Lab

T H E R609 modular • laboratory • program • publisher: H. A. Neidig in • chemistry editor: M. L. Gillette Estimating a Heat of Neutralization prepared by M. L. Gillette, Indiana University Kokomo, H. A. Neidig, Lebanon Valley College, and J. N. Spencer, Franklin and Marshall College Purpose of the Experiment Estimate the heat of neutralization for the reaction of sodium hydroxide with hydrochloric acid. Background Required You should be familiar with basic laboratory techniques for measuring the volume and temperature of solutions, with the concepts associated with stoichiometry and thermochemistry, and with graphing procedures. Background Information Consider the neutralization of hydrochloric acid (HCl) with sodium hydroxide (NaOH) solutions as shown in Equation 1. HCl (aq) + NaOH (aq) → H2O ( l ) + NaCl (aq) (Eq. 1) Neutralization reactions are exothermic; that is, energy is released in the form of heat. The heat of neutralization (∆Hneut) is the heat transferred when one mole of acid reacts with one mole of base. This heat is generally reported in either kilojoules per mole (kJ/mol) or kilocalories per mole (kcal/mol). Example Problem Determine the ∆Hneut for the reaction of nitric acid (HNO3) with NaOH solution. HNO3 (aq) + NaOH (aq) → NaNO3 (aq) + H2O ( l ) (Eq. 2) Before mixing 50 mL of 1.0M HNO3 and 51 mL of 1.0M NaOH, the temperature of the reactants was 23.1 °C. After mixing, the temperature of the reaction mixture was 29.4 °C. The specific heat capacity of the reaction mixture is 3.89 J/g • °C and the density is 1.04 g/mL. Solution (1) Calculate the temperature change for the reaction. ∆T = Tfinal – Tinitial = 29.4 °C – 23.1 °C = 6.3 °C (Eq. 3) Copyright © 1999 by Chemical Education Resources, Inc., P.O. Box 357, 220 S. Railroad, Palmyra, Pennsylvania 17078 No part of this laboratory program may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Printed in the United States of America 01 00 99 — 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 2 THER 609/Estimating a Heat of Neutralization (2) Calculate the amount of heat released by the reaction. mol 1000 mL HNO3 was the limiting reagent. Always use caution in the laboratory. skin.6 × 103 J (3) From the volumes and molarities of the reactants. ask your laboratory instructor for assistance.   amount of energy released. (4) Finally. 0 × 10 mol of HNO3 . 2 × 104 J / mol = –   –2  5.0 × 10 mol In This Experiment You will determine ∆Hneut for the reaction of HCl with NaOH. Many chemicals are potentially harmful. 0 mol NaOH / L ) = 5 . 6)  2 . mol  added. • Record all of your data on your Data and Observations sheet. ° C ) (Eq. J ∆Hneut . 5)  1L  number of moles –2 = ( 50 mL )   ( 1. calculate the number of moles of HNO3 and of NaOH you mixed. . Prevent contact with your eyes. mol 1000 mL  1L  number of moles –2 = ( 51mL )   ( 1. 89 J / g • ° C ) ( 6. 04 g  = ( 101mL )   ( 3 .1 × 10 mol of NaOH. J / g • ° C   1. © 1999 Chemical Education Resources Caution: Wear departmentally approved safety goggles while doing this experiment. number of moles  volume of solution  1 L   molarity of  =     of acid or base. remembering that ∆Hneut is negative for exothermic reactions. mL  1000 mL solution. using Equation 5. calculate ∆Hneut using Equation 6. Procedure Note: • If you are not familiar with the proper technique for inserting a thermometer into a split rubber stopper. 4)     released. 6 × 103 J   = –5 .3 ° C )  mL  = 2. mL   mixture. mol (Eq. J mixture. amount of heat  volume of reaction density of reaction specific heat capacity =  ( ∆T . g / mL   of mixture. using Equation 4. J / mol = –    number of moles of acid or base neutralized. Avoid ingesting any of the reagents. and determine which reagent was limiting. • Dispose of your reaction mixtures and rinses according to your laboratory instructor’s directions. 0 mol HNO3 / L ) = 5 . mol / L (Eq. and clothing. Obtaining the Reactants Caution: 2M HCl is a corrosive. Following the procedure described in Step 4.0 mL of 2M HCl into the cylinder. II. Pour all of the base into a dry 150-mL beaker. Figure 1 Equipment for time–temperature study 4. . as shown in Figure 1. Suspend the thermometer by clamping the stopper to a ring stand. 5. making sure that you can read the temperature markings between 20 and 35 °C. Position the thermometer so that it does not touch the sides or bottom of the cup. Measure 50. suspend a second thermometer in the NaOH solution in the beaker. Positioning the Equipment © 1999 Chemical Education Resources 3.0 mL of 2M NaOH into the cylinder. Immerse the end of the thermometer in the HCl solution in the cup (see Figure 1). toxic solution that can cause skin irritation. 