Hydrogen molecule, dissociation energy

There are many compounds in existence which have a considerable positive enthalpy of formation. They are not made by direct union of the constituent elements in their standard states, but by some process in which the necessary energy is provided indirectly. Many known covalent hydrides (Chapter 5) are made by indirect methods (for example from other hydrides) or by supplying energy (in the form of heat or an electric discharge) to the direct reaction to dissociate the hydrogen molecules and also possibly vaporise the other element. Other known endothermic compounds include nitrogen oxide and ethyne (acetylene) all these compounds have considerable kinetic stability. 

A mixture of hydrogen gas and a hydrocarbon, where the hydrocarbon is 1 to 2% of the mixture, at a pressure less than or equal to 1 atm is subjected to an activation source. This activation source could be a plasma produced with argon, a hot filament, or the heat of combustion produced in a torch using the hydrogen-hydrocarbon mixture as a fuel, to name only a few. This activation energy dissociates the hydrogen molecule into hydrogen atoms ... 

Figure 2 Schematic potential energy curve for the hydrogen molecules with scale at bottom of the curve exaggerated to show relation between n = 0 vibrational energy levels of the four isotopic forms of the molecules. Note that molecules containing a heavy isotope are more stable (have higher dissociation energies) than molecules with a light isotope. Isotope fractionations between molecules are explained by differences in their zero-point energies...

The difference between the zero point energies of isotopomers of the other molecules in the table is much smaller than between the hydrogen isotopomers. Thus the difference between the zero point energies of Li Li and Li2 is less than 0.1 kJ mol unless accuracies greater than 0.1 kJ mol are required, we may disregard isotope effects on dissociation energies for molecules that do not contain hydrogen. 

The amount of energy required to dissociate a hydrogen molecule H2 to two separate hydrogen atoms is its bond dissociation enthalpy. For H2 it is quite large, amounting to -1-435 kJ/mol (-1-104 kcal/mol). The main contributor to the strength of the covalent bond in H2 is the increased Coulombic force exerted on its two electrons. Each electron in H2 feels the attractive force of two nuclei, rather than one as it would in an isolated hydrogen atom. 

Resonance theory can also account for the stability of the allyl radical. For example, to form an ethylene radical from ethylene requites a bond dissociation energy of 410 kj/mol (98 kcal/mol), whereas the bond dissociation energy to form an allyl radical from propylene requites 368 kj/mol (88 kcal/mol). This difference results entirely from resonance stabilization. The electron spin resonance spectmm of the allyl radical shows three, not four, types of hydrogen signals. The infrared spectmm shows one type, not two, of carbon—carbon bonds. These data imply the existence, at least on the time scale probed, of a symmetric molecule. The two equivalent resonance stmctures for the allyl radical are as follows ... 

The species H2 and H3+ are important as model systems for chemical bonding theory. The hydrogen molecule ion H2+ comprises 2 protons and 1 electron and is extremely unstable even in a low-pressure gas discharge system the energy of dissociation and the intemuclear distance (with the corresponding values for H2 in parentheses) are ... 

FIGURE 2.18 The bond dissociation energies of the hydrogen halide molecules in kilojoules per mole of molecules. Note how the bonds weaken as the halogen atom becomes larger. 

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