The Stoichiometry of the Neutralization of Citric Acid An Introductory Laboratory Susan E. Hayes Siena College, Loudonville, NY 12211 and Union College, Schenectady, NY 12308
Currently considerable emphasis is being given a t many colleges and universities to redesigning introductory chemistry laboratories ( I ) . The trend is to move away from the traditional format in which the same set procedure is carried out by each student individually and is then followed by a report or questions designed to fur the ideas illustrated by the laboratory in the students' minds. This style has been widely criticized for the isolation in which each student works and for the few opportunities it provides to pose and solve chemical questions during laboratory. Newer, alternative formats are intended to encourage teamwork and active questioning and learning during the laboratory. Discussion of the experiment is shifted to the end of the laboratory period, after the students' data has been collected. The students can then use their own data to develop the principles and conclusions of the laboratory in a guided discussion. A typical alternative format involves the following stages: 1. The topic and techniques are introduced briefly at the be-
ginning of the period. 2. A question is posed which the students salve experimen-
tally. Often the experimental work is divided between groups of students, so that eachgroup deteminesjust one part of the data. 3. The results are summarized and the class engages in a full discussion of how the results answer the questions originally posed. Students'reports are based on class results as a whole and on the class discussion. Implementing this new format presents several practical problems. Experiments must be redesigned so that results can be gathered within one to two hours, to leave sufficient time for discussion a t the end of the period. The collection of data must be reorganized, so that each student collects only a portion and, because class results now depend on everyone producing good data, so that incorrect results can be weeded out during the laboratory. The traditional lab lecture must be reorganized into two parts; an initial, brief introduction that poses the questions addressed by the laboratory and a n end-of-lab discussion designed to guide the students in drawing conclusions and insights from their results. Experiments in stoichiometry can be redesigned to follow this format by using the continuous variation method, originally described by Job, Ostromisalensky, and Ruff (24). In fact, the continuous variation method has been used in introductory stoichiometry experiments for many years (5-111, although its application in a discovery lab format is more recent (I). In a typical experiment using groups and a discovery-lab approach, each student (or pair) carries out the reaction a t a different stoichiometric ratio, measuring an extensive property, such as the weight of the product, the intensity of its absorption of light, or the heat of the reaction. T h e class d a t a i s pooled a n d t h e correct stoichiometry is found by determining which reactant ratio gives the most product, or in the case of a strongly exo-
Table 1. Reagents and Equipment, Per Pair of Students 1.OO M citric acid, at room temperature 1.00 M sodium hydroxide, at room temperature thymol blue indicator 2 nested polystyrene cups thermometer, graduated to 0.2 *C buret stand, to hold the thermometera 100-mLgraduated cylinder several beakers and a wash bottle We instruct the students to stir by swirling the cup while keeping the thermometer stationary,clamped in the buret stand. Usually fewer thermometers are broken using this configuration.
tbermic reaction, the most heat. I n the experiment described below, we have chosen the calorimetric method because it is very quick (no precipitates to dry and weigh) and requires only polystyrene cup calorimeters, student thermometers, and ordinary laboratory glassware (Table 1).
