Water intermolecular attraction

The flushing action of the toilet commences when the bowl is filled from a reservoir until the water level tops the inverted-U-shaped trap and starts to spill over the other side. Once water starts to flow over the side of the trap, it pulls the rest of the water with it, as a chain would pull the links that are attached to it down to a lower level. With water, intermolecular attractions form the chain that pulls the water molecules one after another. This siphoning action has to initiate quickly so that water will surge over the trap and make a moving seal. 

In the case of nonionic but polar compounds such as sugars, the excellent solvent properties of water stem from its ability to readily form hydrogen bonds with the polar functional groups on these compounds, such as hydroxyls, amines, and carbonyls. These polar interactions between solvent and solute are stronger than the intermolecular attractions between solute molecules caused by van der Waals forces and weaker hydrogen bonding. Thus, the solute molecules readily dissolve in water. 

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. 

The graph in Figure 11-37 shows that adding heat to boiling water does not cause the temperature of the water to increase. Instead, the added energy is used to overcome intermolecular attractions as molecules leave the liquid phase and enter the gas phase. Other two-phase systems, such as an ice-water mixture, show similar behavior. 

Due to the neighbourhood of secondary alcohol groups and remaining hydro-phobic acetyl groups in a not fully hydrolysed polymer, a balanced situation results that dictates the overall water solubility. Temperature plays an important role in that interplay between the intermolecular attracting forces and the polymer water interaction. An optimum in cold water solubility can be observed with a DH of 87-89 mol% for molecular weights between 25,000 and 100,000 Da (degree of polymerisation, DP, 600-2,400). 

These partial charges, or dipoles, can lead to intermolecular attractions that play an important role in such physical properties as melting point and boiling point, and they are quite important in determining solubility as well. The boiling point of water, 100°C, is quite high when compared to values for other small molecules (Table 2.4). 

This high boiling point for water can be attributed to strong intermolecular attractions (called "dipole-dipole interactions") of the type... 

Another characteristic dependent on the intermolecular forces is the surface tension of the liquid. Surface tension results from the unbalanced forces on molecules at the surface of a liquid. Figure 8.11 shows how surface tension results from these unbalanced forces. Consider water as the liquid in Figure 8.11. A water molecule in the interior of the liquid is surrounded on all sides by other water molecules. Attractive intermolecular forces pull the molecule equally in all directions and these forces balance out. A water molecule on the surface experiences an unbalanced force toward the interior of the fluid. This unbalanced force pulls on the surface of the water putting it under tension. This situation is similar to the tightening of the head of drum. The tension causes the surface of the water to act like a thin film. If you carefully use tweezers to place a clean needle on the surface of water, surface tension will allow the needle to float even though the needle is denser than water. 

The mixing of rubbing alcohol and water is an exothermic process, as evidenced by the warmth you feel upon combining the two. At the molecular level, hydrogen bonds are being formed between alcohol molecules and water molecules. Recall from Section 7.1 that the hydrogen bond is a molecule-to-molecule attraction. It is the formation of these intermolecular attractions between alcohol and water molecules that results in the release of heat. 

The intermolecular attractions between the hydrocarbon chains in the interior of the micelle represent an energetically favourable situation but it is not one which is significantly more favourable than that which results from the alternative hydrocarbon-water attraction in the case of single dissolved surfactant molecules. Comparison of the surface tension of a typical hydrocarbon oil with the dispersion component of the surface tension of water (as discussed on page 67) illustrates this point. 

Solutions tend to form when the intermolecular attractive forces between solute and solvent molecules are about as strong as those that exist in the solute alone or in solvent alone. NaCI dissolves in water because ... 

The intermolecular attraction between solute and solvent molecules is known as solvation. When the solvent is water, it is known as hydration. The figure to the left shows a hydrated Na+ ion. 

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