Water bond energy

The heats of formation of the gaseous atoms, 4, are not very different clearly, it is the change in the bond dissociation energy of HX, which falls steadily from HF to HI, which is mainly res ponsible for the changes in the heats of formation. 6. We shall see later that it is the very high H—F bond energy and thus the less easy dissoeiation of H—F into ions in water which makes HF in water a weak aeid in comparison to other hydrogen halides. 

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. 

FIGURE 2.3. The energetics of a heterolytic bond cleavage reaction in a polar solvent. The specific example shown corresponds to the CH3OCH3— CH3 + CH30 reaction in water. The energy of the covalent state does not include the effect of the solvent on this state, but a more consistent treatment (e.g., eq. (2.21) should account for the polarization of the solvent toward the charges of the ionic state. This would result in destabilization of H31. 

In formulating a set of bond-energy values we first calculated the energies of formation of molecules from experimental values of the heats of combustion of the compounds6 and thermochemical data pertaining to the products of combustion—carbon dioxide, water, nitrogen, etc. The same values for the latter quantities were used as previously.4... 

There are two kinds of bond energy. The energy necessary to cleave a bond to give the constituent radicals is called the dissociation energy D. For example, D for H2O—>HO -f H is 118 kcal mol (494kJmol ). However, this is not taken as the energy of the O—H bond in water, since D for H—O H -f O is 100 kcal... 

Bond energies can be used to estimate the energy change that occurs in a chemical reaction. The reaction of molecular hydrogen with molecular oxygen to form gaseous water provides a simple example ... 

The H+-H2O bond has the special feature that although the bond energy is high, the proton will readily hop from one water molecule in the hydration complex to a neighboring water molecule. This hop is a quantum motion and will occur only when the water molecules have a favorable mutual orientation. It will occur predominantly... 

Almost all living creatures require oxygen to act as the ultimate electron acceptor in a series of chemical reactions. In these, oxygen is reduced to the level of water and the bond energy of the substrates thus concommi-tandy oxidized is liberated. Oxygen is able to perform these functions because it can be progressively oxidized by successive one-electron additions, but it is this property that provides the basis for the toxicity associated... 

The end phosphate adds water and is transferred onto another compound, causing thereby the phosphorylation of the latter. An alternative route for the phosphate bond energy release is exemplified by pyrophosphate cleavage of ATP ... 

Fig. 20.11. Substrate quality obtained by comparing basolateral-to-apical with apical-to-basolateral transport of substrates in polarized cell monolayers of MDR1-transfected cell lines [86] plotted versus (A) the log of the air/water partition coefficient, or (B) H-bond energy (arbitrary units, EUh cf. text). Units of the air/ water partition coefficient were [M ]. Compound (concentrations in Ref. [86] in brackets) were clozapine (50 nM) (1) cyclosporin A (2 tM) (2) daunorubicin (3) dexamethasone (2 tM) (4) digoxin (2 pM) (5) domperidone (2 pM) (6) etoposide (7) flunitrazepam (500 nM) (8) haloperidol (50 nM) (9) ivermectin (50 nM) (10) loperamide (2 pM) (11) morphine (2 pM) (12) ondansetron...

When ethanol was substituted for propane, fewer moles of oxygen were required, but fewer moles of carbon dioxide and water were produced. The bond energies of the reactants decreased by (6486 kJ - 4726 kJ) = 1760 kJ, but the bond energies of the products decreased even more by (8498 kJ - 5974 kJ) = 2520 kJ. Therefore, we can deduce that the combustion of ethanol is less exothermic than that of propane and the other alkanes. 

A physical chemist reading from a data book learns that 40.7 kJ mol-1 of energy are liberated when 1 mol of water condenses and will translate this information to say that when 1 mol (18 g) of steam condenses to form liquid water, bonds form concurrently with the liberation of 40700 J of energy. 

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