how do intermolecular forces affect viscosity

A dirty car is covered with a mixture of substances, some of which are polar. These properties are achieved by a careful blend of additives that modulate the intermolecular interactions in the oil, thereby controlling the temperature dependence of the viscosity. 11.4: Intermolecular Forces in Action- Surface Tension, Viscosity, and Capillary Action, 11.3: Intermolecular Forces- The Forces that Hold Condensed Phases Together. Liquids with high viscosity, like honey, are commonly called “thick.”. Because a sphere has the smallest possible surface area for a given volume, intermolecular attractive interactions between water molecules cause the droplet to adopt a spherical shape. around the world, How Intermolecular Forces Affect Phases of Matter. Unless otherwise noted, LibreTexts content is licensed by CC BY-NC-SA 3.0.

Of course, viscosity is resistance of flow and surface tension of lateral force but viscosity is s display of frictional force during laminar flow caused due to intermolecular forces. Attractive interactions between the polar substances and water cause the water to spread out into a thin film instead of forming beads.

Intermolecular forces also cause a phenomenon called capillary action, which is the tendency of a polar liquid to rise against gravity into a small-diameter tube (a capillary), as shown in Figure \(\PageIndex{3}\). How do intermolecular forces affect viscosity? The bonds in honey are hydrogen and oxygen form hydrogen bonding, which are one of the strongest bonds, and this causes strong cohesion forces between them and it causes the flow of honey to be slow/viscous. What will be the shape of the meniscus (convex or concave)? •Viscosity (the resistance of a liquid to flow) increases. Legal.

Adding soaps and detergents that disrupt the intermolecular attractions between adjacent water molecules can reduce the surface tension of water. Capillary action is the phenomenon in which liquids rise up into a narrow tube called a capillary. This is even observable in the zero gravity conditions of space as shown in Figure \(\PageIndex{2}\) (and more so in the video link) where water wrung from a wet towel continues to float along the towel's surface! As a result, an oil that is thin enough to be a good lubricant in a cold engine will become too “thin” (have too low a viscosity) to be effective at high temperatures. Capillary action will pull the ethylene glycol up into the capillary. Typically much stronger than intermolecular forces. The bonds hold the water molecules together, and intermolecular forces make all of the water molecules stick together. 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. Intermolecular Forces: forces between molecules.

Will the oil be pulled up into the tube by capillary action or pushed down below the surface of the liquid in the beaker? What intermolecular forces are present in #CH_3F#? Surface tension is the energy required to increase the surface area of a liquid by a given amount. Higher the intermolecular forces between the liquid particles, harder it is for it to escape into the vapor phase, ie., you need more energy to convert it from liquid to the vapor phase, in other words, higher its boiling point. The height to which the water rises depends on the diameter of the tube and the temperature of the water but not on the angle at which the tube enters the water.

Because they affect the surface properties of a liquid, soaps and detergents are called surface-active agents, or surfactants. The stronger the intermolecular interactions, the greater the surface tension. Hence raindrops are almost spherical, and drops of water on a waxed (nonpolar) surface, which does not interact strongly with water, form round beads. The smaller the diameter, the higher the liquid rises. A typical molecule in the interior of the droplet is surrounded by other molecules that exert attractive forces from all directions. This maximizes the number of attractive interactions and minimizes the number of water molecules at the surface. If, however, the cohesive forces are stronger than the adhesive forces, as is the case for mercury and glass, the liquid pulls itself down into the capillary below the surface of the bulk liquid to minimize contact with the glass (Figure \(\PageIndex{4}\)). Liquids consisting of long, flexible molecules tend to have higher viscosities than those composed of more spherical or shorter-chain molecules. From the strength of this interaction, predict the behavior of the oil and the shape of the meniscus. When filling a glass with water, the glass can be overfilled so that the level of the liquid actually extends above the rim. 9434 views

Figure \(\PageIndex{1}\) presents a microscopic view of a liquid droplet.

Chemical Bonds: forces within molecules. Determine whether the forces interact with the surface of glass.