8-oz pressed polystyrene cup. Place a split rubber stopper on the upper end of a thermometer. Caution: 2M NaOH is a corrosive. toxic solution that can cause skin irritation. Label a dry 100-mL graduated cylinder “acid”.THER 609/Estimating a Heat of Neutralization 3 I. 1. Pour all of the acid into a dry. Label a dry 100-mL graduated cylinder “base”. 2. Measure 51. then at 1-min intervals for 10 min. Your laboratory instructor might ask you to use a clean. quickly pour the NaOH solution. Record all times and temperatures in the “determination 1” column of your Data and Observations sheet. Use the temperature at time 0.1 °C increments. read the temperature to the nearest 0. Measure temperature–time data for the reaction mixture 30 s after mixing. Continue to stir the mixture at a constant rate. IV.0 min as the initial entry for the NaOH solution. and estimate the temperature to the nearest 0. If you are working with a partner. Record your data in the “determination 2” column.THER 609/Estimating a Heat of Neutralization III. Mixing the Reactants and Making Post-Mixing Temperature Measurements 7. If the thermometers are calibrated in 0.05 °C. Take the initial reading of the HCl solution after 30 s. Making Pre-Mixing Temperature Measurements Note: If the thermometers are calibrated in 1 °C increments. following the directions of your laboratory instructor. with stirring. into the HCl solution in the polystyrene cup. dry polystyrene cup for each determination. Record all times and temperatures. Caution: Wash your hands thoroughly with soap or detergent before leaving the laboratory. Alternately read the temperatures of the two solutions every 30 s over a 5-min period. Discard the reaction mixture.1 °C. read the temperature to the nearest degree. 9. After 5 min. one of you can read the temperatures while the other records the data. Do a second determination following the procedure of Steps 1–8. © 1999 Chemical Education Resources 4 . 8.5 °C. 6. and estimate the temperature to the nearest 0. (b) Would your experimental ∆Hneut have been higher or lower if you had mixed the acid and base solutions in the beaker? Briefly explain.” What experimental evidence do you have to verify this statement? 2. One of the first statements in the Background Information is “Neutralization reactions are exothermic. (a) Speculate about why you were directed to mix the HCl and NaOH solutions in the polystyrene cup and not in the beaker. The theoretical ∆Hneut for the reaction of HCl with NaOH is –5. using Equation 7. 7) 3. that is. % =   ( 100% ) theoretical ∆Hneut   (Eq. 1. Calculate the percent error in your determination. THER 609/Estimating a Heat of Neutralization 5 .name partner section date Post-Laboratory Questions Use the spaces provided for the answers and additional paper if necessary. energy is released in the form of heat.59 x 104 J/mol.  theoretical ∆Hneut – experimental ∆Hneut  percent error. © 1999 Chemical Education Resources (c) Would your experimental ∆Hneut have been higher or lower if you had covered your cup with a polystyrene top in which holes were made for your thermometer and stirring rod? Briefly explain. 5. briefly explain how your experimental results would have been affected if you had based your ∆T determination on your recorded temperatures rather than on the extrapolation of your plotted data. (a) Write the complete ionic equations for each of these reactions. Referring to your temperature–time data and graph. HCl–NaOH: (c) Would you expect ∆Hneut for the HCl–NaOH reaction to be the same as that for the HNO3–NaOH reaction? Briefly explain why or why not. HCl–NaOH: HNO3–NaOH: (b) Write the net ionic equations for each of these reactions. Compare the theoretical ∆Hneut for the HCl–NaOH reaction with the calculated result for the HNO3–NaOH reaction in the Background Information.6 THER 609/Estimating a Heat of Neutralization 4. © 1999 Chemical Education Resources HNO3–NaOH: . 0 _________ _________ 15. mL _______________________ ______________________ molarity of HCl solution. mol/L _______________________ ______________________ Temperature–Time Data determination 1 temperature.0 mixture _________ 1.0 _________ _________ 7.0 _________ _________ 14. °C time.5 3.0 _________ _________ 12.0 _________ _________ 11.0 acid determination 2 temperature.5 4. mol/L _______________________ ______________________ volume of NaOH solution.5 1.0 _________ _________ 9.5 _________ _________ 6.0 _________ _________ 8.0 _________ _________ 10. mL _______________________ ______________________ molarity of NaOH solution.0 _________ 0. °C _________ _________ _________ _________ _________ _________ _________ © 1999 Chemical Education Resources mix 5. min base 0.5 acid mixture _________ _________ _________ _________ _________ _________ _________ _________ _________ 3.0 _________ _________ 13.name partner section date Data and Observations determination 1 determination 2 volume of HCl solution.0 4.0 _________ _________ THER 609/Estimating a Heat of Neutralization 7 .0 base _________ _________ 2.5 2. 