Idea Transcript
EXPERIMENT 8
ENTHALPY CHANGES rev 5/11 GOAL In this experiment, you will use a simple calorimeter to investigate changes in enthalpy for related reactions and solutions. INTRODUCTION The heat absorbed or released during a chemical reaction is equal to the enthalpy change (∆H) for the reaction, at constant pressure. Calorimetry is the measurement of heat absorbed or released during chemical and physical processes. In the Preliminary Activity, you will gain experience using a Temperature Probe and a calorimeter as you determine the enthalpy change as a hydrochloric acid solution is neutralized by a solution of sodium hydroxide. 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 lost to the calorimeter and the surrounding air is negligible. Hydrochloric acid will be the limiting reactant in this experiment, and you will accordingly be determining the enthalpy, ΔH, of neutralization of the acid. You will calculate this value by dividing the enthalpy change of the reaction by the moles of the limiting reagent. Hydrogen ions from the HCl react with hydroxide ions from the NaOH in a one-to-one ratio to produce water in the overall reaction: H+(aq) + Cl–(aq) + Na+(aq) +OH–(aq) H2O(l) + Na+(aq) + Cl–(aq) Because this reaction is a neutralization (acid + base), we can refer to the enthalpy change as either an enthalpy of reaction or an enthalpy of neutralization. All reactions will have accompanying enthalpy changes. Some will be endothermic and some will be exothermic. Physical changes also have accompanying enthalpy changes. We often focus on melting (enthalpy of fusion) or boiling (enthalpy of vaporization), but other processes are also interesting. For example, when a substance dissolves in a solvent, we experience the enthalpy of solution. When AlCl3.6H2O(s) is added to water, it dissolves in releases some heat. This heat is the enthalpy of solution. AlCl3.6H2O(s) Al3+ (aq) + 3 Cl-(aq) + 6 H2O(l) After completing the Preliminary Activity, you and your partner will investigate two researchable questions. One of your investigations will explore enthalpy changes in a set of related reactions. The second will explore solubility.
HAZARDS The acids and bases used in this experiment are corrosive. If you get any on your skin, rinse it immediately and thoroughly. Used solutions from this experiment may be flushed down the drain with excess water since they are all harmless salts or neutralized acid/base mixtures.
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PRELIMINARY ACTIVITY 1. If it has not already been done for you, connect the Temperature Probe to the data-collection interface. 2. Place a Styrofoam cup into a 400 mL beaker as shown in Figure 1. Measure out 50.0 mL of ~1.00 M HCl into the Styrofoam cup. Record the precise concentration of the HCl solution. CAUTION: Handle the hydrochloric acid with care. It can cause painful burns if it comes in contact with the skin. 3. Use a utility clamp to suspend a Temperature Probe from a ring stand as shown in Figure 1. Lower the Temperature Probe into the solution. Note: It may take up to 15 seconds for the Temperature Probe to equilibrate at the temperature of the HCl solution.
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4. Measure out 50.0 mL of 1.05 M NaOH solution. CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing. 5. Start data collection. After 3–4 readings at the same temperature have been plotted, add the 50.0 mL of NaOH solution to the foam cup all at once. Stir the reaction mixture gently. 6. Data collection will stop after 150 seconds. 7. Use the Statistics function (STAT button) to display the initial and maximum temperature readings during the reaction. Record these values. If the minimum temperature is not a suitable initial temperature, examine the graph and determine the initial temperature. 8. Rinse and dry the Temperature Probe, Styrofoam cup, and stirring rod. Flush the used solution down the drain with excess water.