When dealing with a solid solute and a liquid solvent, the solute dissolves when the molecules of both are similar enough in polarity. As a result, the relative velocity between two adjacent layers decreases, hence the coefficient of viscosity increases. Many of the commercially available oil additives “for improved engine performance” are highly viscous materials that increase the viscosity and effective SAE rating of the oil, but overusing these additives can cause the same problems experienced with highly viscous single-grade oils. How do intermolecular forces affect solvation? 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. How do intermolecular forces affect freezing point?

Cohesive forces bind molecules of the same type together, Adhesive forces bind a substance to a surface. Based on the nature and strength of the intermolecular cohesive forces and the probable nature of the liquid–glass adhesive forces, predict what will happen when a glass capillary is put into a beaker of SAE 20 motor oil. What intermolecular forces are present in #NH_3#? The higher the viscosity, the slower the liquid flows through the tube and the steel balls fall. The viscosity of a liquid is its resistance to flow. The same phenomenon holds molecules together at the surface of a bulk sample of water, almost as if they formed a skin. Consequently, most modern oils are multigrade, with designations such as SAE 20W/50 (a grade used in high-performance sports cars), in which case the oil has the viscosity of an SAE 20 oil at subzero temperatures (hence the W for winter) and the viscosity of an SAE 50 oil at high temperatures. 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. In this diagram, the solid connections are chemical bonds and the dashed connections are intermolecular forces. Predict what will happen when a glass capillary is put into a beaker of ethylene glycol. (Hint: the surface of glass is lined with Si–OH groups. Surface tension is the energy required to increase the surface area of a liquid by a given amount. The oil will have a convex meniscus similar to that of mercury. Cohesion is the attraction in a molecule of liquid to each other due to intermolecular forces.

Weaker than chemical bonds. The meniscus will be concave. Viscosity is proportional to the strength of intermolecular forces holding molecules together. ), Given: substance and composition of the glass surface, Asked for: behavior of oil and the shape of meniscus. The cohesive forces responsible for its high boiling point are almost solely London dispersion forces between the hydrocarbon chains. Cohesion is the attraction in a molecule of liquid to each other due to intermolecular forces. The bonds in honey are hydrogen and oxygen form hydrogen bonding, which are one of the strongest bonds, and this causes strong cohesion forces between them and it causes the flow of honey to be slow/viscous. Thicker liquids tend to have a higher viscocity. What does “like dissolves like” mean? Such foams are now used universally to fight large-scale fires of organic liquids. London dispersion forces also increase with chain length. Why do intermolecular forces tend to attract? Let's take honey as an example. Others, such as motor oil, molasses, and maple syrup, flow very slowly and have a high viscosity. The oil in an automobile engine must effectively lubricate under a wide range of conditions, from subzero starting temperatures to the 200°C that oil can reach in an engine in the heat of the Mojave Desert in August. In contrast, a molecule on the surface experiences a net attraction toward the drop because there are no molecules on the outside to balance the forces exerted by adjacent molecules in the interior. 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. The viscosities of some representative liquids are listed in Table 11.3.1 and show a correlation between viscosity and intermolecular forces. Although you have been introduced to some of the interactions that hold molecules together in a liquid, we have not yet discussed the consequences of those interactions for the bulk properties of liquids. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. Viscocity is a liquid resistance to flowing. So-called single-grade oils can cause major problems. Due to a combination of these two effects, long-chain hydrocarbons (such as motor oils) are highly viscous. Register now! Surface tension is the energy required to increase the surface area of a liquid by a unit amount and varies greatly from liquid to liquid based on the nature of the intermolecular forces, e.g., water with hydrogen bonds has a surface tension of 7.29 x 10-2 J/m2 (at 20°C), while mercury with metallic bonds has as surface tension that is 15 times higher: 4.86 x 10-1 J/m2 (at 20°C). Note the correlation between the surface tension of a liquid and the strength of the intermolecular forces: the stronger the intermolecular forces, the higher the surface tension.

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