If molecules stick together more, they'll be tougher to break apart; Stronger intermolecular forces → higher melting and boiling points. Viscosity. Viscosity is a . For Review. 1. Intermolecular forces are the relatively weak forces between molecules that hold the The electrostatic attraction between As the strengths of intermolecular forces increase, surface tension, viscosity, melting point and boiling . liquid line shows a direct relationship between the melting point and pressure. Nov 6, Viscosity is more related to IM forces than collisions. The stronger the intermolecular force, the more the molecules will the attracted to one another, leading to a.
Cohesive forces bind molecules of the same type together Adhesive forces bind a substance to a surface The same phenomenon holds molecules together at the surface of a bulk sample of water, almost as if they formed a skin. When filling a glass with water, the glass can be overfilled so that the level of the liquid actually extends Capillary action is the net result of two opposing sets of forces: Water has both strong adhesion to glass, which contains polar SiOH groups, and strong intermolecular cohesion.
When a glass capillary is put into water, the surface tension due to cohesive forces constricts the surface area of water within the tube, while adhesion between the water and the glass creates an upward force that maximizes the amount of glass surface in contact with the water. If the adhesive forces are stronger than the cohesive forces, as is the case for water, then the liquid in the capillary rises to the level where the downward force of gravity exactly balances this upward force.
11.4: Intermolecular Forces in Action: Surface Tension, Viscosity, and Capillary Action
The upper surface of a liquid in a tube is called the meniscus, and the shape of the meniscus depends on the relative strengths of the cohesive and adhesive forces. Capillary action of water compared to mercury, in each case with respect to a polar surface such as glass. Differences in the relative strengths of cohesive and adhesive forces result in different meniscus shapes for mercury left and water right in glass tubes.
Mark Ott Polar substances are drawn up a glass capillary and generally have a concave meniscus. Fluids and nutrients are transported up the stems of plants or the trunks of trees by capillary action. Plants contain tiny rigid tubes composed of cellulose, to which water has strong adhesion.
Because of the strong adhesive forces, nutrients can be transported from the roots to the tops of trees that are more than 50 m tall. The moisture is absorbed by the entire fabric, not just the layer in contact with your body. Some liquids, such as gasoline, ethanol, and water, flow very readily and hence have a low viscosity.
Others, such as motor oil, molasses, and maple syrup, flow very slowly and have a high viscosity. The two most common methods for evaluating the viscosity of a liquid are 1 to measure the time it takes for a quantity of liquid to flow through a narrow vertical tube and 2 to measure the time it takes steel balls to fall through a given volume of the liquid.
The higher the viscosity, the slower the liquid flows through the tube and the steel balls fall. The viscosities of some representative liquids are listed in Table Because a liquid can flow only if the molecules can move past one another with minimal resistance, strong intermolecular attractive forces make it more difficult for molecules to move with respect to one another.
This effect is due to the increased number of hydrogen bonds that can form between hydroxyl groups in adjacent molecules, resulting in dramatically stronger intermolecular attractive forces. There is also a correlation between viscosity and molecular shape.
Liquids consisting of long, flexible molecules tend to have higher viscosities than those composed of more spherical or shorter-chain molecules. London dispersion forces also increase with chain length. Due to a combination of these two effects, long-chain hydrocarbons such as motor oils are highly viscous. Viscosity increases as intermolecular interactions or molecular size increases.
Motor Oils Motor oils and other lubricants demonstrate the practical importance of controlling viscosity. Viscosity decreases rapidly with increasing temperatures because the kinetic energy of the molecules increases, and higher kinetic energy enables the molecules to overcome the attractive forces that prevent the liquid from flowing. So-called single-grade oils can cause major problems. If they are viscous enough to work at high operating temperatures SAE 50, for examplethen at low temperatures, they can be so viscous that a car is difficult to start or an engine is not properly lubricated.
Of course, the arrangement is much more ordered in the solid than in the liquid where the molecules are in constant motion. The larger the dipole, the larger the attractive force. Physical properties, such as boiling point and melting point, reflect the size of the intermolecular forces the larger the forces, the higher the boiling point, etc.
Effects of Intermolecular Forces - ChemistNATE | Lessons
This is all due to the intermolecular attractions between molecules. These interactions depend on two factors: Also, cations interact more strongly with dipoles because they tend to be smaller. Since there are also repulsive forces between ions and molecules electrons do not want to get too close to each other, and nuclei do not want to get too close to each otherand since these ions and molecules are already relatively close together in the liquid and very close in the solid state, then we can understand that there is little room for compressibility.
Compare this to gases.
Dispersion Forces Nonpolar molecules have uniform and symmetrical distribution of electrons and therefore, no dipole moment. However, if we think that the electrons have freedom to move about, we can think that at any given time, the molecule can have a nonuniform distribution of electron density, giving it a fleeting temporary dipole, called an instantaneous dipole.