Enthalpies of neutralization

Although not commonly used, thermometric titrations have one distinct advantage over methods based on the direct or indirect monitoring of plT. As discussed earlier, visual indicators and potentiometric titration curves are limited by the magnitude of the relevant equilibrium constants. For example, the titration of boric acid, ITaBOa, for which is 5.8 X 10 °, yields a poorly defined equivalence point (Figure 9.15a). The enthalpy of neutralization for boric acid with NaOlT, however, is only 23% less than that for a strong acid (-42.7 kj/mol... 

D. J. Eatough, J. J. Christensen, R. M. Izatt. Determination of the Enthalpy ofSolution ofTris-(hydroximethyl)aminomethane in 0.1 M HCl Solution and the Enthalpy of Neutralization ofHClO4 with NaOH at Low Ionic Strengths by Use of an Improved Titration Calorimeter. J. Chem. Thermodynamics 1975, 7, 417—422. 

A chemist wants to determine the enthalpy of neutralization for the following reaction. 

The chemist uses a coffee-cup calorimeter to neutralize completely 61.1 mL of 0.543 mol/L HCl(aq) with 42.6 mL of NaOH(aq). The initial temperature of both solutions is 17.8°C. After neutralization, the highest recorded temperature is 21.6°C. Calculate the enthalpy of neutralization, in units of kJ/mol of HCl. Assume that the density of both solutions is 1.00 g/mL. Also assume that the specific heat capacity of both solutions is the same as the specific heat capacity of water. 

The technique of titration calorimetry has been successfully used to determine the nature and abundances of a variety of acidic functional groups in proteins ( ). Several investigators have made rather limited efforts to use titration calorimetry to study humic substances, usually as a method to determine the cation exchange capacity or titratable acidity of humic substances (23, 24). Choppin and Kullberg ( ) have recently used titration calorimetry to determine the enthalpies of neutralization of acidic functional groups in humic substances and have combined that data with pH titration data to obtain AG, AH, and AS values... 

The tabulated enthalpy of ionization (AH,) of the functional group is calculated from the experimentally determined enthalpy of neutralization (AH) using the equation AH = AH + AH, where AHy = + 55.8 kJ/mole is the enthalpy of ionization of water. 

The enthalpy of formation was calculated from the enthalpy of hydrolysis of K(cr), the enthalpies of solution of HCl(g) and KCl(cr), and the standard enthalpy of neutralization of a strong acid and strong base. Extensive measurements are available for all these quantities. Values used in the calculation, in addition to the enthalpy of formation for HCl(g), are summarized below. 

The neutralization of an acidic functional group is usually accompanied by the evolution of heat. For the reaction of OH with an acidic functional group with pKa < 9, the reaction is essentially quantitative (reacted base = added base) and the heat evolved is directly proportional to the quantity of added base. The slope of heat versus moles of reacted base is the enthalpy of neutralization of the acidic functional group. If the acidic functional group is rather weak ipKa > 9), the reaction with added OH is incomplete and the amount of heat evolved is not proportional to the quantity of added base. For simple, well-defined organic acids of this type, nonlinear regression methods can be used to simultaneously determine both the enthalpy of neutralization... 

Figure 25.1 Variation of the net enthalpy of neutralization A f/(NaOH) AjH(HCl) 5t, , with the total amount of oxygen. (Adapted from Ref. [8].)...

Thermochemical equations for reactions taking place in solution must also specify the concentrations of the dissolved species. For example, the enthalpy of neutralization of a strong acid by a strong base is... 

The enthalpy of neutralization of a strong acid with a strong base is —55.2 l /mol of water formed. If 100.0 ml of 1.00 M NaOH and 100.0 ml of 1.00 M HCI, both at 25.0°C, are mixed, assume that the heat capacity of the resulting solution is 4.18 J/g °C and that the density of that solution is 1.02 g/mL. (a) Write a balanced chemical equation for the reaction, (b) Determine the number of moles of water that will be formed, (c) Determine the quantity of heat that the reaction will release, (d) How much heat is absorbed by the resulting solution (e) Calculate the mass of the solution. 

The enthalpy of neutralization for the reaction of a strong acid with a strong base is -56 kJ/mol water produced. How much energy will be released when 200.0 mL of 0.400MHCI is mixed with 150.0 mL of 0.500 M NaOH ... 

Fig. 25. Variation of the enthalpy of neutralization of the total acid groups with the total amount of oxygen on the surface [300]. Reprinted with permission fiom F. Stoeckli, C. Moreno-Castilla, F. Carrasco-Marin and M.V. Lopez-Ramon, Carbon, 39 (2001) 2231...

Enthalpy of neutralization AH one mole of water formed by the neutralization of an acid by a base CH3C00H(aq) + NaOH(aq) CH3C00Na(aq) + H20(l)... 

Although the calculated heats of formation refer to the gas phase, relative enthalpies of neutral hydrocarbon isomers often do not change much in the liquid... 

For sulfuric acid, a dibasic acid, the enthalpy of neutralization equation is... 

However, where neutralizations involve a weak acid, a weak base, or both, then the enthalpy of neutralization will be smaller in magnitude than -57 kJ mol, that is, slightly less exothermic. 

For example, the enthalpy of neutralization for ethanoic acid and sodium hydroxide is -55.2 k] moh because some of the energy released on neutralization is used to ionize or dissociate the acid ... 

Strong and weak acids can also be distinguished by measuring and comparing their enthalpies of neutralization (Chapter 5). 

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