Enthalpy atom combination

In a Born-Haber cycle, we imagine that we break apart the bulk elements into atoms, ionize the atoms, combine the gaseous ions to form the ionic solid, then form the elements again from the ionic solid (Fig. 6.32). Only the lattice enthalpy, the enthalpy of the step in which the ionic solid is formed from the gaseous ions, is unknown. The sum of the enthalpy changes for a complete Born-Haber cycle is zero, because the enthalpy of the system must be the same at the start and finish. 

The selected values for the reaction enthalpies were combined with ancillary literature data to yield the atomization energies AH° q and the standard heats of formation AHf 298,15 of the gaseous di-rare-earth carbides (table 10). 

When a mole of a compound is produced from its constituent atoms in the gas phase at 1 atmosphere pressure and 25 °C, energy is released. The standard state heat (or enthalpy) of atom combination, is the difference in enthalpy of product and... 

Table 1. Standard state enthalpies of atom combination,...

For molecules, why are all of the values for enthalpies of atom combination negative ... 

One can also imagine the process in which a mole of a substance is broken apart into its constituent gas phase atoms. This is precisely the reverse of an "enthalpy of atom combination reaction," and, in this case, energy will be consumed. For example, the value of AH° for the reaction... 

For any given chemical reaction, if the sum of the enthalpies of atom combination for all of the reactants is more negative than the sum of the enthalpies of atom combination for all of the products, will the value of A7/° for the reaction be positive or negative Explain your reasoning. 

Model 2 Bond Strength and Enthalpies of Atom Combination. 

Use enthalpies of atom combination and entropies of atom combination to determine if any of the following reactions have an equilibrium constant greater than 1. Find the reaction with the greatest equilibrium constant. Find the reaction with the smallest equilibrium constant. 

TABLE A.3 Standard-State Enthalpies, Free Energies, and Entropies of Atom Combination... 

When atoms combine to form molecules, energy is released as covalent bonds form. The molecules of the products have lower enthalpy than the separate atoms. When hydrogen atoms combine to form hydrogen molecules, for example, the reaction is exothermic, it evolves 436 kj of heat for every mole of hydrogen that is produced. Similarly, when chlorine atoms combine to form chlorine molecules, the reaction evolves 243 kJ mol of chlorine produced ... 

The two possible initiations for the free-radical reaction are step lb or the combination of steps la and 2a from Table 1. The role of the initiation step lb in the reaction scheme is an important consideration in minimising the concentration of atomic fluorine (27). As indicated in Table 1, this process is spontaneous at room temperature [AG25 = —24.4 kJ/mol (—5.84 kcal/mol) ] although the enthalpy is slightly positive. The validity of this step has not yet been conclusively estabUshed by spectroscopic methods which makes it an unsolved problem of prime importance. Furthermore, the fact that fluorine reacts at a significant rate with some hydrocarbons in the dark at temperatures below —78° C indicates that step lb is important and may have Httie or no activation energy at RT. At extremely low temperatures (ca 10 K) there is no reaction between gaseous fluorine and CH or 2 6... 

The atom and bond concepts dominate chemistry. Dalton postulated that atoms retained their identities even when in chemical combinations with other atoms. We know that their properties are sometimes transferable from one molecule to another for example, the incremental increase in the standard enthalpy of formation of a normal hydrocarbon per CHj group is —20.6 1.3 kJmol . We also know that more often there are subtle modifications to the electron density. 

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