The heat of solution of citric and of acetic acid is calculated for various concentrations from the calorimetric experiments of Richards and his coworkers. From these data the heat of electrolytic dissociation of the two adds is calculated. By extrapolation, the heat effect at infinite dilution is evaluated. It is pointed out that for both acids the value of this last heat effect is in good agreement with the value which was obtained by Bafes and Pinching for citric acid and by Harned and Ehlers for acetic .add from experiments on the change of the E.M.F. of suitable cells by temperature. Calculation of the heat of dilution from the heat of electrolytic dissociation at infinite dilution is possible at sufficiently low concentrations.Richards and Mair carefully investigated the heat of neutralization of the first hydrogen ion of citric acid and of acetic acid with sodium hydroxide. The results were communicated in two papers I ) , which were completed and published after the death of Richards. The authors tried in vain to discover a relationship between the degrees of dissociation of the acids and their heats of dilution. They therefore concluded from their results: "that some other factor or factors must be largely responsible for the heat of dilution (in the range studied), although they do not preclude the possibility that there is a heat effect due to ionization masked by some larger effect".In the following it is pointed out that their data make it possible to calculate the heat effect of the electrolytic dissociation of these acids, at least at low concentrations.AH, designates the molecular heat effect of the neutralization at a certain concentration as measured, using one mole of both reactants, and AH,o the heat of-the reaction H + ( a q ) + OH-(aq) = H,O(aq) at infinite dilution. This quantity is, of course, independent of the nature of acid and base. AH,, AHb and AH, stand for the molecular
The velocity of the reaction between formic acid and chromic acid has been measured in mixtures of sulphuric acid and water, containing from 0 -8 7 % sulph. ac. at temperatures between 273" and 353" K.This velocity is nearly proportional to the concentrations of the reactants In very dilute sulph. ac. the acceleration caused by this substance is about proportional to its concentration, in more concentrated solutions it is greater. Between 60 and 87% there is an optimum .Between 5 and 60% sulph. ac. the velocity constants at 293" increase to the 33000 fold, they can be calculated from H a m m e t t and D e yr u p 's acidity function Ho (which is based on measurements with basic indicators at 298" K). At other temperatures the relation of H a m m e t t and D e y r u p fails to represent the experimental results. It is supposed that the relative acidity, at least in dilute sulph. ac., changes with the temperature. Between 13.5 and 40% sulph. ac. there exists a simple relation between the composition of the solvent and the velocity of the reaction at any temperature.The temperature coefficients in most solvents used change very little with the temperature, in consequence the formula of A r r h e n i u s does not hold.Information about the temperature coefficient of the velocity of the oxidation in solutions with 50-95.5 "/o sulph. ac. was obtained with adipic acid 1 ) and succinic acid *), while oxalic acid furnished data for all the concentrations from 0-95 % 3).For succinic and adipic acid the formula of A r r h e n i u s does not hold; for oxalic acid no test was possible, as the velocities were determined at two temperatures only. Experiments over a larger range of temperature encounter difficulties in this case. Therefore other substances, which allow a test of the formula of A r r h e n i u s for oxidations in dilute sulph. ac. were looked for. Until now only formic acid has proved satisfactory; the method used was the one described in the second communication on this subject.
The velocity ( v ) of the reaction between oxalic acid (1) and chromic acid (2) has been measured in mixtures of sulph. ac. and water. containing from 0 to 95.8 yo sulph. ac. a t 303.0" K and 323.0" K. T h e most appropriate method to investigate the influence of the solvent was to calculate n i and n? from v :-= k X clnl X q n Z . n t was found to vary between 1 and 2, n2 between 1 and 1.51. In consequence, the effect of the composition of the solvent varies with the concentration of the reactants. The addition of sulph. ac. to water decreases the velocity of the reaction until a minimum is reached, a t * 70 yo sulph. ac.an optimum exists. The initial stage of the reaction shows, in solvents which contain more than 60% sulph. ac. abnormal (too low) velocities, a phenomenon which is not caused by the effect of any product of the reaction, nor by any reaction between the solvent and the reactants. It is supposed to be the consequence of the slow formation of a compound between the reactants, prior to the actual decomposition. The temperature co?fficients are largely dependent on the composition of the solvent, they vary between 4.32 (water) and 8.8 ( 7 0 % sulph. ac.) for 20" (303.0-323.0" K). An optimum is found a t 70%. a minimum a t 80 yh sulph. ac. In water and diluted sulph. ac. no complete oxidation can be obtained as part of the oxalic acid is bound by the reduction product of the chromic acid. Taking this into consideration the initial stage of the reaction in water may be represented by the formula for a termolecular reaction, which can also be applied to solutions in 13.4% sulph. ac.. where complete oxidation is obtained.T h e oxidation of succinic, glutaric and adipic acid has been investigated in solvents from 60 to 99.0 96 sulph. ac. In more dilute sulph. ac. the velocity, even at 373' K is so low that measurements become inpracticable. I ) For part I see Rec. trav. chim. 56, 873 (1937). The volume V in the first experiment recorded in this paper (p. 874) was 6.88. not 5.82 as stated. The calculation of k was made with the right value.--~ __
The values of E and log. A in kT = A e ‐E/RT. which have been published for the oxidation of succinic acid with chromic acid in sulphuric acid of varying strength, allow one to calculate kT for any temperature. The maximum which was found in the kT‐acid concentration curve must show a shift to lower sulphuric acid concentrations with decrease of the temperature. At −40° it is found very near to the composition SO3 3 aq. This, and the fact that the speed of oxidation increases rapidly with the acid concentration as soon as the composition SO3 5 aq. has been reached, leads to the conclusion that SO3 3 aq. is one of the active components of the system. Comparison of the speed of decomposition of chromic acid, the speed of oxidation of organic substances and the speed of decomposition of organic substances by sulphuric acid, leads to the conclusion that a second hydrate, probably 2 SO3 1 aq. is active too. It is known that reactions which are dependent on the concentration of this substance have an abnormally high temperature coefficient. The superposition of the effects caused by these two hydrates explains the behaviour of solutions of sulphuric acid in water at different temperatures in a number of cases.
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