Chap11-part3

Chap11-part3 - Intermolecular Forces Affect Physical...

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Unformatted text preview: Intermolecular Forces Affect Physical Properties -Viscosity - Surface Tension Viscosity Viscosity is determined by the ease with which molecules in a fluid can move around each other; i.e., by the strength of intermolecular attractions in the fluid. The units of viscosity are mass/length-time (e.g., kg/m-sec or gm/cm-sec). 1 Poise = 1 gm/cm-sec. Viscosity decreases with increasing temperature. Viscosity describes a fluid's internal resistance to flow and measure of fluid friction Viscosity can be measured either by examining the rate at which the liquid flows through a tube under gravitational pressure or by examining the rate at which steel spheres sink through the fluid. Bestsynthetic.com/viscosity Viscosity For structurally similar compounds, viscosity increases with molecular size. Which substance has a higher viscosity and why? CH 3 CH 2 CH 2 OH HOCH 2 CH(OH)CH 2 OH Which substance has a higher viscosity and why? CH 3 CH 2 CH 2 OH HOCH 2 CH(OH)CH 2 OH Glycerol, because of its higher capacity for hydrogen bonding. at 20 o C, water 0.010 gm/cm-sec 1-propanol 0.023 gm/cm-sec glycerol 15 gm/cm-sec Surface Tension Molecules in the liquid interior interact with other molecules in all directions. Molecules at the surface of a liquid interact only with other molecules along the surface or in the interior of the liquid. The net result is that molecules at the surface experience at net inward force, and the surface area is minimized. For example, water droplets form spheres, which have the lowest ratio of area to volume. Surface tension is a measure of the amount of energy it takes to increase the surface area of a liquid and comes in units of energy/area (e.g., J/m 2 ). Intermolecular interactions that bind similar molecules together are call cohesive forces. Interactions that bind molecules to a surface are called adhesive forces. The relative importance of cohesive vs. adhesive forces can be determined by examining the shape of the meniscus of the liquid in a tube. The pulling of water into a glass tube is called capillary action . water mercury in glass in glass **glass is negatively charged** **contact angle measurements** water in glass negative meniscus adhesive > cohesive water in hydrophobic tube positive meniscus cohesive > adhesive protein solution in glass more negative meniscus adhesive >> cohesive mercury in glass positive meniscus cohesive > adhesive Surface Tension Phase Changes...
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Chap11-part3 - Intermolecular Forces Affect Physical...

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