1. Remember to include units with all calculated results. Extrapolate each of these lines to the line of mixing. Figure 2 Plot of typical temperature–time data Figure 3 Extrapolation of temperature–time data to find ∆T © 1999 Chemical Education Resources To determine ∆T. draw a vertical line of mixing (See Figure 3). Then draw the best straight line through the post-mixing data. . Plot the temperature vs elapsed time as shown in Figure 2. draw the best straight lines through the data points for each solution taken prior to mixing.8 THER 609/Estimating a Heat of Neutralization Calculations and Conclusions Show your calculations in the space provided. ignoring the first two or three post-mixing data points. Then. (Remember that ∆H is negative for exothermic reactions. using Equation 5.00M HCl. determination 1 _________________ determination 2 _________________ 3.0 mL of 2. Assume the final volume of the reaction mixture was 101.04 g/mL. using Equation 4. For each determination.0 mL of 2. calculate the amount of heat transferred when you mixed 51. calculate the number of moles of NaOH that reacted when you mixed 51.0 mL of 2.0 mL of 2. average ∆Hneut _________________ THER 609/Estimating a Heat of Neutralization 9 .0 mL of 2.name partner section date 2. For each determination. and its specific heat capacity was 3. Find the temperature change (∆T ) for each determination.) © 1999 Chemical Education Resources determination 1 _________________ determination 2 _________________ 7. determination 1 _________________mol HCl determination 2 _________________mol HCl _________________mol NaOH _________________mol NaOH 5. calculate the average ∆Hneut for the reaction of 1 mol of HCl and 1 mol of NaOH.0 mL of 2. For each determination. using Equation 6.89 J/g • °C.00M HCl with 51. calculate the number of moles of HCl and of NaOH that were present when you mixed 50. If you did two determinations. calculate ∆Hneut for the reaction of 1 mol of HCl and 1 mol of NaOH. For each determination.00M NaOH. determination 1 _________________ determination 2 _________________ 4. The average initial temperature (Tinitial) of the reactant solutions is the average of these solutions’ temperatures at the intersection of the extrapolated lines and the line of mixing. its density was 1.00M NaOH with 50.00M HCl. determination 1 _________________ determination 2 _________________ 6. The final temperature (Tfinal) is the intersection of the extrapolated post-mixing line and the line of mixing.00M NaOH and 50.0 mL. . partner section date © 1999 Chemical Education Resources name THER 609/Estimating a Heat of Neutralization 11 . . partner section date © 1999 Chemical Education Resources name THER 609/Estimating a Heat of Neutralization 13 . . 04 g/mL.89 J/g • °C and 1.0 mix 11. What hazards should you know about when you work with: (a) 2M NaOH? (b) 2M HCl? 2.0 mL of 1.0 mL of 1. min temperature. (a) How will you determine the initial temperature of your HCl and NaOH solutions (∆Tinitial) if the temperatures of these solutions are not exactly the same? (b) Thermometers respond relatively slowly. respectively.0 27.7 7.9 5.0 28.0 28.1 2. 4.9 10. Briefly explain how you will overcome this difficulty when you calculate ∆T in this experiment. a student mixed 30. Define the following terms as they apply to this experiment: (a) exothermic reaction (b) ∆Hneut 3.4 14.0 28.0 4. temperature.0 21.0 3.0 27.0 21. min THER 609/Estimating a Heat of Neutralization 15 .name partner section date Pre-Laboratory Assignment 1.00M NaOH and recorded the temperature–time data shown below.0 28.0 21.0 13.7 8. °C 0.0 27.3 © 1999 Chemical Education Resources time. Assuming the initial temperatures of the HCl and NaOH solutions were the same.2 12.0 28.0 27.00M HCl with 31. While performing an experiment similar to the one described in the Procedure.2 1.0 28.8 6.8 9. calculate ∆Hneut for the reaction. °C time.0 28. and the specific heat capacity of the reaction mixture and its density are 3. THER 609/Estimating a Heat of Neutralization ISBN 0-87540-609-2 © 1999 Chemical Education Resources 16 .