Discussion: Return to the prelab side of the room with your lab partner. Working as a pair, answer these questions in your lab notebook. Consult the Introduction for help. 1. Subtract the initial temperature from the final temperature to determine the temperature change, t, for the process. 2. Determine the total mass of the product solution (assume its density is 1.00 g/mL). 3. Use the equation below to calculate q, the amount of heat energy change observed for the solution. Cp = 4.18 J/g°C. q = Cp•m•t 4. Find H for the reaction (H = –q because heat lost by the reaction is gained by the solution). 5. Record in your notebook the volume and concentration of NaOH and HCl that you used above. Explain how you can tell just by looking at the numbers that HCl is the limiting reagent in our reaction. 6. Calculate moles of HCl used in the reaction. (because HCl is the limiting reagent) 7. Use the results of the Step 4 and Step 5 calculations to determine H/mol HCl (that is H/mol limiting reagent). 8. For Investigation 1, you will explore a Researchable Question that compares two or more related reactions. Consult the list of available materials below. List two possible Researchable Questions. 9. For Investigation 2, you will explore a Researchable Question that compares enthalpy of solution for two or more compounds. Consult the list of available materials below. List two possible Researchable Questions. 2
10. Participate in the discussion led by your instructor. As directed by the instructor, choose Researchable Questions from the list generated by the class for each Investigation. Available Materials for Investigation 1: Mg(s) (a metal that will react with acids) Fe(s) (a metal that will react with acids) 1.05 M NaOH(aq) (a base) 1.05 M KOH(aq) (a base) 1.0 M H3PO4(aq) (a moderately strong acid; note 3 moles H+) 1.0 M HC2H3O2(aq) (a weak acid) 1.0 M H3BO3(aq) (a very weak acid; note 3 moles H+) 1.0 M HCl(aq) (a strong acid) Available Materials for Investigation 2: CaCl2 (anhydrous, s) CaCl2.2H2O(s) NH4Cl(s) NH4NO3(s) NaCl(s) NaNO3(s) CuSO4(anhydrous, s) CuSO4.5H2O (s)
INVESTIGATIONS 1 AND 2 1. Prepare separate Research Plans in your notebook for your two investigations. The sections should be a. Researchable Question b. Variables (Specify controls that are held constant, independent variables that you vary, and dependent variables that change in response. You will want 2 or 3 variations to explore your Researchable Question.) c. Materials List (just the chemicals) d. Procedure (Include the balanced equations for your reaction. For Investigation 1, calculate the limiting reagent before proceeding. You may not use more than 60 mL of any solution for a single trial. It is fine to use less. For Investigation 2, think about reasonable amounts of solid and solvent.) e. Data Table f. Calculated Results (an outline of how you will use your data)
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2. Get your plans approved by the instructor, make any needed changes, and then conduct your investigations. 3. After you complete all work, clean up, return to the prelab side of the room and complete two Investigation Report forms (available at the Instructor’s bench). 4. Complete needed calculations to find your results. 5. Prepare to share the key points of each orally with the rest of the class. 6. During the class discussion, you will be asked to briefly share your investigations with the rest of the class. You may modify or expand upon your conclusions based upon class discussion. Take notes on the investigations of others in your lab notebook since the lab report will include questions on the work of other groups. (All the Investigation Report forms will be available for viewing on Inquire the day after you complete this experiment.)
FOR YOUR LAB REPORT RESULTS AND DISCUSSION For Investigation 1, write several paragraphs that describe your investigation, including a summary of each of the points in your research plan. Include a clean table that clearly shows your data and calculated results, including units. Now describe what you can conclude from your investigation. Make sure that you refer to your data as the basis for your conclusions. If your data is inconclusive or if you had experimental difficulties, describe these. Now prepare a similar discussion of your Investigation 2. QUESTIONS 1. Write several paragraphs, summarizing things your class collectively learned from the investigations. Use examples from this experiment to illustrate your points. Your writing must be clear, grammatically correct, and chemically correct. Consult the Investigation Report Forms on Inquire to find the information you need. Do not choose your own investigation or one exactly like yours. 2. Use the Investigation Report Forms to address the following points. a. Find an Investigation 1 that you found interesting. Describe the question being investigated, the procedure used, the data collected, the results calculated, and what you can conclude from it. b. Find an Investigation 2 that you found interesting. Describe the question being investigated, the procedure used, the data collected, the results calculated, and what you can conclude from it.
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