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CALORIMETRIC STUDIES OF NEUTRALIZATION REACTIONS JOHN G. MILLER, ARTHUR I. LOWELL, and WALTER W. LUCASSE University of Pennsylvania, Philadelphia, Pennsylvania
TAERMOCAEMI~AL studies of a wide variety
of types
Siuce AH, involves the severance of a bond in the weak
(4) are readily adapted for simplified undergraduate ex- acid, its sign would be expected to be opposite to that
perimentation. They may include determination of the heat changes in strictly chemical processes, such as the decomposition of hydrogen peroxide and numerous other chemical reactions, and also the physisal aspects of thermochemistry, such as heats of solution and dilution. Neutralization reactions, however, are m&t frequently employed to exemplify the methods and calculdtions of thermochemistry. The speed of such reactions, the drill in preparing solutions of acids and bases of definite concentration, and the importance of the results in connection with the classroom work on the theory of solutions have made these reactions almost the exclusive choice of nearly every laboratory text. It is, perhaps, unfortunate that acid-base studies are uhally restricted to strong electrolytes. Use of weak acids and bases and of polybasic acids not only requires fuller understanding in the selection of indicators for the preparation of the solutions but also demands more thought in the interpretation of the thermochemical results. From his work with strong acids and bases, the student concludes that these substauces are completely ionized and, in simplest form, writes the fundamental reaction:
of AH2. In this event, the sum of AHl and AH2, that is, the heat content change of the complete reaction HA
+ OH-
-
H,O
+ A-
would be smaller in magnitude, as is found in most cases, than - 13.8kg.-cal. Such a picture, however, fails to explain the ocourrence of values greater than -13.8 kg.-cal. and gives an erroneously simple interpretation of the heat of ionization. It must be emphasized that the solvent plays an important part in all acid-base reactions and an additional step may be introduced. Thus, for a weak acid and strong base, the steps are given more accurately as H + +A-
and the total reaction is HA +OH-
-- + -+
HA-H++AH+.zH20 +A- .yH1O (z 1)H20
+ (Z + y)H,O H + %HsO + OH-
+ (Z + y)H.O
(Z
1)HsO +A- .yH?O
The first two steps, normally written as a single one, taken together show the effect of the solvent. It is the sum of the heat content changes for these two steps that is commonly considered the heat of ionization in H + + OHHnO; AH = -13.8 kg.-cal. (at 20°C.) solution. This quantity includes, in addition to the Because of the constancy of the value of AH Per mol of heat absorbed to break the bond in the acid, all of the water formed, irrespective of the strong acid or base effects on the heat of ionization due to the medium in used, he is unprepared to find such values of the heat which the acid is dissociated. A convenient way to content change as -16.1 kg.-cal. for the reaction be- show this is to write the process in two steps, as above, tween hydrogen fluoride and potassium hydroxide, stressing thus one of the major effects, that of solvation -12.3 kg.-oal. for that between hydrogen chloride and of the ions. It is best, however, not to imply that this ammonium hydroxide, and -11.9 kg.-cal. for that be- solvation involves a definite number of molecules of tween formic acid and ammonium hydroxide. water nor that it is the only effect due to the solvent. The additional factors which are involved with weak Since the solvation of the ions is an exothermic procacids and bases may be emphasized by experimentation ess, the heat content change of the complete reaction is and by consideration of the effects of the additional the algebraic sum of three two of which repprocesses which take place in such cases. If, in ad&- resent heat evolved. The apparent h,eat content change tion to the formation of water from its ions, the only of the major reaction, the formation of water, may be reaction is the breakdown of the weak electrolyte, the increased or decreased depending upon the dominance value of the over-all heat change will deviate from of the effect of solvation or of dissociation. Indeed, -13.8 kg.-call. by an amount corresponding to the the role of the solvent explains why some neutralizasimple heat of ionization. Thus, the steps for the tion reactions, such as that of sulfuric acid with a strong reaction of a weak acid with a strong base might be base, are more exothermic than the strong acid-base written: reactions in which all of the reactant ions are already fully solvated, the strong electrolytes being completely HA-Ht+A-; AH, ionized when dissolved. The solvent effect is particuH + OHH20; AH%
