Ammonia standard enthalpy

The standard enthalpy of this reaction is equal to the difference PA(K) — PA(B). Thus, the determination of PA(K) requires PA B). The proton affinity of B will rely, in turn, on the proton affinity of some other molecule and so on. A scale of relative values of proton affinities is thus built. To derive absolute data, a reliable anchor must be found. The one most frequently used is the proton affinity of ammonia, /M(NH3), which is now accepted to be 853.6 kJ mol-1 [67]. This is in excellent agreement with the result of a benchmark calculation by Martin and Lee,P (NH3) = 853.1 1.3 kJmol-1 [70],... 

The most stable state of nitrogen in acidic solution is the ammonium ion, NH4(aq), which is isoelectronic with CH4 and H30+. It is a tetrahedral ion with strong N-H bonds. The mean N-H bond enthalpy in NH4(aq) is 506 kJ mol 1 (that of the O-H bonds in H30 + is 539 kJ mol" ). The enthalpy of hydration of the ammonium ion is — 345 kJ mol V This value placed into the Born equation (3.32) gives an estimate of the radius of the ammonium ion of 135 pm, a value insignificantly different from its thermochemical radius of 136 pm. The value is comparable to that estimated for the smaller H30+ ion (99 pm) from its more negative enthalpy of hydration (— 420 kJ mol -see Section 2.6.1). The proton affinity of the ammonia molecule is of interest in a comparison of its properties with those of the water molecule. The proton affinity is defined as the standard enthalpy change for the reaction ... 

In the synthesis of ammonia by the reaction N2(g) + 3H2(g) - 2NH3(g), which is crucial to the economies and well-being of all nations, the standard enthalpy of reaction is —92.22 kj-mol 1 at 298 K. However, the industrial synthesis is run at 450°C. What is the standard reaction enthalpy at the latter temperature ... 

STRATEGY The synthesis of ammonia is exothermic, so we expect the equilibrium constant to be smaller at the higher temperature. To use the van t Hoff equation, we need the standard reaction enthalpy, which can be calculated from the standard enthalpies of formation in Appendix 2A. 

Because the free-energy change for any process at constant temperature and pressure is AG = AH — TAS, we can calculate the standard free-energy change AG° for a reaction from the standard enthalpy change AH0 and the standard entropy change AS°. Consider again the Haber synthesis of ammonia ... 

Using the standard enthalpies of formation listed in Table 9.4, calculate the standard enthalpy change for the overall reaction that occurs when ammonia is burned in air to form nitrogen dioxide and water. This is the first step in the manufacture of nitric acid. 

Use the following standard enthalpies of formation to estimate the N—H bond energy in ammonia. Compare this with the value in Table 13.6. 

The resulting ionic compound is a white solid which melts between 373 and 375 °C (and decomposes to the imide and ammonia above this upper limit as noted above, eventually decomposing to lithium nitride). Experimentally, the standard enthalpy of formation for lithium amide, AHf, is -176 kJ mol ... 

Fig. 6.40 Schematic illustration of the process whereby hydrogen can be transformed into ammonia, which is stored as Mg(NH3)6Cl2. The Mg(NH3)6Cl2 can be transported safely and when needed ammonia is released and decomposed into molecular hydrogen and nitrogen. The standard enthalpies of formation and decomposition are indicated [243]. If all...

The value of AH is an approximate measure of the stability of a substance relative to the elements from which it is made. The standard enthalpies of formation of graphite, diamond, water, ethyne (acetylene, C2H2), ammonia and sodium chloride are shown in Fig. 13.6. The reference states of elements define an energy baseline or sea level . Compounds such as ethyne, for which AHf is positive, and which therefore possess a greater enthalpy than their constituent elements, appear above sea level and are called endothermic compounds. Compounds such as water, ammonia and sodium chloride, for which AHf is negative and which therefore possess a lower enthalpy than their constituent elements, appear below sea level and are called exothermic compounds. 

Calculate the enthalpy of reaction for the ammonia synthesis at 450 C and a pressure of 600 atm using the value of the standard enthalpy of reaction at 450 °C in the ideal gas state calculated in Example 12.1. For the calculation of the residual enthalpies, the group contribution equation of state volume-translated Peng-Robinson (VTPR) should be applied. The required VTPR parameters are given in Appendix K. 

A copy of Appendix 8 of Catalyst Handbook is presented, the nomograph of selected properties of ammonia. Also given are standard free energy of formation, standard enthalpy of formation, and standard state entropy of ammonia. Numerical data are from D. R. Stull, E. F. Westrum Jr., and G. C. Sinke, The Chemical Thermodynamics of Organic Compounds, Wiley, New York (1969). 

Ammonia borane dissolves in water to form a solution stable in the absence of air. However, hydrogen is released upon its acid catalyzed hydrolysis in aqueous solution (Eq. (7.2)) [124]. The reaction in Eq. (7.2) is exothermic with a standard enthalpy change of — 156kJ/ mol at 25 C [125]. 

The entropy of a substance, unlike its enthalpy, can be evaluated directly. The details of how this is done are beyond the level of this text, but Figure 17.4 shows the results for one substance, ammonia. From such a plot you can read off the standard molar entropy at 1 atm pressure and any given temperature, most often 25°C. This quantity is given the symbol S° and has the units of joules per mole per kelvin (J/mol-K). From Figure 17.4, it appears that... 

In the manufacture of nitric acid by the oxidation of ammonia, the first product is nitric oxide, which is then oxidized to nitrogen dioxide. From the standard reaction enthalpies... 

For a long time efforts to improve the efficiency of industrial ammonia production concentrated on synthesis gas production, and major progress was achieved over the years. In ammonia synthesis itself considerable progress was made in converter design and recovery of the reaction enthalpie at high temperature, but there has been no substantial improvement in the catalyst since the 1920s. The standard commercial iron... 

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