Intermolecular forces Joining

In these compounds, as in water, the principal intermolecular forces are hydrogen bonds. When a substance like methyl alcohol dissolves in water, it forms hydrogen bonds with H20 molecules. These hydrogen bonds, joining a CH3OH molecule to an H20 molecule, are about as strong as those in the pure substances. 

Such hydrogen bonds create an intermolecular force that joins multiple water molecules together in a group and affects their behavior. Hydrogen bonds are actually just extremely strong dipole-dipole interactions. This is why it takes a lot of energy to boil water compared to other liquids that do not contain hydrogen bonds. 

Differences in properties are a result of differences in attractive forces. In a covalent compound, the covalent bond between atoms in molecules is quite strong, but the attraction between individual molecules is relatively weak. The weak forces of attraction between individual molecules are known as inter-molecular forces, or van der Waals forces. Intermolecular forces, which are discussed at length in Chapter 13, vary in strength but are weaker than the bonds that join atoms in a molecule or ions in an ionic compound. 

The primary structure of a protein is its amino acid sequence. Secondary structure is the shape defined by hydrogen bonds joining the CO and NH groups of the amino acid backbone. Tertiary and quaternary structures are the three-dimensional folded arrangements of proteins that are stabilized by hydrogen bonds and other intermolecular forces. 

Till now in this chapter we have very briefly been looking at the different types of bonds that can exist between two atoms (intramolecular forces) and at the different types of forces that can act between molecules (intermolecular forces). In the following sections we are going to look more detailed into the different types of chemical bonds. That way we among other aspects will be able to explain why it is beneficial for some atoms to join in a chemical bond and why this is not the case for other atoms. 

Figure 8.1 7 I Carbon atoms within each sheet in the graphite structure are joined by strong covalent bonds, with a fairly short C—C distance. Because each carbon atom forms four bonds within its own sheet, intermolecular forces must be responsible for holding the sheets together. This is apparent from the much larger spacing between layers. Because the forces holding the layers to one another are weak, sliding of one layer relative to the next is fairly easy, and this explains the softness and lubricating properties of graphite.

An example of the positive deviation from Raoult s law is a solution made of acetone (CH3COCH3) and carbon disulfide (CS2). (a) Draw Lewis structures of these molecules. Explain the deviation from ideal behavior in terms of intermolecular forces, (b) A solution composed of 0.60 mole of acetone and 0.40 mole of carbon disulfide has a vapor pressure of 615 mmHg at 35.2°C. What would be the vapor pressure if the solution behaved ideally The vapor pressure of the pure solvents at the same temperature are 349 mmHg for acetone and 501 mmHg for carbon disulfide, (c) Predict the sign of AT/join-... 

The forces of attraction between molecules are known as intermolecular forces. Intermolecular forces vary in strength but are generally weaker than bonds that join atoms in molecules, ions in ionic compounds, or metal atoms in solid metals. Compare the boiling points of the metals and ionic compounds in Figure 5.8 (on the next page) with those of the molecular substances listed. Note that the values for ionic compounds and metals are much higher than those for molecular substances. 

The many forms of so-called amorphous (non-crystalline) carbon such as charcoals and lampblack are all actually microcrystalline forms of graphite. The latter has a covalently bonded layer structure comprising a network of joined flat hexagonal Ce rings where the separation of the layers is reported to be 3.35A. This is about equal to the sum of the Van der Waals (intermolecular) radii, indicating that the forces between layers should be relatively slight, as is evidenced by the observed softness and lubricity of the material. 

Atoms in individual polymer molecules are joined to each other by relatively strong covalent bonds. The bond energies of the carbon-carbon bonds are on the order of 80 to 90 kcal/mol. Polymer molecules, like all other molecules, are attracted to each other by intermolecular secondary forces. 

Surface tension is defined as the force per unit length exerted by one surface. For an adhesive to adequately wet a plastic, it should have a surface tension lower than the plastic s surface tension. After intermolecular contact is achieved through good wetting, adhesion is primarily the result of formation of electrostatic forces including Van der Waak forces and hydrogen bonds. Since these bonds are approximately 10 times weaker than covalent bonds, they are only effective for a very short distance around 2-5 molecular diameters. Contact between adhesives and plastics surfaces to be joined must therefore be excellent. 

Our model repre.sentation of the oriented fiber is given in Fig. 3. The nodes in the figure represent the elementary repetition units of the polymer chains, i.e. methyl units for polyethylene. For very long chains, each node is made to correspond to more than one repetition unit (Termonia et al., 1985). The nodes are joined in the x- and z-directions by secondary bonds having an elastic constant Ki. These bonds account for the intermolecular vdW forces in polyethylene or hydrogen bonds in nylon. Only nearest-neighbor interactions are considered. In the y-direction, stronger forces with elastic constant K account for the primary bonds, i.e. C-C bonds in polyethylene. 

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