-
+
-
121
122
JOURNAL OF CHEMICAL EDUCATION
lady well shown with polybasic acids. In allowing the reaction to take place in steps, successive ionization liberates further ions to become solvated with resultant thermal effects. Finally, the fundamental concepts are still more clarified when studies of reactions of strong, weak, and polybasic acids with strong bases are combined with the parallel use of a weak base, such as ammonium hydroxide, and with experiments on the heat of solution of salts. For example, the heat of reaction of hydrochloric acid with ammonium hydroxide and the heat of solution of ammonium chloride are related. The fact that the solution of ammonium chloride in water is an endothermic reaction indicates that the solvation of the ammonium ion is negligible and thus the heat of reaction of hydrochloric acid with ammonium hydroxide, being virtually due to only two opposing factors, is understandably low. Although the hydroxyl ion is not commonly so written, it probably is solvated and this should be taken into account in certain cases. In the neutralization of ammonium hydroxide, however, the heat of solvation of the hydroxyl ion is canceled out in the over-all process, just as that of the hydrogen ion is in a weak acid-strong base reaction. Experimental studies of the heat of reaction of the following acids with, first, a strong base (sodium hydroxide) and, then, a weak base (ammonium hydroxide) have been found convenient and present a varied set of effects: a strong acid (sulfamic), a weak acid (acetic), a stronger weak acid (monochloroacetic), and two dibasic acids (oxalic and tartaric). In each case the heat effect is conveniently large at concentrations desirable for work. All of the reactants are readily available and reasonably soluble, and preparation and standardization of the solutions are facilitated by the fact that four of the acids are stable solids. In laboratories in which acetic acid of known concentration is available for other experiments, conductance for example (3), this might be used as a basis upon which to standardize sodium hydroxide and this in turn to prepare standard hydrochloric acid. Using these two, solutions of any acids and bases, strong or weak, can be prepared of desired concentration. It is instructive, however, to use sulfamic acid both as a representative strong acid in thermal study and as a primary basis for standardization
plete reaction; 0.5 N tartaric acid was treated with equal volumes of 0.25 and 0.5 N base. SuIfamic Acid: This substance, NH2SO3H, was chosen because it is a strong acid of unusual qualities, being a stable, nonhygroscopic solid. Because no values of the specific heat and density are given in the literature for 0.5 N sulfamic acid, 0.25 N sodium sulfamate, nor 0.25 N ammonium sulfamate, each measurement permitted calculation of only one value of the heat of neutralization on the assumption that the product of the density and specific heat of each solution is equal to unity. In two experiments, the values of AH obtained for the sodium hydroxide reaction were -13.07 and -13.01 kg.-cal. in the temperature range from 27 to 30°C. No values are reported in the literature for comparison, but it is seen that the typical strong acid behavior is shown. With ammonium hydroxide two values for. AH in the range from 26 to 2 9 T . were obtained, -11.44 and -11.43 kg.-cal., the difference from the strong-base value being comparable with that shown by other strong acids upon reaction with ammonium hydroxide. Acetic Acid: With acetic acid and sodium hydroxide the average value of AH a t 26'C. was -12.65 kg.-cal. and a t 29.1°C. was -12.67 kg.-cal., which compare favorably with the literature values and attest the reliability of the apparatus and the method. The reaction with ammonium hydroxide, giving - 10.94 kg.cal. at 26.2"C. and - 11.43 kg.-cal. at 29.0°C., indicates a high temperature effect due to change in the heat of iqnization and primarily to the low specific heat of the salt solutions involved. Monochloroacetic Acid: With monochloroacetic acid and sodium hydroxide an average AH value of -13.73 kg.-cal. was found at 24.6'C., which is about 4 per cent lower than the value given in the literature for 18' C. The relatively high value of the heat content change can be attributed to the dual effect of solvation and fairly high ionization. Using the same acid with ammonium hydroxide, the energy of dissociation of the weak base came into play and led to a vdue of - 12.2 kg.