4 /OH F H -C -H H@+ -C'. 'OH H-C -H I
ofl
C
\
OH
The Experiment Figure 1. The structure of We used this experiment to in- citric acid. traduce stoichiometry to a group of precollege summer students in a 90-minute laboratory. After describing acid-base neutralization and the heat it produces, we explained that the maximum increase in temperature would be obtained when just enough base is added to neutralize allthe acidic hvdroeens Dresent in solution. Then we described the structure of citric acid (Fig. 1)and posed the following questions to be answered by the students: "
"
A
1. How many moles ofsodium hydroxide react with one mole
of citric acid? 2. How many of the hydrogens in citric acid are acidic? 3. Which groups (COOH, OH, or CH,) do you think contain
the acidic hydrogens? In the experiment, 1.00 M concentrations of both sodium hydroxide and citric acid were used to insure large, easily measured, temperature changes. The students were grouped in pairs and eacb pair assigned two of the eight different ratios tested by the class. Each ratio gave a final volume of 60 mL, so that the calorimeter constants of the polystyrene cups and the heat capacities of the solutions were constant throughout and could be neglected. The reagents and equipment used are listed in Table 1. The students added a few drous of thvmol blue to their stock acid and base solutions, tdreducethe chances of solution mix-UDSand to pive a n au~roximatemeasure of the pH of eacb solution after neutr&ation. They were shown how to measure volumes to 0.1 mL and to read the thermometers to 0.02 "C. We instructed the students to use the Volume 72 Number 11 November 1995
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Table 2. The Temperature Increase Measured by Two Classes on Mixing Sodium Hydroxide with Citric Acid mL NaOH added 20.0 25.0 30.0 35.0 40.0 45.0 48.0 50.0 55.0
m~hC1 IT, adoed class 1 PC)" 40.0 35.0 30.0 25.0 20.0 15.0 12.0 10.0 5.0
\ J.
class 2 PC)B
St0 ch ometq. moles haOrl mo es c tr c ac d
4.7 4.5 6.78
6.3 7.7 8.2 9.0 6.9 6.7 2.7
211 5l7 111 715 211 311 411 511 Ion
7.6 8.2 6.7 6.3 3.8 'Each temperature is the average of two student measurements. When the measurements differed by mare than half of a degree, the runs were repeated.
Discussion The success of this laboratory rests on the assumption that the enthalpies for the successive neutralizations of citric acid are similar and highly exothermic. Because citric acid is a weak acid, the molecule is largely cmvdent in solution, and the reaction enthalpies that lead to the temperature increase need not he equal. Each successive neutralization can be treated a s the sum of two reactions, the ionization of the COOH group (1)and the neutralization of H30+ (5): R-COOH + H,O + R-COO- + H30i AH,= 4.2 kJlmol(1) AH2=2.4 kJlmal AH,= -3.3 kJlmol AH --54.1 kJlmol(2) H30t + OH- 7- 2 H20 Fortunately, a s shown above, the differences between the heats of ionization for citric acid are quite small relative to the heat of neutralization (12).These variations apparently are not large enough to affect the linearity of the temperature rises on either-side of the equivalence point,
average of t h e temperatures of the two solutions before mixing a s the initial temperature. (Typically, this difference is less than 1 "C.) As soon as each pair obtained the temperature increase for a given ratio i t was tabnlated on the board. Because all of the ratios were run in duplicate, incorrect results were spotted easily and suspect experiments were rerun immediFirst Class ately.' Each student also was asked to put A Second Class a portion of their final solution in a test tube. These tubes were arranged i n a rack, from the smallest NaOH volume to t h e l a r g e s t so t h a t t h e whole class could see the increase in p H from v e r y acidic ( p i n k color) through weakly acidic (yellow color) to basic (blue). This showed visually a t what ratios excess NaOH had been added. Once all measurements were completed and seen to be reasonably accurate, the group discussed a t what volume the maximum temperature rise mrnoles of HCL added would be expected if the sodium hydroxide to citric acid ratio were 1:1, 15 20 25 30 35 40 45 50 55 60 2:1, 3:1, 4:1, and 5:l (Table 2). The combined results were plotted and Figure 2. Results from two classes, showing the temperature increases that the students measshown using a n overhead projector, a s ured on mixing varying volumes of sodium hydroxide and citric acid to give 60-mL solutions. shown i n Figure 2. Although the individual points show some scatter, the maximum falls clearly a t a 3:l ratio, a t least within the accuracv of our measurements (Fiz. 2). establishing the 3:l stoichiometry of the reaction. With This contrasts with the results of Mahoney, Sweeney, and some guidance the students then concluded that there Davenport for continuous variation plots for several minmust be three acidic hydrogens in a citric acid molecule. eral acids (61, where simificant positive and negative dein all^, the class compared the number of acidic protons viations were seen near the equivalence point. These difwith numbers of each type of hydrogen i n citric acid (1OH, ferences have two 4 CH2, and 3 COOH) and agreed that the COOH group 1. The acids that gave deviations, H2S0, and H3P04,have probably contained the acidic H's. The discussion was conionization energies that are significantly larger than eluded by asking the students to think of experiments that those of carboxylic acids: -21 kJ for HS04-and-7.5, +3.3 would further establish the acidic nature of the COOH and +15 kJ for each successive ionization of H2POL(12). . . 2. The measurements made bvMahonev. ".Sweenev. ", and Davenport are undoubtedly more precise than ours, the preci'In fact, this in-class comparison of results, plus the ease and sion of our precollege students measurements being at speed with which a measurement can be repeated, means that poor best 0.5 "C. student technique can be detected and corrected immediately.
