Heat of neutralisation

From the strongly acid taste, the heat of neutralisation with dilute sodium hydroxide 3 and the electrical conductivity of the solution,4 it appears that tetrathionic acid is a fairly strong acid,5 comparable with dithionic acid in this respect. The heat of formation6 is given by the equation ... 

Using the dihydrogen salt, NaH2As03, von Zawidzki2 determined the equivalent conductivity in the presence of N/32 arsenious acid in order to diminish hydrolytic dissociation, and concluded that it resembled the salt of a monobasic acid. At extreme dilutions the equivalent conductivity increased, apparently owing to hydrolysis and oxidation. This view that arsenious acid is essentially a feeble monobasic acid is supported by Thomsen s thermochemical values for the heats of neutralisation of the acid (see p. 140). 

The following values for the heats of neutralisation at 15° C. of arsenic acid by aqueous ammonia, and for the heats of dissolution of the ammonium arsenates, have been obtained.2 The solutions of arsenic acid for the determination of the former contained J mole of H3As04 per litre, and those of ammonia were of such concentrations that 1 volume of the acid was neutralised by an equal volume of the base. 

The heat of formation of sodium orthoarsenate in aqueous solution from the elements is 381,500 calories,1 and that of the solid 360,800 calories.2 The heat of neutralisation (3Na0H.H3As04.aq.) is 35,920 calories,8 and the heat of formation from the oxides is 2... 

Enthalpy of neutralisation (molar heat of neutralisation) Changes of state... 

The following results were obtained from a neutralisation reaction between 1 mol dm-3 hydrochloric acid and 1 mol dm-3 sodium hydroxide. This experiment was carried out to measure the heat of neutralisation of hydrochloric acid. The temperature rise which occurred during the reaction was recorded. 

The conductivity increases at first with the temperature as is usual the rate of increase then diminishes and the conductivity reaches a maximum at about 50° C., the exact temperature varying with the concentration and being 57° C. in the case of normal acid.5 The conductivity then decreases. It is supposed that the effect of the normal increase in ionic mobility with temperature is diminished and finally reversed by the opposite effect of decreasing dissociation. Since the dissociation constant decreases with rise of temperature the dissociation into ions must take place with evolution of heat, i.e. the heat of ionisation is positive. Therefore the neutralisation of the acid with alkali must result in a production of heat greater than the heat of formation of water from its ions, which may be taken as 13-52 Cals, per mol. If the heat of dissociation is Qa Cals, per gram-ion and the undissociated portion of the free acid is 1 - a, then the total heat of neutralisation Qn will be given by the equation... 

Basicity.—The heats of neutralisation indicate a dibasic acid (per atom of phosphorus) as will be seen from the following1 figures —3... 

The heat of neutralisation, Qn, of phosphoric acid (1 mol) with NaOH (n mols) in dilute solution has been determined with the following results —... 

The heat of dissociation, Qd, probably is positive, since dissociation diminished with rise of temperature. This agrees with the fact that the heat of neutralisation, Qa, of the first hydrogen ion, viz. 14-8 Cals., is somewhat greater than that of a completely dissociated strong monobasic acid, viz. 13-5 Cals. The following calculation also shows a quantitative agreement —3... 

With regard to heats of neutralisation, 1 mol of the acid, to which n equivalents of strong alkali were added, gave the following successive amounts of heat, the sum of which is the total heat of neutralisation at each step —... 

Wartenberg et al. (1937) have determined the heat of formation of CaCr04 using a heat of neutralisation calorimeter and have reported the value - 49.75 kCal for A//. They have not reported the limits of error in their measurement. But there is agreement between their value and the value reported in the present work (Prasad Abraham, 1970) taking into account the error of 3000 calories, estimated for the results ofWartenberg. Subsequently, Jacob et al. (1992) re-estimated the value by EMF method and reported a value of 47.68 1.2 kCal. 

The heat of neutralisation of ferrocyamc acid with 4 molecules of potassium hydroxide is... 

