Lab

Estimating a Heat of Neutralizationprepared 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 withhydrochloric acid.

Background Required

You should be familiar with basic laboratory techniques for measuring the vol-ume and temperature of solutions, with the concepts associated withstoichiometry 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 ofheat. The heat of neutralization (Hneut) is the heat transferred when one moleof acid reacts with one mole of base. This heat is generally reported in either kilo-joules 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 ofthe reactants was 23.1 C. After mixing, the temperature of the reaction mixturewas 29.4 C. The specific heat capacity of the reaction mixture is 3.89 J/g C andthe 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)

T H E R

609m o d u l a r l a b o r a t o r y p r o g r a m i n c h e m i s t r ypublisher: H. A. Neidig editor: M. L. Gillette

Copyright 1999 by Chemical Education Resources, Inc., P.O. Box 357, 220 S. Railroad, Palmyra, Pennsylvania 17078No part of this laboratory program may be reproduced or transmitted in any form or by any means, electronic or mechanical, includingphotocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.Printed in theUnited States of America 01 00 99 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

(2) Calculate the amount of heat released by the reaction, using Equation 4.

amount of heatreleased, J

volume of reactionmixtur

=

e, mLdensity of reaction

mixture, g / mLs pecific heat capacityof mixture, J / g C ( ,T C ) (Eq. 4)

=

=

( ).

( .1011 04

3 89mLg

mLJ / g C ) (6.3 C )

2.6 10

3 J

(3) From the volumes and molarities of the reactants, calculate the number of moles ofHNO3 and of NaOH you mixed, using Equation 5, and determine which reagentwas limiting.

number of molesof acid or base, mol

volume of solu=

tionadded, mL

1 L

1000 mLmolarity of

solut

ion, mol / L

(Eq. 5)number of moles

of HNO molmL

L

1000 mL3 ,( ) ( .=

50

11 0 5 0 10 2mol HNO / L ) mol

number of molesof NaOH mol

3 = .

,

=

= ( ) ( . .

511

1 0 5 1 10 2mLL

1000 mLmol NaOH / L ) mol

HNO3 was the limiting reagent.

(4) Finally, calculate Hneut using Equation 6, remembering that Hneut is negative forexothermic reactions.

Hneut J / mol amount of energy released, J

number o, =

f moles of acid or base neutralized, mol

(Eq. 6)

=

=

. .

2 6 105 2 10

34J

5.0 10 molJ / mol

2

In This Experiment

You will determine Hneut for the reaction of HCl with NaOH.

Procedure

Note: If you are not familiar with the proper technique for inserting a ther-mometer into a split rubber stopper, ask your laboratory instructor forassistance.

Dispose of your reaction mixtures and rinses according to your lab-oratory instructors directions.

Record all of your data on your Data and Observations sheet.

2 THER 609/Estimating a Heat of Neutralization

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Caution: Wear departmentally approved safety goggles while doing this experiment.Always use caution in the laboratory. Many chemicals are potentially harmful. Prevent

contact with your eyes, skin, and clothing. Avoid ingesting any of the reagents.

I. Obtaining the Reactants

1. Label a dry 100-mL graduated cylinder acid. Measure 50.0 mL of 2M HClinto the cylinder. Pour all of the acid into a dry, 8-oz pressed polystyrene cup.

2. Label a dry 100-mL graduated cylinder base. Measure 51.0 mL of 2M NaOHinto the cylinder. Pour all of the base into a dry 150-mL beaker.

II. Positioning the Equipment

3. Place a split rubber stopper on the upper end of a thermometer. Suspend thethermometer by clamping the stopper to a ring stand, as shown in Figure 1.

4. Immerse the end of the thermometer in the HCl solution in the cup (see Fig-ure 1). Position the thermometer so that it does not touch the sides or bottom ofthe cup, making sure that you can read the temperature markings between 20and 35 C.

5. Following the procedure described in Step 4, suspend a second thermometerin the NaOH solution in the beaker.

THER 609/Estimating a Heat of Neutralization 3

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Caution: 2M HCl is a corrosive, toxic solution that can cause skin irritation.

Caution: 2M NaOH is a corrosive, toxic solution that can cause skin irritation.

Figure 1 Equipment for timetemperature study

III. Making Pre-Mixing Temperature Measurements

Note: If the thermometers are calibrated in 1 C increments, read the temperatureto the nearest degree, and estimate the temperature to the nearest 0.5 C.

If the thermometers are calibrated in 0.1 C increments, read the temperature tothe nearest 0.1 C, and estimate the temperature to the nearest 0.05 C.

6. Use the temperature at time 0.0 min as the initial entry for the NaOH solu-tion. Take the initial reading of the HCl solution after 30 s. Alternately read thetemperatures of the two solutions every 30 s over a 5-min period. Record alltimes and temperatures in the determination 1 column of your Data and Obser-vations sheet. If you are working with a partner, one of you can read the temper-atures while the other records the data.

IV. Mixing the Reactants and Making Post-MixingTemperature Measurements

7. After 5 min, quickly pour the NaOH solution, with stirring, into the HCl solu-tion in the polystyrene cup.

Continue to stir the mixture at a constant rate. Measure temperaturetimedata for the reaction mixture 30 s after mixing, then at 1-min intervals for 10 min.Record all times and temperatures.

8. Discard the reaction mixture, following the directions of your laboratory in-structor.

9. Do a second determination following the procedure of Steps 18. Yourlaboratory instructor might ask you to use a clean, dry polystyrene cup for eachdetermination. Record your data in the determination 2 column.


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Caution: Wash your hands thoroughly with soap or detergent before leaving thelaboratory.

Post-Laboratory QuestionsUse the spaces provided for the answers and additional paper if necessary.