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Journal of Chemical Education
I t is likely that if they measure the temperature change more precisely, students will find that the citric acid curve does show a slight deflection a t the equivalence point. Because the sum of the three heats of ionization is endothermic, t h e t e m p e r a t u r e increases measured should be slightly smaller than those predicted for a fully ionized acid. However, this should not affect the value of the experiment because the more sophisticated student able to make more precise measurements also should be able to interpret this more complex result. As described above, this laboratory is short, having been designed to fit into a 90-minute period. For the standard three-hour laboratory, the experiment can be extended by having the students design further experiments of their own. For example, the students could propose experiments to confirm that COOH is the acidic group, such as measuring the equivalence points for other carboxylic acids or alcohols. Because the basic experiment is so short, they 2Bcca~setnc on zallon energ es of most carboxyl c ac as gener. aly lc ocMoen 4.8 KJ mot ana 4 . 8 nJ mo (1210-1 assmpuon of essenl a y eqba neals ol neulra zal on w~llbe lrde for most carooxylic acids.
should be able to complete these experiments during the same laboratory period and also have time to discuss these additional results.' Acknowledgment I would like to thank Charles Scaife of Union College for suggesting an experiment of this type as both suitabl;? and sufficiently brief. I also thank Meng- Fona, - who assisted i n developing the laboratory. Literature Cited 1. Kildahl, N.: Berks, L. H
J Ckem Educ. 1993, 70, 671-673. 2. Job. P. Compl. Rend 1925,180, 928:Ann. Ckem.. (Patisl lSetie 10)1928.9, 113-203; ISetie 11)1936.6.97-144. 3. Oafmmiralensky, I., J. Rusa P k ~ Ckem. s Soc. 1910,421332,1500:Ber Deut Climn Get. 1911.44, 268. 4. Ruff,0. Ckem. Zty 1910. 13, 1003:ZAgnaw. Chsm. 1910.23. 1839-1831;Z Pkysth. Chem 1911,76.21-57;Eer Deul Chem. Ges. 1911,44, 5448. 5. Gil, V. M. 8.; Oliveira, N. C. J. Chrm. Edvc 1990.67, 473478. 6. Mahoney, D. W: Sweeney, J. A ; Davenport, D. A. J Chem Edue 1981.58,730.781. 7. Kalbus,L. H.; Petmcd, R. H. J. Chem Educ. 1969,46, 776778. 8. Masaeuer J. R.: C0to.M. V.: Casas. J. S. J. Chem Educ 1975.52. 387. 9. ~ a & . ~ h hEduc. . 1919.56477-478. 10. DeMoura,J.M.:Marcello,J.A. J. Chem Educ. 1987.64. 452453. 11. Seaife, C. W J.; Besehley 0.T.. Jr Ckernisiv m the Lobomlory: Saundeis College Publishine: Philadelohia. 1987: o 427. , 12. Chtistensen.J . J.; Hausen, L. D.; Izatt, R. M. Hondbwk ofPmion Ionbetion Heals and Reloled Thennodynamic Buontitles:Wiley: New York,1976.
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