Atomization, enthalpy change thermodynamics

The modem process for manufacturing nitric acid depends on the catalytic oxidation of NH3 over heated Pt to give NO in preference to other thermodynamically more favour products (p. 423). The reaction was first systematically studied in 1901 by W. Ostwald (Nobel Prize 1909) and by 1908 a commercial plant near Bochum. Germany, was producing 3 tonnes/day. However, significant expansion in production depended on the economical availability of synthetic ammonia by the Haber-Bosch process (p. 421). The reactions occurring, and the enthalpy changes per mole of N atoms at 25 C are ... 

There are several ways of converting H29% to Af//29g and subsequently to Ahyd//298. The atomization method is illustrated in Fig. 3.9, which shows a thermochemical cycle for determination of Af//29g of a hydrocarbon. The top horizontal line represents the thermodynamic state of nuclei and electrons, the bottom horizontal line represents elements in their standard states and the verticals, of which there are six, represent enthalpy changes. Each of the three total enthalpy changes in the top half of the figure H29i represents a fall from the top state to the state of... 

When the lattice is formed from the gaseous ions the lattice enthalpy of NaCI is released, Le. AH = -784 kJ mol", thus overcoming the imbalance of the ionization processes to make the compound thermodynamically stable. There is a significant, but relatively small, contribution from van der Waals forces to the lattice enthalpy. The enthalpy changes accompanying the formation of NaQ from its elements are shown in Figure 5.8. To transform the elements into their gaseous atomic states the enthalpies of atomization are included in the formation process. 

A proper thermodynamic description of electrochemical phase formation is necessarily complicated by the need to consider the changes in energy and entropy associated with the process as a whole. Fortunately, electrocrystallisation has many features in common with the growth of a solid phase from its supersaturated vapour, and it is useful to examine this simpler process first. In the case where two phases but only one component are present, phase formation involves the incorporation of atoms or molecules into a pre-existing surface of the solid, and the enthalpy change is therefore related to the transition... 

A thermodynamic breakdown of the standard enthalpy change for the conversion of elemental halogens into halogen atoms at 25 °C. 

Entropy is a measure of the degree of randomness in a system. The change in entropy occurring with a phase transition is defined as the change in the system s enthalpy divided by its temperature. This thermodynamic definition, however, does not correlate entropy with molecular structure. For an interpretation of entropy at the molecular level, a statistical definition is useful. Boltzmann (1896) defined entropy in terms of the number of mechanical states that the atoms (or molecules) in a system can achieve. He combined the thermodynamic expression for a change in entropy with the expression for the distribution of energies in a system (i.e., the Boltzman distribution function). The result for one mole is ... 

Equation (16-2) allows the calculations of changes in the entropy of a substance, specifically by measuring the heat capacities at different temperatures and the enthalpies of phase changes. If the absolute value of the entropy were known at any one temperature, the measurements of changes in entropy in going from that temperature to another temperature would allow the determination of the absolute value of the entropy at the other temperature. The third law of thermodynamics provides the basis for establishing absolute entropies. The law states that the entropy of any perfect crystal is zero (0) at the temperature of absolute zero (OK or -273.15°C). This is understandable in terms of the molecular interpretation of entropy. In a perfect crystal, every atom is fixed in position, and, at absolute zero, every form of internal energy (such as atomic vibrations) has its lowest possible value. 

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