1. One of the first statements in the Background Information is Neutralization reactions are exothermic;that is, energy is released in the form of heat. What experimental evidence do you have to verify this state-ment?

2. The theoretical Hneut for the reaction of HCl with NaOH is 5.59 x 104 J/mol. Calculate the percent error

in your determination, using Equation 7.

percent error, % =theoretical experimentalneut H H

Hneut

neuttheoretical

( % )100 (Eq. 7)

3. (a) Speculate about why you were directed to mix the HCl and NaOH solutions in the polystyrenecup and not in the beaker.

(b) Would your experimental Hneut have been higher or lower if you had mixed the acid and basesolutions in the beaker? Briefly explain.

(c) Would your experimental Hneut have been higher or lower if you had covered your cup with apolystyrene top in which holes were made for your thermometer and stirring rod? Briefly explain.


name partner section date

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4. Referring to your temperaturetime data and graph, briefly explain how your experimental results wouldhave been affected if you had based your T determination on your recorded temperatures rather than onthe extrapolation of your plotted data.

5. Compare the theoretical Hneut for the HClNaOH reaction with the calculated result for theHNO3NaOH reaction in the Background Information.

(a) Write the complete ionic equations for each of these reactions.

HClNaOH:

HNO3NaOH:

(b) Write the net ionic equations for each of these reactions.

HClNaOH:

HNO3NaOH:

(c) Would you expect Hneut for the HClNaOH reaction to be the same as that for the HNO3NaOHreaction? Briefly explain why or why not.


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Data and Observations

determination 1 determination 2

volume of HCl solution, mL _______________________ ______________________

molarity of HCl solution, mol/L _______________________ ______________________

volume of NaOH solution, mL _______________________ ______________________

molarity of NaOH solution, mol/L _______________________ ______________________

TemperatureTime Data

determination 1 determination 2temperature, C temperature, C

time, min base acid mixture base acid mixture

0.0 _________ _________

0.5 _________ _________

1.0 _________ _________

1.5 _________ _________

2.0 _________ _________

2.5 _________ _________

3.0 _________ _________

3.5 _________ _________

4.0 _________ _________

4.5 _________ _________

mix

5.5 _________ _________

6.0 _________ _________

7.0 _________ _________

8.0 _________ _________

9.0 _________ _________

10.0 _________ _________

11.0 _________ _________

12.0 _________ _________

13.0 _________ _________

14.0 _________ _________

15.0 _________ _________



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Calculations and ConclusionsShow your calculations in the space provided. Remember to include units with all calculated results.

1. Plot the temperature vs elapsed time as shown in Figure 2.

To determine T, draw a vertical line of mixing (See Figure 3). Then, draw the best straight lines through the datapoints for each solution taken prior to mixing. Then draw the best straight line through the post-mixing data, ig-noring the first two or three post-mixing data points. Extrapolate each of these lines to the line of mixing.


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Figure 2 Plot of typical temperaturetime data

Figure 3 Extrapolation of temperaturetime data to find T

2. Find the temperature change (T ) for each determination. The average initial temperature (Tinitial) ofthe reactant solutions is the average of these solutions temperatures at the intersection of the extrapolatedlines and the line of mixing. The final temperature (Tfinal) is the intersection of the extrapolatedpost-mixing line and the line of mixing.

determination 1 _________________ determination 2 _________________

3. For each determination, calculate the amount of heat transferred when you mixed 51.0 mL of 2.00MNaOH and 50.0 mL of 2.00M HCl, using Equation 4. Assume the final volume of the reaction mixture was101.0 mL, its density was 1.04 g/mL, and its specific heat capacity was 3.89 J/g C.


4. For each determination, calculate the number of moles of HCl and of NaOH that were present when youmixed 50.0 mL of 2.00M HCl with 51.0 mL of 2.00M NaOH, using Equation 5.

determination 1 _________________mol HCl determination 2 _________________mol HCl

_________________mol NaOH _________________mol NaOH

5. For each determination, calculate the number of moles of NaOH that reacted when you mixed 51.0 mL of2.00M NaOH with 50.0 mL of 2.00M HCl.


6. For each determination, calculate Hneut for the reaction of 1 mol of HCl and 1 mol of NaOH, using Equa-tion 6. (Remember that H is negative for exothermic reactions.)


7. If you did two determinations, calculate the average Hneut for the reaction of 1 mol of HCl and 1 mol ofNaOH.

average Hneut _________________



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Pre-Laboratory Assignment

1. What hazards should you know about when you work with:

(a) 2M NaOH?

(b) 2M HCl?

2. Define the following terms as they apply to this experiment:

(a) exothermic reaction

(b) Hneut

3. (a) How will you determine the initial temperature of your HCl and NaOH solutions (Tinitial) if thetemperatures of these solutions are not exactly the same?

(b) Thermometers respond relatively slowly. Briefly explain how you will overcome this difficultywhen you calculate T in this experiment.

4. While performing an experiment similar to the one described in the Procedure, a student mixed 30.0 mLof 1.00M HCl with 31.0 mL of 1.00M NaOH and recorded the temperaturetime data shown below. Assumingthe initial temperatures of the HCl and NaOH solutions were the same, and the specific heat capacity of the re-action mixture and its density are 3.89 J/g C and 1.04 g/mL, respectively, calculate Hneut for the reaction.

time, min temperature, C time, min temperature, C

0.0 21.7 8.0 28.2

1.0 21.8 9.0 28.1

2.0 21.9 10.0 28.0

3.0 mix 11.0 28.0

4.0 27.2 12.0 27.9

5.0 28.0 13.0 27.8

6.0 28.4 14.0 27.7

7.0 28.3



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ISBN 0-87540-609-2

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