-cal. at 24.3'C. Oxalic Aeid: When the concentration of acid was such that only the first hydrogen was removed by the addition of sodium hydroxide, the heat content change was found to be -13.87 kg.-cal. (28.7T.), and when U,Q I n obtaining the data given below, the method of both were removed, -14.03 kg.-cal. (25.2'C.). Using procedure and calculation was the same as that de- ammonium hydroxide, the heat of dissocation of the scribed in an earlier paper (4).' For the studies with base was such as to render the corresponding values of monobasic acids, 0.5 N solutions of acid and base were AH, -12.01 kg.-cal. (27.8'C.) and -12.90 (25.1°C.). used in all cases: For the dibasic acids, stepwise re- These values appear to agree with literature values. action was achieved by treating 0.5 N base with an Tartaric Acid: In the work with tartaric acid the equal volume (200 cc., as throughout) of 1.0 N oxalic concentration of the base rather than that of the acid acid to cause half neutralization and 0.5 N acid for com- was altered to achieve partial and complete neutralization. Parenthetically, it might be noted that in ther'The numerical data given in the earlier paper, except in mochemistry the term "neutralization" is not used with Figure 4, are in large calories and should have been designated the same regard for hydrolysis as in titration and does as kg.-cal.; the equations for calculation, however, are in terms of small coalories. For consistenoy with Figure 4, the signs for not imply that the solutions are neutral a t the end the heats of solution of sodium acetate should have been reversed, that for the anhydrous being -3.86 kg.-cal. and for the tn- ~ o i u t . Sufficientdata were available in the case of the hydrate 4.85 kg.-cal.
(Continued on page 128)
CALORIMETRIC STUDIES OF NEUTRALIZATION REACTIONS (Continuedf ~ o mpage 1$2)
tartaric acid-sodium hydroxide iyuretns ro calculate the use of an approximate calculation in several cases. thrrrnnl rllunee :it i)orl~the initial rind the final tcnmera- Finally, the high temperature coefficient of ionization of tures. When the concentration of the sodium hy- weak electrolytes is reflected in large differences in the droxide was such as to remove only the &st hydrogen, heat of reaction at different temperatures whenever such the heat content change was -12.69 kg.-cal. a t 24.3OC. electrolytes are involved. Thus, the heat of dissociaand -12.67 kg.-cal. a t 25.g°C., indicating almost no tion of ammonium hydroxide, 1240, 790, and -470 kg.temperature coefficient over this range. With sodium cal. at 18', 25', and 51' C., respectively, is typical and hydroxide of sufficient concentration for complete nen- wduld suggest sensitivity to temperature for any retralization, however, the values at the lower (21.7"C.) action in which it enters. In the above experiments, and higher (24.7OC.) temperatures were found to be however, the expected order of magnitude of the heat - 11.21kg.-cal. and - 13.90 kg.-cal., respectively. The content change is shown in all cmes and from such difference at the two temperatures is due largely to the studie's the average student receives a clearer insight abnormally low value of the specific heat of solutions of into the nature of the heat of formation of water from normal sodium tartrate (5). The values for its ions, the heat of ionization, and of solvation. and com~letereaction of tartaric acid with ammonium hydroxide were found to be -11.32 kg.-cal. (25.7'C.) LITERATURE CITED and - 11.29 (24.7'C.), respectively. "~uantitativeAnalyWhenever temperature and other conditions made it ( 1 ) BOOTH,H. S., AND V. R.DAMERELL, sis," McGraw-Hill Book Company, NewYork,l944, p. 181. possible, comparison of the results of check experiments ( 2 ) B ~M. J., ~G. F. sMITH, ~ AND ~ L. F. ~ A,,Dn IETH, , Eng. of the above studies showed excellent agreement; Chem., Anal. Ed., 10, 690 (1938). J. CHEM.EDUC.. however. several factors combine to make difficult anv (3) ~. MILLER,J. G.. AND W. W. LUCASSE, . 22.. rigid evkuation of the data. Where comparable work 565 (i945). on Q . ,+ ~,D. ", B., J. G. MILLER,AND W. W. LUCASSE, ibid., appears in the literature, it is usually of an early date (4) PAPISON, 'U, O'J ( ' 3 " ) . &h probable inaccuracy of experim+ation. Further- (5) RICRAEDS, T. w.,AND F. T. GUCKER, J . Am. Chem. SOC., 47, more, lack of specific heat and denwty values force the 1889 (1925).
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