Enthalpy of nitrating mixtures

From diagrams it is possible to calculate the relative enthalpies of nitrating mixtures and of the heats generated during the mixing of acids. The following examples illustrate how to use the diagrams ... 

Nitrating mixtures Composition of nitrating mixtures Utilization of spent acid Enthalpy of nitrating mixtures Heats of dilution Mixing the acids... 

We can therefore report that AH = —208 kj because the enthalpy of the reaction mixture decreases by 208 kj in this reaction (Fig. 6.18). An endothermic process absorbs heat, and so when ammonium nitrate dissolves in water the enthalpy of the system increases (Fig. 6.19). Note that AH < 0 for exothermic reactions, whereas AH > 0 for endothermic reactions. 

Thermal Properties of Nitration Acids. Heals of Solution. To determine the heat evolved during the actual process of nitration of a hydrocarbon by mixed acids, it is necessary to consider not only the heat of nitration but also various heats of solution. These may be obtained from the enthalpy chart developed by McKinley and Brown (Fig. 4-5). For each of the three components, the enthalpy is taken as zero at the standard state, consisting of the pure liquid component at a temperature of 32 F and a pressure of 1 atm. Plotted against the same abscissa but against different ordinates are the specific heat data for the system. From this figure, containing both 32 F relative enthalpies and specific heats, the enthalpy of any liquid mixtures of these components at ordinary temperatures can be readily calculated by reading the desired relative enthalpy at 32 F and the specific heat from the chart. 

FIG. 1. Enthalpies of mixing, AH, and enthalpy interaction parameters, AHM/N1N2, in liquid potassium nitrate-lithium nitrate mixtures at 3 5 C. (From Kleppa and Hersh ). 

FIG. 2. Plot of enthalpy interaction parameters (evaluated at Ni = No = 0.5) against 6 = [(d - d2)/(di + d2)]- for binary liquid alkali nitrate mixtures. (From Kleppa and HershlO). 

Hersh and Kleppa found that to a first approximation the resixlts for the binary alkali nitrate, chloride and bromide mixtures could be represented by the same coefficient Q in Eq. (9) This was interpreted to indicate that the principal contribution to this term is Coulombic in origin, and that in these salts nearest-neighbour cation-anion polarization makes only a modest contribution to the enthalpy of mixing. 

C) Commercial instant ice packs are available that contain a mixture of ammonium nitrate and water separated by a barrier. When the ice pack is twisted, the barrier breaks and the two substances mix. The temperature rapidly decreases as the ammonium nitrate dissolves in the water. Describe the changes in enthalpy, entropy, and free energy during this process. 

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