings are no longer changing, you may stop the trial early. g. Examine the graph as before to determine and record the initial and maximum temperatures of the reaction. 1. Rinse and dry the Temperature Probe, Styrofoam cup, and the stirring rod. Dispose of the solution as directed. Part III Conduct the Reaction Between Solutions of HCl and NH3 12. Measure out 50. 0 mL of 2. 0 M HCl solution into a nested Styrofoam cup (see Figure 1). Lower the tip of the Temperature Probe into the cup of HCl solution. 13. Measure out 50. 0 mL of 2. 0 M NH3 solution, but do not add it to the HCl solution yet. 14. Conduct this reaction in a fume hood or in a well-ventilated area. Repeat Step 10 to conduct the reaction and collect temperature data. DATA TABLE |Reaction 1 |Reaction 2 |Reaction 3 | |Maximum temperature (°C) | | | | |Initial temperature (°C) | | | | |Temperature change (? T) | | | | DATA ANALYSIS 1. Calculate the amount of heat energy, q, produced in each reaction.
Use 1. 03 g/mL for the density of all solutions. Use the specific heat of water, 4. 18 J/(g•°C), for all solutions. 2. Calculate the enthalpy change, ? H, for each reaction in terms of kJ/mol of each reactant. 3. Use your answers from 2 above and Hess’s law to determine the experimental molar enthalpy for Reaction 3. 4. Use Hess’s law, and the accepted values of ? H in the Pre-Lab Exercise to calculate the ? H for Reaction 3. How does the accepted value compare to your experimental value? 5. Does this experimental process support Hess’s law? Suggest ways of improving your results.

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