Attraction energy

We have now discussed three types of intermolecular forces dispersion forces, dipole forces, and hydrogen bonds. You should bear in mind that all these forces are relatively weak compared with ordinary covalent bonds. Consider, for example, the situation in HzO. The total intermolecular attractive energy in ice is about 50 kj/mol. In contrast, to dissociate one mole of water vapor into atoms requires the absorption of928 kj of energy, that is, 2(OH bond energy). This explains why it is a lot easier to boil water than to decompose it into the elements. Even at a temperature of 1000°C and 1 atm, only about one H20 molecule in a billion decomposes to hydrogen and oxygen atoms. 

Net forward rate for folding at lamellar thickness / e Pairwise nearest neighbour attractive energy... 

The attractive energies 4D(cr/r)6 and ae2/2 r4 have two important effects on the vibrational energy transfer (a) they speed up the approaching collision partners so that the kinetic energy of the relative motion is increased, and (b) they modify the slope of the repulsive part of the interaction potential on which the transition probability depends. By letting m °°, we have completely ignored the second effect while we have over-emphasized the first. Note that Equation 12 is identical to an expression we could obtain when the interaction potential is assumed as U(r) = A [exp (— r/a)] — (ae2/2aA) — D. Similarly, if we assume a modified Morse potential of the form... 

Fig. 1. The Lennard Jones 12 6 pair potential plotted for a pair of CH2 united atoms using the OPTS united force field. Enonbond = 4e((o /r) (o /r) ), where s is the well depth for the potential and cr is the distance at which the repulsive energy exactly cancels the attractive energy...

It is of interest to note that in this model the anisotropy in the attractive energy is determined by the same parameter, 7, as that controlling the anisotropy in the repulsive energy. In these expressions for the contact distance and the well depth their angular variation is contained in the three scalar products Uj Uj, Uj f and uj f which are simply the cosines of the angle between the symmetry axes of the two molecules and the angles between each molecule and the intermolecular vector. 

The electrons and nuclei in a molecule balance these three interactions in a way that gives the molecule its greatest possible stability. This balance is achieved when the electrons are concentrated between the nuclei. We view the electrons as shared between the nuclei and call this sharing a covalent bond. In any covalent bond, the attractive energy between nuclei and electrons exceeds the repulsive energy arising from nuclear-nuclear and electron-electron interactions. 

For H2 to be a stable molecule, the sum of the attractive energies must exceed the sum of the repulsive energies. Figure 9A shows a static arrangement of electrons and nuclei In which the electron-nucleus distances are shorter than the electron-electron and nucleus-nucleus distances. In this arrangement, attractive interactions exceed repulsive interactions, leading to a stable molecule. Notice that the two electrons occupy the region between the two nuclei, where they can interact with both nuclei at once. In other words, the atoms share the electrons in a covalent bond. 

Fusion is a more attractive energy source than fission. Whereas fission occurs for only a few rare, extremely heavy... 

For ion-pair formation the electrostatic attraction energy = N iZ j(Q°yi4neQer (per mole of ion pairs) should be larger than the ion pair s mean thermal energy (i.e., at least 2RT). This condition yields for the critical distance of ion pair formation in aqueous solutions at 25°C ... 

Comparing and Contrasting Nonionic compounds do not exist in crystal lattice structures but rather as individual particles, which are affected by other particles. In other words, nonionic compounds experience forces between particles. Based on what you learned in Part B about the melting points of ionic versus nonionic compounds, how do you think the attractive energy between particles compares with the energy of the crystal lattice ... 

Fig. 1 Illustration of the DLVO theory interaction of two charged particles as a function of the interparticle distance (attractive energy curve, VA, repulsive energy curve, VR and net or total potential energy curve, Vj).

Table 2.5 Attractive Energies in Simple Covalent Compounds...

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