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Midterm 2 Notes.doc

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CHEM 120
Ariel Fenster

Chem 120 Alanna Houston Midterm 2 Notes CHAPTER 12 Surface Tension: - Interior molecules have more neighbours to be attracted to with intermolecular forces than surface molecules. - Increased attraction means a lower state of energy than the surface molecules. - Therefore, there is a tendency for liquids to maintain the lowest surface area compared to volume. (spherical drops of rain) - Spheres have the smallest surface area to volume ratio than any other geometric shape - To increase surface area means that interior molecules have to be moved to make surface molecules which means work is done - Steel needles remain suspended on the water because energy is required to spread the surface of the water overtop of the needle. - The gain in potential energy is greater than can be offset by the loss of the potential energy by the needle falling under the force of gravity. - Definition: energy or work required to increase the surface area of a liquid (represented by gamma) Units: J/m^2 - When there is increased temperature, the motion of the molecules increases and therefore the surface tension decreases because intermolecular forces decrease with the movement and therefore less energy is required to extend the surface - Cohesive forces: forces between liquid molecules - Adhesive forces: forces between the liquid molecules and the surface - If cohesive forces are stronger than adhesive forces, the water does not spread out and remains in its spherical shape (water droplets on a greased slide) - If adhesive forces are stronger than cohesive forces, the water spreads out (water droplets on a clean slide) The energy requirement for spreading the drop into a film is met through the work done by the collapsing drop. - Detergent lowers the surface tension of water and dissolves the grease to expose a clean surface. (Lowering the surface tension means lowering the energy required to break the droplets so it spreads more easily – these are known as wetting agents) Capillary Action: - Water seems to climb up the tube because of adhesive forces between the glass tube and the water molecules - The pressure below the meniscus falls slightly and atmospheric pressure pushes a column of water up the tube to eliminate the pressure difference. Chem 120 Alanna Houston - The smaller the diameter of the tube, the higher the liquid level because its magnitude is also directly proportional to surface tension. - Interface between water and air is called the meniscus and in liquid mercury, the meniscus is convex because the cohesive forces in mercury, consisting of metallic bonds between Hg atoms are strong, mercury does not wet glass. The effect of meniscus formation is greatly magnified. Because of this, the mercury in
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