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PODCAST 4.7

IB Chemistry Northwestern High School Dr. J. Venables. PODCAST 4.7. Some Properties of Liquids. Viscosity Viscosity is the resistance of a liquid to flow. A liquid flows by sliding molecules over each other. The stronger the intermolecular forces, the higher the viscosity. Surface Tension

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PODCAST 4.7

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  1. IB Chemistry Northwestern High School Dr. J. Venables PODCAST 4.7

  2. Some Properties of Liquids • Viscosity • Viscosity is the resistance of a liquid to flow. • A liquid flows by sliding molecules over each other. • The stronger the intermolecular forces, the higher the viscosity. • Surface Tension • Bulk molecules (those in the liquid) are equally attracted to their neighbors.

  3. Viscosity

  4. Surface Tension

  5. Surface Tension • Surface molecules are only attracted inwards towards the bulk molecules. • Therefore, surface molecules are packed more closely than bulk molecules. • Surface tension is the amount of energy required to increase the surface area of a liquid. • Cohesive forcesbind molecules to each other. • Adhesive forcesbind molecules to a surface.

  6. Meniscusis the shape of the liquid surface. • If adhesive forces are greater than cohesive forces, the liquid surface is attracted to its container more than the bulk molecules. Therefore, the meniscus is U-shaped (e.g. water in glass). • If cohesive forces are greater than adhesive forces, the meniscus is curved downwards. • Capillary Action: When a narrow glass tube is placed in water, the meniscus pulls the water up the tube. • Surface molecules are only attracted inwards towards the bulk molecules.

  7. Phase Changes • Examples – Name each of the following phase changes: • solid  gas: • liquid  gas: • solid  liquid: • gas  solid: • gas  liquid: • liquid  solid:

  8. Phase Changes • solid  gas: sublimation • liquid  gas: vaporization (boiling, evaporation) • solid  liquid: melting • gas  solid: deposition • gas  liquid: condensation • liquid  solid: freezing

  9. Phase Changes

  10. Energy Changes Accompanying Phase Changes • Sublimation: Hsub • Vaporization: Hvap • Melting or Fusion: Hfus • Deposition: Hdep • Condensation: Hcon • Freezing: Hfre

  11. Energy Changes Accompanying Phase Changes • Sublimation: Hsub> 0 (endothermic). • Vaporization: Hvap> 0 (endothermic). • Melting or Fusion: Hfus> 0 (endothermic). • Deposition: Hdep< 0 (exothermic). • Condensation: Hcon< 0 (exothermic). • Freezing: Hfre< 0 (exothermic).

  12. Energy Changes Accompanying Phase Changes • Generally heat of fusion (enthalpy of fusion) is less than heat of vaporization: • it takes more energy to completely separate molecules, than partially separate them.

  13. Energy Changes Accompanying Phase Changes • All phase changes are possible under the right conditions. • The sequence • heat solid  melt  heat liquid  boil  heat gas • is endothermic. • The sequence • cool gas  condense  cool liquid  freeze  cool solid • is exothermic.

  14. Heating Curves • Plot of temperature change versus heat added is a heating curve. • During a phase change, adding heat causes no temperature change. • These points are used to calculate Hfus and Hvap. • Supercooling: When a liquid is cooled below its melting point and it still remains a liquid. • Achieved by keeping the temperature low and increasing kinetic energy to break intermolecular forces.

  15. From origin to “a” = From a to b = From b to c = From c to d = From d up =

  16. From origin to a = solid From a to b = melting/freezing point From b to c = liquid From c to d = boiling/condensation point From d up = gas

  17. Vapor Pressure • Explaining Vapor Pressure on the Molecular Level • Some of the molecules on the surface of a liquid have enough energy to escape the attraction of the bulk liquid. • These molecules move into the gas phase. • As the number of molecules in the gas phase increases, some of the gas phase molecules strike the surface and return to the liquid. • After some time the pressure of the gas will be constant at the vapor pressure.

  18. Dynamic Equilibrium: the point when as many molecules escape the surface as strike the surface. • Vapor pressure is the pressure exerted when the liquid and vapor are in dynamic equilibrium. • Volatility, Vapor Pressure, and Temperature • If equilibrium is never established then the liquid evaporates. • Volatile substances evaporate rapidly. • The higher the temperature, the higher the average kinetic energy, the faster the liquid evaporates.

  19. Vapor Pressure and Boiling Point • Liquids boil when the external pressure equals the vapor pressure. • Temperature of boiling point increases as pressure increases. • Two ways to get a liquid to boil: increase temperature or decrease pressure. • Pressure cookers operate at high pressure. At high pressure the boiling point of water is higher than at 1 atm. Therefore, there is a higher temperature at which the food is cooked, reducing the cooking time required. • Normal boiling point is the boiling point at 760 mmHg (1 atm).

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