Hydrogen molecule bond enthalpy

A measure of the stability of a molecule is its bond enthalpy, which is the enthalpy change required to break a particular bond in 1 mole of gaseous molecules. (Bond enthalpies in soUds and liquids are affected by neighboring molecules.) The experimentally determined bond enthalpy of the diatomic hydrogen molecule, for example, is... 

The partial Lewis structure that follows is for a hydrocarbon molecule. In the full Lewis structure, each carbon atom satisfies the octet rule, and there are no unshared electron pairs in the molecule. The carbon-carbon bonds are labeled 1, 2, and 3. (a) Determine where the hydrogen atoms are in the molecule. (b) Rank the carbon-carbon bonds in order of increasing bond length, (c) Rank the carbon-carbon bonds in order of increasing bond enthalpy. [Sections 8.3 and 8.8]... 

The H — H bond enthalpy (436 kJ/mol) is high for a single bond. = (Table 8.4) By comparison, the Cl— Cl bond enthalpy is only 242 kj/mol. Because H2 has a strong bond, most reactions involving H2 are slow at room temperature. However, the molecule is readily activated by heat, irradiation, or catalysis. The activation generally produces hydrogen atoms, which are very reactive. Once H2 is activated, it reacts rapidly and exothermically with a wide variety of substances. 

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 ... 

Heats of reactions can be measured by the calorimetric and kinetic methods, using photo- and mass-spectrometry. Bond dissociation enthalpy calculated from the thermal effect of the reaction at ambient pressure is close to the bond energy because PV is small, for example for the hydrogen molecule PV 2.5 kJ/mol. Finally, the difference between the dissociation energy at 0 K and that at room temperature is also very small for the hydrogen molecule the difference is AE 1 kj/mol. 

The bond enthalpy (bond energy) is the amount of energy required to break one mole of a specific covalent bond between two atoms in one mole of gaseous molecules. Measurement of bond enthalpies can be performed using a mass spectrometer (Chapter 2). The concept of bond enthalpy is illustrated in Figure 5.27 using the hydrogen molecule. 

For the hydrogen molecule, the thermochemical equation describing the bond dissociation enthalpy is ... 

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. 

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