Intermolecular+Forces

toc

=INTERMOLECULAR FORCES=  Forces of attraction and repulsion between molecules of matter ( see MOLECULE, ). Molecular behavior depends to a great extent on the balance (or lack of it) of the forces that pull the molecules together, or push them apart, and a study of these phenomena has been an important part of the development of physical chemistry in the 20th century ( see CHEMISTRY, PHYSICAL, ).The forces of attraction account for the COHESION, (q.v.) of molecules in the liquid and solid states of matter, and are called long-range forces or van der Waals forces after the Dutch physicist Johannes Diderik van der Waals. These forces are responsible for many physical and chemical phenomena such as ADHESION,, FRICTION,, DIFFUSION,, SURFACE TENSION,, and VISCOSITY, (qq.v.).The phenomenon of short-range or repulsive forces, which occurs between molecules that do not interact chemically, is more complex. These forces tend to become significant when the electron clouds of the molecules overlap, and the corresponding energy is called exchange or overlap energy. The repulsive exchange energy is responsible for the mechanical rigidity or impenetrability of molecules and limits of compressibility of matter. Exchange forces can also be attractive, as with atoms that have a chemical reaction with one another. These attractive exchange forces are called valence forces, and they play an important part in the whole field of chemistry. See also ATOM AND ATOMIC THEORY.; CHEMICAL REACTION,; GASES,.

Video Example:
media type="custom" key="3640875"

COHESION
P henomenon of intermolecular forces holding particles of a substance together. Cohesion differs from adhesion in being the force of attraction between adjacent particles within the same body; adhesion is the interaction between the surfaces of different bodies. The force of cohesion in gases can be observed in the liquefaction of a gas, which is the result of a number of molecules being pressed together to produce forces of attraction high enough to give a liquid structure.Cohesion in liquids is reflected in the surface tension caused by the unbalanced inward pull on the surface molecules, and also in the transformation of a liquid into a solid state when the molecules are brought sufficiently close together. Cohesion in solids depends on the pattern of distribution of atoms, molecules, and ions, which in turn depends on the state of equilibrium (or lack of it) of the atomic particles. In many organic compounds, which form molecular crystals, for example, the atoms are bound strongly into molecules, but the molecules are bound weakly to each other.

Video Example of Cohesion and Surface Tension Video
media type="custom" key="3640895"

Attraction between the surfaces of two bodies. The term is sometimes used to denote the tendency of two adjacent surfaces, which may be of different chemical compositions, to cling to each other, whereas cohesion is used to refer to the attraction between portions of a single body. For example, if a sheet of glass is lowered into water and withdrawn, some water will cling to the glass (adhesion) but the rest will be pulled back into the main body of water (cohesion).The force of attraction is attributed to electromagnetic interactions produced by fluctuations in the distribution of electrons in the molecules of the facing surfaces. The distance between the molecules of the facing surfaces is a determining factor in the amount of force exerted. A surface that may appear smooth to the naked eye actually may be too rough to hold its molecules close enough to a facing surface to produce an electromagnetic bond. Gauge blocks, used for taking accurate measurements, have such smooth surfaces that their facing surfaces can be made to stick to each other by twisting them together.In the human body, when tissues or organs that are normally separated unite or grow together, the process is called adhesion. It may occur after inflammation or during healing after a surgical operation.

How is this an example of Adhesion?
media type="custom" key="3640901"

CAPILLARY ACTION
The movement of a liquid along the surface of a solid caused by the attraction of molecules of the liquid to the molecules of the solid.**//A Closer Look//** The paper towel industry owes its existence to //capillary action,// both for the way paper towels soak up liquids and for the trees out of which the towels are made. Molecules of water are naturally attracted to each other and form temporary hydrogen bonds with each other; their attraction for each other on the surface of a liquid, for example, gives rise to //surface tension.// But they are also attracted in a similar way to other molecules, called //hydrophilic// molecules, such as those in the sides of a narrow glass tube inserted into a cup of water, in the fibers of a towel, or in the cells of tree tissue known as //xylem//. These attractive forces can draw water upward against the force of gravity to a certain degree. However, they are not strong enough to draw water from the roots of a tree to its highest leaves. An additional, related force, referred to as //transpiration pull,// is required to do that. As water evaporates from the tiny pores, or //stomata,// of leaves, water from adjacent cells is drawn in to replace it by //osmosis.// Again, intermolecular attractive forces cause other water molecules to follow along, ultimately drawing water up from the roots of the tree. The American Heritage® Science Dictionary Copyright © 2005 by Houghton Mifflin Company. Published by [|Houghton Mifflin Company]. All rights reserved.

Read this Wikipedia example and tell us if the information is accurate!
http://en.wikipedia.org/wiki/Capillary_action

Condition existing at the free surface of a liquid, resembling the properties of an elastic skin under tension. The tension is the result of intermolecular forces exerting an unbalanced inward pull on the individual surface molecules; this is reflected in the considerable curvature at those edges where the liquid is in contact with the wall of a vessel. More specifically, the tension is the force per unit length of any straight line on the liquid surface that the surface layers on the opposite sides of the line exert upon each other. See COHESION, .The tendency of any liquid surface is to become as small as possible as a result of this tension, as in the case of mercury, which forms an almost round ball when a small quantity is placed on a horizontal surface. The near-perfect spherical shape of a soap bubble, which is the result of the distribution of tension on the thin film of soap, is another example of this force; surface tension alone can support a needle placed horizontally on a water surface. See also CAPILLARITY, .Surface tension is important at zero gravity, as in space flight: Liquids cannot be stored in open containers because they run up the vessel walls.

Surface Tension Video
media type="custom" key="3640923"

More video examples
http://www.expertvillage.com/video/9867_surface-tension-demo.htm

Web Resources
http://hyperphysics.phy-astr.gsu.edu/Hbase/surten.html