We may, for example, surround the calorimeter with a bath of dilute acid and run in alkali from a burette. The rate at which the alkali is added can be regulated so that the heat of neutralisation produces at every instant the same rise in temperature in the bath as is produced by the reaction in the interior of the calorimeter. Richards and his colleagues have made some exceedingly accurate calorimetrical determinations in this way.f... 

The heats of neutralisation of dilute acids and bases, determined by Thomsen, Berthelot, and others, are worthy of special notice. The figures given in the following table are for each Vorlesungen, iii. p. 85. 

When this is the case, the heat of reaction must be quite independent of the nature of the anion and of the cation, aa these are not affected by the reaction. This is clearly true for nitric and hydrochloric acids with all the bases given in the table. For sulphuric and carbonic acids, however, the conditions for the validity of the theory are apparently not fulfilled. In the first case, the heat of dilution of sulphuric acid amounts to 2000 cal., and this amount must be subtracted from the figure given in the table, as it is evolved when the alkali and acid are mixed. In the second case, carbonic acid is so weak an acid that it is practically undissociated. The heat necessary for the dissociation into ions therefore uses up part of the heat of neutralisation. From the table it follows that the electrolytic dissociation of J mol. HgCOg requires 13700 — 10200 = 3500 calories. The constant heat of neutrahsation 13700 cal. is the heat of ionisation of water, i.e, the quantity of heat required for the dissociation of water, and liberated on the combination of its ions. 

According to recent experiments by Woermann this heat of neutralisation, is slightly greater for dilute solutions of KOH than for NaOH. Woermann gives the following heats of neutralisation at 0° C. (ice calorimeter) ... 

At room temperature these heats of neutralisation are about 1000 cal. smaller than at 0°. 

The first column contains the temperature in degrees centigrade, the second the experimental values of /i, and the third the heat of dissociation calculated by van t Hoff s equation. The fourth column contains the values of the heat of neutralisation of strong acids and bases determined calorimetrically by Wormanmt... 

Table A-86 Determination of Af//° (Na2Se20y, cr, 298.15 K.) from the heat of neutralisation of NaySeyOvCcr) with NaOH(aq). The sources of the data are indicated.

The affinity of It at normal concentration for OH aho at noinial concentration, in acgieoin solution, amount to H I log kT or + o i volt-faraday, or rH,6ao calorics prt graitwnn or pet grant mole of water formed. The heat of the same million, whi< li, it has already been shown, is necessarily the heat of neutralisation t l one equivalent of a strong add with a strong alkali, is 14,000 alone approximately. 

Neutralisation of aqueous acids. Hydrated lime, whether Ca(OH)2, Ca(OH)2 MgO, or Ca(OH)2 Mg(OH)2, reacts readily with aqueous acids. The rate of reaction depends in part on the particle size. The heats of neutralisation of calcium hydroxide with strong acids are similar to those of other strong bases. The values for the reaction of calcium hydroxide with sulfuric and hydrochloric acids are 31,140 and 27,900 cal/mole respectively. The corresponding heats of reaction of magnesium hydroxide are reported as 31,220 and 27,690 cal/mole [19.2] (see also section 28.3). 

The heat Of neutralisation is independent of the nature of the acid or base. For this reason, it is assumed to be the heat released due to the recombination of the hydrogen ion from the acid and hydroxide ion from the base to form water. 

The heat of neutralisation is the amount of heat evolved when one gram equivalent of an acid is neutralised by one gram equivalent of a base to give one gram equivalent of a neutral salt. Thus, in dilute solution, an acid or base is considered to be completely ionised, and the neutralisation reaction is Na + ) + OH(-) + H( + ) + Q(-) ==> Na(+) + a(-) + P... 

Ortho and / flra-substituted acids give different straight lines owing to chelation and steric hindrance. To account for slopes less than —1, we must assume that the formation constant of the ion pair, the medium effects, or both depend on the strength of the acid. Mead found linear correlations between heats of neutralisation of amines with trichloroacetic add in benzene and base strength in water primary, secondary and tertiary amines fell on parallel lines. 

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