T H E Heat O F Neutralization O F Strong Acids and Bases in H I G H L Y Dilute Aqueous Solutions

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T H E HEAT O F NEUTRALIZATION O F STRONG ACIDS AND BASES IN H I G H L Y DILUTE AQUEOUS SOLUTIONS]

Can. J. Chem. Downloaded from www.nrcresearchpress.com by 86.145.230.179 on 01/15/13 For personal use only.

ABSTRACT Heats of neutralization of sulphuric and hydrochloric acids by sodium hydroxide have been determined, using a microcalorimeter of the Tian-Calvet type. The range of concentrations employed was 5x10-" N to 3x10-* N,which is sufficiently low to permit an accurate extrapolation to be made down to zero concentration. The extrapolated values obtained were: for 11:SO4-NaOH, 13.48 3~0.05 lccal. per gm-equiv. and for HCI-NaOH, 13.523Z0.05 kcal. per gm-equiv. These values, which correspo~ld the process H30++ OIi- + 2H20, are signifito cantly higher than the values of 13.32-13.37 kcal. obtained on the basis of previous calorimetric studies i n a llluch higher concentration range. The present values, however, agree well with those calculated by Harned and co-worlrers and by Everett and Wynne-Jones from electrochemical data. INTRODUCTION

Although heats of neutralization of strong acids by strong bases have been measured on very many occasions, there has remained a distinct discrepancy between the results obtained by direct calorimetry and those calculated from electrochemical data. The classical calorimetric studies of this problem are those of Richards and Rowe (12) and of Richards and Hall ( l l ) , who worlted with a variety of strong acids and bases, a t concentratiolls (after mixing) of 1 A4 and higher. Vario~isworliers, including Rossini (13), Pitzer (lo), and Kegeles (8), have employed heat ol dilution and other thermochemical data to calculate from Richards' results the heat of neutralization a t infinite dilution and a t 25" C., and have obtained valuqs ranging from 13.32 to 13.37 lical. per gm-ecluiv." Pitzer (10) also made a calorimetric determination of his own from which he obtained a value of 13.36&0.016 Itcal. per mole a t infiilite dilution. Later calorimetric determinations a t hiqher concentrations, such as those of Bender and Rierinan (I) and of Bierman and Weber (2), have provided some support for the 13.32 value. The value for the heat of neutralization a t iilfiilite dilution is the heat of the process H30+ OH- 4 2H20,and may also be obtained from the temperature dependellce of the e.m.f. of various cells. Harned and Owen (6) analyzed the res~ilts such experiments and arrived a t a value of 13.48 Itcal. for the process. of Using somewhat more reliable methods of analysis of the same results, Everett and Wynne-Jones ( 5 ) and Harned and Robinsoil (7) arrived a t values of 13.49 and 13.51 1;cal. respectively. The probable errors of the electrochemical values are about 0.05 lical.; those of the calori~lletric values are more difficult to estimate, since they involve corrections for dilutions. The calorimetric values are seen to be lower than the electrochemical ones by 0.15 to 0.2 lical , which is outside the range of errors of the electrochemical values.

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'ilifanuscript received April 25, 1956. Co~ztribz~tio?~ the Departnzent of Chemistry, University of Ottawa, Ottawa, Canadn. frolit * A l l calories qzcoied i n this paper are "ther~nocltemical" calories, eqzlal to 4.18.40 absolute jozcles.

16'78

CANADIAN JOURNAL O F

CHEMISTRY. V L O.

34. 1958

Can. J. Chem. Downloaded from www.nrcresearchpress.com by 86.145.230.179 on 01/15/13 For personal use only.

The weakness of the previous calorimetric worli is that, in order for the heats liberated to be sufficiently large to be measured accurately, the determinations must be macle a t fairly high concentrations, and under these conditions the corrections for dilution are substantial. With the use of modern microcalorimetric techniques it is, however, possible to make determinations of heats of neutralization a t very much lower concentrations than have hitherto been employed. The microcalorimeter employed in the present...
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